Exhaust Back Pressure Myth DEBUNKED!

[@raymondr7769]

As an engineer who grew up working on my own cars, these types of videos are awesome. They bridge the gap between the book smart engineers and the street smart technicians with the hands on experience.

[@januarioqueiroz3122]

👉😊👍

[@johnalcala]

As I posted above, I've been in a Fox body mustang GT that lost a lot of torque after he added headers. It was sad actually

[@j4ke2fl0w3]

@@johnalcala like he explains in the video, you still need proper tubing to create enough velocity without adding backpressure. too thin tubing restricts air flow and puts backpressure. too much air flow will not let the exhaust have enough speed

[@agems56]

That's because like we used to say about souped up Fords is " But it's still a Ford"

[@thecraftsman8133]

@@johnalcala lost a lot of torque? How did you measure that? Dyno? What about HP?? Did you gain or lose? Did it change the power curve up or down? I have never seen a Fox Body or any other "lose torque" from adding headers....& I have known a few. Wonder if you tuned? Or deleted the O2 sensors like I have seen a few bone heads do thinking they can delete and don't tune. Also your headers shorty, mid or long tube? Diameter of tubing? Its hard to believe you lost actual measurable torque. You didn't just "think" you lost torque and was just disappointed you didn't gain 35HP instantly like the advertisement on the Box said did you? And that lack of throwing you and pinning you to the back of your seat didn't happen so you just assumed or thought that was the outcome? Because gaining that "advertised" HP & torque on the box is supposed to include doing a handful of other mods to the engine to get that Maxed out HP gain from those "amazing" headers, such as tune for starters.... after freeing up the exhaust with headers, now your moving more exhaust and now its running lean and have to add more fuel. You are not running optimum air/fuel mixture. You aren't getting the most out of the headers if you don't do other mods, now your intake isn't keeping up, you need more fuel also.....tune tune tune tune after EVERY MOD YOU DO. if its a carburetor then make your adjustments add more fuel and free up the restriction at the intake to help the headers do what they are supposed to do, free up restriction

[@erasmus_locke]

I think the real reason the old timers told you to have back pressure is so that your car would be slower than theirs at the drag strip

[@xemanuelli]

They said that and then hit the strip on open headers 😂

[@markim5087]

Na, as a old timer I guess being 56 and wrenching since 88-89, the back pressure thing came around from heavier trucks ,or cars which need low end TQ at a lower RPM , for better TQ drivability, and what happens on say a blown supercharger D motor removing the back pressure will make it feel like the bottom end fell out of it, so that’s where it came from, plus dyno weren’t everywhere like today, to prove the facts, I’ve had plenty of supercharged motors which I regretted removing the factory mainfolds , granted it sounds better,and made more power overall, but since the power band was moved up , the drivability sucked it wasn’t as much fun why the vehicle was heavy , if I’d added a smaller pulley and retuned it ,then yes I’m pretty sure it would of felt better..

[@Cjbarker2]

​@@markim5087 I've noticed that most headers are designed to perform best at higher-than-stock RPM, and if you follow the ecomodder guys, they'll go out of their way to build a manifold that produces the most optimized scavenging possible with the cam/port/displacement combination. This results in vacuum/max flow at peak design RPM, but it does produce a restriction for higher-RPM use

[@robcarey4278]

An engine does need some back pressure for torque I know this for my own experience if you go to big of a pipe you lose low-end torque and they had to keep on going higher RPM to get any power why on God's green earth would he say something like this small engines do require backpressure not a lot mind you just a little

[@bobtee7899]

🤣🤣

[@drrrrockzo]

I have been trying to explain this to people for years, now I can just send them your video. Awesome content!

[@chestrockwell8328]

Same here... there are other videos as well but Mike has some "cred". Silly how even today people mention needing back pressure on a four stoke engine. On two stroke "back pressure" is really reversion pulse tuning otherwise two strokes don't need "back pressure" either.

[@daviddntait]

Exactly the same thing I was gonna say! I always tell people, if back-pressure is a good thing, go tell the top fuel dragster guys to put mufflers on instead of zoomie headers only!

[@frederickcook87]

Likewise, but there are folk out here still saying you need back pressure to make HP 😖

[@ryurc3033]

But really , how many people think what he said was I need 3 inch straight pipe on my stock Honda Accord

[@leftyo9589]

@@daviddntait and truthfully it is just a restriction, not back pressure, as you arent pumping air in from the other side.

[@zackrichardson2732]

How can you not just feel totally comfortable with everything he’s saying. The wisdom spoken by a man that has obviously years under his belt. He reminds me of an old Hawaiian surfing dude sitting on the beach, describing the waves and how to surf.

[@fattslapper1819]

Bro are you all out of drugs now? Or do you still think he’s jesus?

[@dudesweetpro]

This guy is so knowledgeable and always chill. He’s OG with all the jdm stuff and his builds are amazing!

[@RT22-pb2pp]

I believe engineers who really understand the science of things, call me crazy.

[@danielsvamp]

@@RT22-pb2ppNo need to be an engineer to understand the science and physics of these things.

[@nitramleinad9622]

Or Mr. Miagi...

[@lunoirxx]

I believe this myth mostly still persists from the fact that a lot of people don't realize that you need to retune the fueling on your engine once you make your exhaust more free flowing. People remove mufflers or install a new aftermarket exhaust and lose power because they just leaned out their engine. Happens in the motorcycle world I come from too.

[@working2bselfsufficient724]

100% is the tuning. People didn't know needed retune when change pressure either way to more or less. Once returned less pressure is almost always better as far as performance goes. However if want a quiet motor/exhaust you'll end up with a lot pressure after adding everything on. Cats mufflers etc which are needed to run quietly and for some states inspections if your state has them.

[@LifeInJambles]

I mean tuning is important when you're doing things that change your mixture obviously, but he literally explains the main issue with oversized tubing clearly in the video. Even properly tuned, the larger your tubing the higher the RPM your scavenging effect will be effective at. If you're running a track-only NA rig then you still need to pick the right size tubing, but it'll be larger than what you'd want for a street rig, especially a daily. Not getting any scavenging will result in imperfect exhaust clearing which means you can't pull as much fresh air in and can't squirt as much fuel. Basically as he explained it's a VE issue. No exhaust performs perfectly at every RPM so your tubing diameter should be tuned like anything else to the powerband you're looking for. Even if you tune the fuel to match, larger tubing raises your powerband and hurts your bottom end power.

[@LifeInJambles]

This all basically goes out the window when you're dealing with a turbo. On a turbo application you basically want the least restriction possible post-turbine, so an open, oversized exhaust tube can only help performance if you can live with the sound.

[@imoffendedthatyouareoffended]

Odd, after removing the mufflers off my wrx it felt better, it definitely didn’t lean out though. Not even slightly. Backfires like a mfer.

[@lunoirxx]

@@imoffendedthatyouareoffended Do you know your AFR? Lean backfires can happen too. Could be that your ECU is able to compensate for the increase in flow. Not all fuel injection systems can do that though.

[@TheLDORDAZ]

This man is a legend, and the fact that he’s out here sharing his knowledge and actually working on projects is awesome!

[@skivijimmy]

He might be a legend but it's so irritating to watch him with that stupid smirk on his face

[@powar7746]

​@@skivijimmy bro? 💀

[@SAMPLETEXT285]

​@@skivijimmy then dont watch him its a pretty simple concept but i know that can be hard to comprehend for someone like you who might be a little IQ deficient or lacking in intelligence hence why instead of just clicking off you complain in the comments but i want you to try it at least once or twice i promise you that you'll be better off doing that. And also nobody asked for your opinion on him either just saying.

[@fuzzyfireman1]

My thought is that in the old days, people would go full flow exhaust without re-jetting the carb. It would run leaner and lose power. I know because of a 1983 thunderbird that I put a 351w in. I ran dual exhaust with a h-pipe. it had slowed down. Once I listened to a local garage friend of mine (running that garage since the 60s) I drilled out the jets slightly, corrected timing, and installed colder plugs. It made a huge difference.

[@motoiq]

Bingo you get it!

[@kimkleiner8456]

I remember finding a book in high school and i believe the title was "the design and building of the british sports car" and by sports car they really mean racing car. It was written by a pair of british mechanical engineers and what i found fascinating was that it was filled with formulas and tables that at first i was pretty sceptical about. For example and pertinent to this subject they had a formula that you could enter displacement, compression ratio, stroke and maybe rpm that you wanted to optimize and it would give you diameter and length for both intake and exhaust. Now this was 50+ years ago so i don't remember the formula or variables exactly but i was amazed that these formulas existed. As a gear head and at that time heading toward a career as a mechanical engineer it reinforced that math could be a handy tool to be utilized toward that goal.

[@michaelhurley3333]

Probably David Vizards, he's pretty sharp!!!, Also on U tube!

[@MrSilus2000]

The nerds try to forumulize what the real mechanics know by feel.

[@Broprotato]

​@@MrSilus2000Cringe, without the nerds you would have no cars to fix or tune by "feeling"

[@MrSilus2000]

@@Broprotato Inventions don't come from nerds. You actually need imagination and balls. History lesson

[@Broprotato]

@@MrSilus2000 Sure bud, complexe machines just engineer themselves with passion and goodwill. No math, no physics and chemistry is required. You are just creating your own facts.

[@fishandgameman]

Great explanation! You are so correct! The whole purpose of cam overlap is to let the air/fuel charge begin flowing into the combustion chamber while the exhaust valve is open, creating a negative pressure due to the exhaust pulse velocity leaving the header pipe. AF can begin filling the chamber an eternity of time, with respect to crank position, before the downward piston movement can pull the AF charge. Proper fluid dynamics is paramount to VE absolutely.

[@geemy9675]

sounds a lot like how 2 strokes scavenge exhaust gases with fresh gases. it might benefit from pressure waves bouncing off changing diameter in the pipe, but obviously only at certain rpms. might work if you're tuning your engine only for top end power

[@j.thomas7128]

And ya don't get negative pressure without scavaging. To have scavaging, there must be some backpressure even if it is as minimal as a 4 into 1 header; as opposed to a true open pipe for each cylinder. Run any motor on a dyno stand with a header and short collector or down pipe vs no header at all as in zero backpressure... tell me your results.

[@geemy9675]

​@@j.thomas7128 first, remember this is especially about tuned engine with valve overlap . you're right its negative pressure that can help scavenging, but you cant generate negative pressure with positive pressure( back pressure). back pressure does the opposite of what you want for scavenging. It's exhaust gas velocity/inertia that keeps pulling fresh air in the cylinder during valve overlap when intake valve just opened even though exhaust gas still has more pressure than intake. The exhaust gas "wants" to go back to the lower pressure (intake), but because its already going fast towards the exhaust, it only slows down, but keeps moving through the pipe, and sucks gas behind it. with no header, exhaust velocity is lost because gas expands in every direction, so during valve overlap, the exhaust gas will probably go backward in the intake. if you have no valve overlap, open headers shouldn't have such a big impact, but still, inertial scavenging can help squeeze the most gasses out of the cylinder before intake opens. In fact, velocity is not the only parameter that can affect power, if you want a very complete explanation why back pressure is not good, watch kzbin.info/www/bejne/oJuzloOVg9NleJY maybe what you meant is that back pressure is needed as a by-product of velocity, but the best compromise is still highest velocity with lowest back pressure

[@j.thomas7128]

@@geemy9675 i understand your explanation. I think the point was missed. You don't have scavaging or boosted exhaust gas velocity unless you have a collective header or collector pipe. Any collecting header or pipe will add some, even if very minimal, backpressure. You won't have the same gas velocity or scavenging without a collector where the just fired cylinder helps to evac the exhaust from the previously fired cylinder. An engine with no header or exhaust manifold will have no as in zero backpressure. Adding a collective header will add some, if minimal, backpressure and some amount of backpressure is needed to create these scenarios. Do you think this reasoning is faulty?

[@fishandgameman]

@@j.thomas7128 Yes maybe, but barely measurable and for an almost immeasurable amount of time, as some of the exhaust pulse from another cylinder may dam a header pipe at the collector. I think if that were to become measurable or rob performance it would be related to post collector downstream exhaust piping-too small, too large, or turbulent flow. When you consider how many times a valve opens or a piston moves one stroke at 3000 RPM in seconds, there are going to be variables in velocity causing different flow dynamics. I installed a WM8-22XE cam in my M8 107 Harley. It made a huge difference over stock performance with a good tune. I thought it was great and at peak performance, but I never could seem to get the engine to spin up achieving RPM over ~4500-5000 and it took a while. I didn’t know until later it was sluggish as hell. This back pressure myth is huge amongst Harley peeps, and they all parrot the same shit they hear. It didn’t make sense to me, so I rid the factory exhaust of a great big engineered restriction upstream from where it goes from one back into two before the Tab Performance mufflers. The change was unbelievable! Not a little or placebo effect, incredibly better. Immediate taps on the rev limiter in in the first 4 or 5 gears and the only thing preventing it in higher gears is my sense for self-preservation. Speed kills, 😆. That cam has a lot of overlap. Unequivocally, the M8 doesn’t need back pressure. It destroys performance at least in my case. Take it for what it’s worth, this was my own experience. ✌️

[@Tyronetrinity]

This guy has a smile on his while explaining engine stuff. Thats how u know he love what he is doing❤

[@Lifeisagamekid]

Or he’s stoned doing it lol

[@crawford323]

Gases have mass and moving gasses have inertia. Gasses have viscosity which means they will stick to each other and to other materials, in this case the exhaust pipes and components. Gasses are compressible. Gasses obey the laws of thermodynamics. These things can complicate design efficiency efforts. Why are expansion chambers necessary on some two cycle engine exhaust systems?

[@danielsvamp]

@@crawford323Expansion chambers are for reflecting exhaust gases back towards the cylinder, the exhaust gases contain some fuel which got sucked all the way through the cylinder (which is good for cylinder filling). In an optimal situation the expansion chamber is tuned exactly for the exhaust port's dimensions to have the returning pulse enter the cylinder just before the piston closes the port. With increasing engine speed the returning pulses grow stronger and stronger until you overcome the "hill" on the powerband. After peak power the pulses return too late to get into the cylinder (because of the port closing too soon (piston moving too fast)). By having a shorter pipe the pulses return quicker and the power moves higher into the rpm-range which can greatly increase peak power, though this also significantly decreases torque at lower engine speeds. Sorry if this is hard to read or if I'm rambling, I know I am, I'm crazy about this stuff. Video from motoiq: kzbin.infoqf9eFYPYXUY?feature=share, great video from 2stroke stuffing: kzbin.info/www/bejne/eYvTn2honMZ0Y5I

[@edwardmoreno9435]

And that’s why I love this man

[@razor6888]

He has no idea what he is talking about...... this is a views and money video.. dont take advice from here. Any Tech worth his/her salt will tell you that. Pure Garbage.

[@TomJones-uf5sl]

This is one of the best explanations of exhaust flow. I have always focused on keeping velocities high in the exhaust and intake. Usually my race cars and motorcycles have always run at the front of the pack as a result. It's counterintuitive sometimes - smaller intake runners often seems wrong - but the results are provable on the dyno and the track. Same goes for hot street cars and race cars exhaust.

[@sahzhree7757]

Smaller intake runners always seem wrong, until you realise early race and rally cars (especially euro) often featured independ throttle bodies, with basically 1 - 2 inch runners.

[@xxxxxxxxxx6903]

To be honest I didn't even watch this video. But based on the video's title, it seems he just wrote off the huge potential NA gaines from exhaust scavenging? No exhaust back pressure = little to no scavenging! Not to mention in anything less than max-effort combos, exhaust back pressure is a design tool to optimize compromises between performance, mileage, NVH, and emissions.

[@ALOSOOON]

The venturi effect. A liquid or gas gains velocity after transitioning from a small opening into a larger one.

[@ALOSOOON]

​@Xxxxx Xxxxx Watch the vid. You can have scavenging without back pressure. When the exhaust leaves the tailpipe it pulls.

[@truth4reel]

Resonance tuning?

[@LarryLinton]

I've never seen anyone look so happy when talking about exhaust gasses 😅

[@thehouseofhorsepowerautobo4506]

Haha! that's what I was thinking too 🤣🤣

[@paulwarmoth3708]

How would a person not like this guy? He gives great information in a manner that anyone can understand plus he is cheerful!

[@SixFigureStearns]

He’s quite literally an Engine Guru.

[@seansmith1725]

Engine Buddah

[@PutsOnSneakers]

Smoke weeed erryday

[@chrispitre2439]

Guru Mike

[@shorty808100]

he's 1/2 right 1/2 wrong

[@motoiq]

It will be hard, Smokey has been dead for some time now.

[@froat1]

A lot of folks confuse the return shock wave of a tuned exhaust with back pressure.

[@thomaslabrum8182]

I can see why, if an engineer using the lingo is explaining exhaust pulse resonance to some average joe that just puts gas in the tank, I can see the takeaway being “more back pressure”

[@daynasmum529]

Perfect example is the expansion chamber on a 2 stroke - it does give an amount of back-pulse if tuned right to the engine - preventing fresh incoming charge going out the exhaust port increasing efficiency.

[@tcknapp1984]

I love this guy. He'd make a phenomenal teacher/instructor.

[@kopronko]

Yes, But, People Must be aware of what a Terrible Idiotic Deception is the catalytic converter , and even much worse is the Egr valve. !!! ! !!!

[@samarmas5082]

I just watch the video because I felt happy watching this guy explain something complicated yet so chill.

[@dale116dot7]

I’m a ECM designer, this all makes sense, this is a perfect explanation that doesn’t dig into the thermodynamics and compressible flow so deeply as to be intimidating. A lot of work I do is on turbocharged engines and it’s quite a lot different as you mentioned, especially if the ECM has control of boost and if you’re lucky enough to have access to the full calibration tools like I do. The one time I actually want exhaust restriction with a turbo is when I want to balance pressures with an EGR system… imagine balancing boost pressure, manifold pressure, wastegate flow, turbine flow, EGR flow, and exhaust pre-turbine pressure. That software is tricky to tune, in this case I’m using EGR to manage exhaust temperature at WOT. One thing I’m always amazed at is the low restriction and conversion efficiency of modern catalytic converters. I remember those old pebble bed converters… they were awful. One thing to not forget when calculating exhaust system flows, restrictions, or pressures is that the temperature is high enough to affect the calculations a lot. Thanks for a great video.

[@motoiq]

Who do you work for? I curious thing I have found that was not intuitive is how EGR can help control detonation on a turbo car. The other thing I found interesting is how much a cat hurts power on a highly tuned naturally aspirated engine with a lot of overlap, more than what the backpressure gains would suggest. I think an out-of-phase-reflected wave is generated by the face of the brick and causes reversion. I think I am going to do a more scholarly version of the video based on all the idiotic comments this video gets.

[@panicow6005]

So are y’all saying straight pipe is better or worse for turbo cars

[@jimbrown3720]

What would you consider a modern catalytic converter ?

[@dale116dot7]

@@jimbrown3720 After roughly 1998, when the chemistry and oxygen storage was optimized for dual loop feedback and active oxygen purge after a lean excursion or fuel cut event.

[@dale116dot7]

@@motoiq I can’t really say for legal reasons who I work for but there was a SAE paper by Cummins that considered this. EGR was needed more to limit exhaust system temperature, but the combination of EGR and spark timing allowed enough margin from detonation and overheated exhaust valves at the target torque/boost. Keep in mind that these are truck/bus engines that go for half a million miles.

[@GeorgeKennedy-u4r]

Had exhaust work done on my Toyota a while back and I was worried that they cut out all the flanges and welded the whole pipe front to back. Thought the engine needed back pressure but I noticed a huge power increase (with the small motor) after that repair. Thanks for the reassurance.

[@X739LEG]

why did i just watch a dude for 12 minutes that had the look of someone who was about to tell a naughty joke only to give me facts then go off the deep end the last 10 seconds with the jokes. this dude rules and im now a fan. also i learned something!

[@Danteneon]

Thank you so much for this video. I have been explaining this to folks since the 80s. I am still surprised how many people (even in the industry) don't know this.

[@pipedynamics116]

Many in the industry still believe the back pressure myth, it can be frustrating debating with them

[@Comet-hn3gm]

@@pipedynamics116 Don't bother. Make more power and go faster, while they perpetuate a lie.

[@dormantmenace]

Cause it's bullshit.. open pipes get you more peak numbers. But back pressure puts the torque curve were it's usuable for average spirited driver. Not all on top like a you on the track or drag strip

[@willbeasy2898]

Same here. Seems to fall on deaf ears.

[@AdmiralRustyShackleford]

@@dormantmenace learn the difference between backpressure and optimal velocity

[@yodab.at1746]

Depends on what you refer to as back pressure. The harmonic of the exhaust pulse is used in conjunction with valve timing so the pulse arrives back in the exhaust port to keep the incoming charge from leaking into the exhaust port just before the valve shuts. If the action of the gas flowing out of the cylinder is being pulled by momentum or the flow of exhaust from the previous cylinder's exhaust, the incoming charge will be at risk of following the exhaust gas out. The exhaust note changes because of theses pulses especially when the engine is "on cam". Back pressure in this sense doesn't effect overall volumetric efficiency, because it's not an overall restriction in gas movement, just a pulse at the right time.

[@fppro1679]

Nope. Just plain old back pressure. That would be pumping loss. These are not harmonic shock waves traveling at the speed of sound. This is just plain old gas holding up the piston from coming up freely! Purely arcane pneumatics and a little bit of physics about expanding gases!

[@yodab.at1746]

@@fppro1679 lol. Not the speed of sound. Its resonance. Its part of why you get a power band. And I don't mean Led Zeppelin

[@fppro1679]

@@yodab.at1746 the resonance you're referring to, is it related to an acoustic event going through the gas, not the gas itself flowing out of the tube. The event itself is like sound or it is sound itself. Essentially traveling through the gas. What I'm talking to you about is pumping loss which is completely different because pumping loss is about the volume of gas being restricted and causing pumping loss, two distinctly different things i. E. Gas volume and flow, and shockwave.

[@Thekickbackartist]

We are all dumber for having read the above nonsense

[@johnbower]

You are spot on, in the past I have made stainless steel exhaust pipes kept them as straight as possible, built the silencer boxes with straight through perforated tube surrounded with fiberglass wool, and found the engines run smoother due to no back pressure in the exhaust system.

[@razor6888]

LMAO

[@Starscream8896]

@@razor6888whats funny

[@capicolaspicy]

I am an old far, 71 of this year, but thankfully born with a natural grasp of science and physics. Got into so many arguments in my youth with so many people fold into believing the old myth that you need some back pressure son! This was a really well done video and I learned a few more things from you, so thank you so much for your knowledge and sharing it!

[@perceive8159]

I agree, to explain why, my son had a 2002 Ford Escape with a rattling cat in one of 2 cats built into the exhaust manifolds which had the v6 3.0 lit. The downstream cat was rattling also. At that time replacement cats were insanely expensive, so I decided to remove the cat material out of each component. On the manifolds I zip cut a square opening and removed the cat material and wielded the tops back on, did the same on the downstream cat and installed a performance muffler with a 2.5 in and 2.5 out and kept the back resonator muffler and all other tubing in-between I used 2 1/4. Granted I got a check engine light doing this stuff but that wasn't the concern at this point, it was to keep the costs down for my son and emissions inspection was removed at that time. Now did we feel any difference in performance, you bet. I personally had a 2008 ford escape v6 in top running condition and hands down his escape ran faster off the line Reved up quicker, it would even light up the front tires a bit of the line. So the point of this is the back pressure was removed, the volumetric efficiency of the air pump (engine) increased dramatically! Nothing like a actual experience to prove the point and support the video shop talk on this topic 😎

[@smokincooks7661]

You just admitted to breaking US federal law, if that's a road-going vehicle. Enforcement is lax, but it's a jerk move to selfishly pollute to save money.

[@captainchaos3053]

There is also the BIG problem of adjustability. If you remove the back pressure from an engine and assuming it can get enough fuel from the carbs to prevent it from running too lean the ignition timing may need to change extremely. You must size your pipes accordingly and remember the the limitations of the rest of your components. It's horses for courses. Newer engines are very different with fuel injection and electronic ignition being much cheaper and easier to tune to your requirements.

[@adogmcdizzle]

Great video!!! As an engineer this was very interesting. I had heard the addage about back pressure, but never really thought about it until now. A friend of mine works on exhausts for a major OEM. I am going to quiz him on some of your points next time we catch up.

[@kevinedwards7206]

what did he say bro?

[@simonh5110]

One thing I heard from an old guy made a lot of sense to me. He said, why build the baddest set of heads that flow the most, just to restrict them?

[@imoffendedthatyouareoffended]

Gained a new subscriber, love when you supply information in a simple effective manner for everyone to understand. This is the perfect video to send someone when they start talking about back pressure.

[@clintvosloo7694]

In defense of the old timers I think what they meant was exactly the fact it needed some resistance for gas velocity. .it probably got misconstrued with time that you needed lots of back pressure instead of the right size to allow the gas to escape smoothly

[@JorgeHernandez-lu1mi]

In defense of all the engineers who I went to school with, the old-timers were wrong. You don't need back pressure in a four-cycle, multicylinder engine's exhaust system, you need backflow prevention between cylinders that coincide on intake and exhaust. The way to do that is to tune acoustics into the system to do that. Back pressure defeats the effect of pulse acoustics.

[@codyharney2997]

You want each exhaust pulse pulled by the pulse in front of it. You don't want to have to shove the pulse into a tube that's already filled with other pulses that haven't left the tube yet. Pull>push

[@neiliewheeliebin]

What you just said doesn't even make sense back pressure does not equal velocity whatsoever

[@clintvosloo7694]

@@neiliewheeliebin it does to lamens. .my point exactly .

[@neiliewheeliebin]

@@clintvosloo7694 It needs adequate diameter pipes to maintain velocity not back pressure, in some instances it can be literally sucking gasses out of the next cylinder in naturally aspirated engines with over 100% volumetric efficiency

[@CamerOneiric]

Sending this to my roommate (a jeep owner) who just two days ago made this claim and even went so far as to say that without back pressure "you lose 100 lb-ft real quick" in a Jeep 😂 Thanks for coming in with the right information.

[@thecraftsman8133]

Only in a jeep??? Lol...kick your roommate out! Lol...its funny how a lot of people go off of superstition also. There must be some kind of curse with jeeps. If you do something like whatever they lose 100ftlbs instantly or in his case "real quick"! Where do some of these people gain their info? Tell your roommate he needs to crack a book and stop listening to idiots. Kinda shows how Naive he is...good luck. Hey, now that you know he believes he will loose 100ftlbs of torque REAL QUICK and belives in angels and unicorns, you know he is naive. he definitely needs to get his info from somewhere else. You never know a person till you live with them.

[@isaacgarcia8686]

@@thecraftsman8133 Idiots are everywhere.

[@ajosepi1976]

I was told that the back pressure thing started in NHRA when they were trying to make power with flat head Ford V8s. If they ran no pipes they made less power than headers. Then it kind of got out of hand with people not really understanding what was going on, but they understood just enough to make it work. Granted this was told to me from an old NHRA crew chief about 30 years ago, so I am sure he knew what he was talking about, but maybe I don't remember so well. Good video.

[@motoiq]

He is probably right.

[@Nabraska49]

I got to say how incredibly well you are at explaining things .. the right information well said piece after piece .. a rare gift.. one thing you could of put down with your explanation is to briefly cover overlap and that would have made your explanation flawless..

[@tlomz470]

To be fair to the back pressure idea, I think it's more of a misattribution than just a malicious lie. The proper flow of the exhaust system through adequately sized tubing (as you described) is probably what a lot of those people have in mind, but how it's expressed can get lost in the weeds to the average layman.

[@WhalerGA]

Yes, definitely. With naturally-aspirated engines, effective exhaust scavenging (ex: through the use of appropriately-sized long tube headers) is helpful in the generation of a healthy power curve. Forced-induction engines are a different story: With forced induction, the bigger and more open the exhaust, the better.

[@AdamantineAxe]

Probably the thought that no headers=no back pressure=worse low end

[@thomaslabrum8182]

I would say it probably had to do with exhaust pulse resonance, old headers were designed with fit over anything else, and a bigger tube on a shit header probably caused turbulence in the exhaust. A mis-timed exhaust pulse will actually prevent scavenging (the vacuum effect) from pulling the remaining exhaust gasses from the cylinder. Nowadays your stock or even aftermarket headers are already formed with exhaust resonance in mind, so everything is different than it used to be.

[@pablovargas2778]

I recently did a muffler delete on my 2007 v6 mustang (which has kit h-pipe with dual tips) After the delete, i noticed some loss of power (not that I had much anyways😂) like in Hp and torque and thought it was because I didn’t have enough back pressure. So would the actual logical explanation maybe be that my exhaust pipe may be to thick in diameter that it’s not flowing at a fast velocity? Thus reduction in power? My car engine is basically stock btw.

[@flowmastaflam]

It's true. The theories were always correct, but the jargon was slightly misrepresenting.

[@SpecialEDy]

There's a simpler, more intuitive explanation. It's pumping loses. If there is pressure in the exhaust, the crankshaft has to push the piston upwards to force that exhaust out, which steals power from the crank.

[@motoiq]

actually if you look at the Otto cycle PV diagram most of the energy is used during blowdown.

[@SpecialEDy]

@@motoiq blowdown is power lost, when the valve opens before BDC, this means some of the heat/pressure is not able to drive the piston to BDC and some energy is lost rather than extracted. If we look at the intake side of the engine, there is probably 85-95% volumetric efficiency on most production head designs. The exhaust side usually has smaller valves, inferior porting, and inferior manifold design, though a shorter dynamic head(mathematical pump head, not physical cylinder head), and a higher speed of sound(probably 2-3 times if I had to guess). So, though exhaust is much easier to expell than intake is to draw, there is still restriction which the piston must drive against. There will always be a loss on a real world engine during the exhaust stroke, scavenging helps at the expense of blowdown. This is especially true of Turbocharged engines. The exhaust turbine is essentially being driven by the crankshaft via the exhaust stroke. The higher the backpressure, the more force required to drive the piston against it. Which is why exhaust is so important. Backpressure creates pumping loses, scavenging reduces loses, theoretically scavenging below crankcase pressure could actually pull the piston upwards and add power to the crankshaft.

[@pepperfish_]

Great explanation, it's pertinent to mention that the knock characteristics change with the flow of the exhaust. I've noticed a major change in my CL9 project when I increased the DC header collector size from 2 1/4 to 3". The tune actually works now ;). makes power all the way to 7600.

[@tiles3458]

I cant wait to install my weapon r header and 3 inch single exit exhaust! I have hondata so I know it'll change the characteristics quite a bit.

[@pepperfish_]

@@tiles3458 make sure to either disable or adjust the knock sensitivity, headers are "noisier" than oem manifolds and can still fool the knock sensor into cutting back on timing. FYI.

[@gwot]

I have read so many articles, even sketchy forum posts about backpressure and none of them explain to me exactly why it is needed. I understand what it does, just couldn't understand why people say you need it. Thank you for finally explaining it to me.

[@williamdawson3792]

Definitely need it

[@gwot]

@@williamdawson3792 You are prime example of what I read online, just a statement of "need it" no explanation as to why and reasoning.

[@NavySturmGewehr]

I've often wondered if there would be an advantage to slowly increasing the diameter of the exhaust down the temperature gradient.

[@charlesmyers9765]

No, not only would it have a negative effect on gas velocity but exhaust gas, though miniscule, cools as it travels down the pipe. So what your thinking is counter intuitive.

[@terborgh8757]

This is gold content in the time of bite size and dumbed down yt clips. I love it that is unpolished, presenter makes mistakes like looking into the wrong camera, but the content and info is spot on.

[@infinition]

Thank you so much. I had always misunderstood this concept. I've noticed lower engine response at low rpms on small NA engines with huge exhausts. I always attributed that to lack of back pressure. I'm really thankful that you've cleared that up

[@LifeInJambles]

I've got a lot of respect for you just now tbh. Not often you'll see somebody say "looks like I was wrong about how things worked, thanks for correcting me" on the internet.... especially about this specific topic.

[@jacksin3323]

Great info. I always rebuild my exhausts in my cars, and even make my own resonator pipes and muffler systems. I was a sound engineer and a firearm supressor designer for years so its a good background.

[@motoiq]

What company did you work for?

[@razor6888]

LMAO

[@Nighthawk745]

Mike, I really appreciate you sharing your tuning knowledge, some of what you said went over my head. Please can you make an in depth course on different tuning topics like exhaust fabrication for highest power, engine building, suspension set up. That would be awesome!

[@motoiq]

check out some of the stuff we have already done, here and on motoiq.com

[@xaviermauerhofer4222]

I’ve got a mate telling me to not get an exhaust cut out on my diesel ute. When I heard him say that, I was for sure thinking ‘wtf is he saying’. I always thought that you’d want a good escape route for fumes so you don’t cause blockages etc. This vid just helps me make my case 🎉🎉

[@Choppin_broccoli]

I am loving this series, Mike is hands down one of the greats in motorsports.

[@zillaspec9244]

thanks mike! this has helped me finalize the exhaust design and piping diameter for my e92 m3 with a 4.4L stroker. since the factory piping is around 2.5" iirc, so with that said, the design would utilize a slightly larger diameter piping off the headers, to a smooth Y merge, to a larger single pipe all the way back. this should be the ticket here but I'll know when it comes down to meeting with my tuner again. with many revisions of course :)

[@kopronko]

People Must be aware of what a Terrible Idiotic Deception is the catalytic converter !!! ! !!! And even Worse is the Egr valve. !!! ! !!!

[@B_r_i_n_k]

I love how Mike looks like he's always holding back the biggest laugh because he just heard the funniest joke.

[@truethought369]

I was in mechanics for meny years. You are spot on. Intake and Exhaust works better without any restrictions. Like anything that needs to breathe! The only way to improve this is by forcing more air into the engine.

[@mikeplat4745]

I absolutely love your content, there are many who will not understand how you are explaining it but I do one hundred percent, Master tech for Ford Motor Company of Canada for about twenty years now. I appreciate your knowledge and how you explain it as it rings out crystal clear to me and you have helped tremendously on multiple builds my son and I are doing currently as they require different exhaust requirements. (North American race motor big and small blocks in vehicles way too small to JDM cars all over the map) Keep up the great work and thank you.

[@calvinevans8305]

Those pulses happen in the intake manifold as well. Richard Holdener explains the concepts very well.

[@RaceMentally]

Doesn’t he prove back pressure is good depending on what you’re trying to accomplish?

[@calvinevans8305]

@@RaceMentally low to no back pressure is ideal. The pipe diameter needs to be big enough to flow with no back pressure but not to big because the exhaust will lose velocity. The idea is to give the combustion chamber and cylinder the most efficient airflow during the intake and exhaust strokes especially with overlap from a performance camshaft. When both intake and exhaust valves are open briefly the exhaust flowing out of the combustion chamber helps draw fresh air from the intake valve. This is called scavenging. Back pressure counteracts this effect. It's like the difference between coasting down hill and riding uphill on a bicycle.

[@wernerxldata]

It is true for two strokes, the back pressure created by the exhaust cone sends back fresh mixture in the cylinder while the exhaust port is still open. The length and cone shape of the exhaust can be used to determine at what rpm the back pressure is most effective.

[@jeffreystroman2811]

Excellent point, although the effect is only for a short duration of any piston cycle, the rest of the time the above is applicable, I imagine.

[@wernerxldata]

@@jeffreystroman2811 It is at the part of the compression stroke, when the exhaust port is still open, the piston pushes fresh mixture out of the port and the pulse wave from the exhaust pushes it back into the cylinder.

[@brettturner7057]

Sorry, not true for 2-strokes either. What a properly "tuned" exhaust for a 2 stroke does is create reflected pressure waves that when timed correctly, hit the exhaust port to reduce the amount of incoming mixture that escapes during port overlap. This is resonant tuning, NOT backpressure. I think the issue here is exactly as Mike explains in the video - backpressure is the wrong word, or at least conjures up the wrong image in people's minds. On 4 strokes, a slight amount of backpressure is a byproduct of creating velocity. You can create backpressure with a crappy manifold or restrictive mufflers, but that doesn't create velocity. The same is true in the argument for 2-strokes. Backpressure is the wrong word. Sure, the reflected wave in a 2 stroke exhaust is a pressure wave, but that's not the same as "backpressure".

[@wernerxldata]

@@brettturner7057 You're right, back pressure is the wrong term here.

[@markgarland9000]

"Pulsating hot gas generator"! Sounds like a job for Tums! I started driving in '69 and would hear the "back pressure" argument. First time I've ever heard it explained! Thank you.

[@darryladams519]

I remember reading a Hot Rod article from back in the 80's or early 90's they did a dyno test using different exhaust. With H pipe, X pipe and true duals as well. The true duals ( one pipe from the header to tail pipe for each side) made to most hp and torque.

[@luipars]

This is also the reason why dragsters have one individual (very short) exhaust pipe per cylinder.

[@irelandgrt]

When I drove my WS6 with open headers it would barely get out of its own way. When I switched from a 3" Y-pipe to a 2.5" my car got noticeably more powerful in the lower rpms.

[@moskavado]

Should have shortent the lenght of the intake to mathc the increase output flow then you would see an increase in horsepower up in the rpm range but at the cost of low end torque. You just lost torgue without the hp gain the way you did

[@alexbrown4820]

Was it tuned for open headers?

[@Big_ol_bass]

That's a tuning issue.

[@NoOne56488]

The issue with oem exhausts are they're made for noise and emission standards, and not for the performance of the engine it self. Take a Nissan Patrol in Australia, by upgrading exhaust after the cats to a 3inch mandrel bent one you gain torque and a bit of hp. The standard exhaust has a section which is flat and a diameter of 1.9 inches, which for a 5.6l V8 is nuts, it also has hard bends and the mufflers has sharp bends and a value in it. Went after market with a good 3 inch and I've gained 30nm and 6kw and fuel usage dropped a bit as well. Yes, the car has a nice V8 note to it now, it's not overlay loud and it's made of 2mm thick 304 grade stainless steel which will last the life of the car.

[@v1-vr-rotatev2-vy_vx31]

Yes my old school buddy and back pressure,,, we'll have to show this to him,, tried to explain it to him that turbo cars needed a 3-in exhaust when the boost is turned up,,, still didn't quite get it.

[@ACommenterOnYouTube]

Old school V8 Muscle guys said back pressure was GOOD because back pressure AKA that restriction in your system helped create low to mid range power. When they did upgrade the exhaust system and placed headers, that MOVED the low mid peak torque to a higher point in the rpm's, a point where they NEVER reved to, EVER. So now what they experienced was a LOSS of low end mid range peak torque giving them the impression they lost power when they really didn't. SO they said, back pressure is good and is needed. My 240sx ka24de motor had a LOT of low end torque with stock exhaust system but had sh*t for top end power. When i slapped a 4-1 header, high flow cat and 2.25 inch catback, i LOST my low end torque but gained a lot of top end power. Now that engine was breathing and felt better. Sure take off was a lot slower but as it reved, it was smooth and got faster as i get to 7k rpm where as before, it would launchg hard and fast but be dead by 6k rpm ...

[@ryanhorsley9965]

His guidance is consistent with what any chemical engineer would learn in a fluid flow class. Subscribed.

[@valkman761]

Chemical engineer in a fluid flow class, LMAO

[@JacopoSkydweller]

@@valkman761 What? They take 2-4 Thermodynamics classes, and multiple fluids classes. What do you think air and steam are if not a fluid?

[@valkman761]

@@JacopoSkydweller fluid dynamics is what's relevant here and presentation isn't really consistent with what someone would learn in a "fluid flow class". The premise of less is more here is really not consistent with anything other than wide open throttle scenario

[@HSGAutomotive]

Dude is the Bobby Lee of mechanics. His whole vibe is spot on 😂

[@dougmyers6013]

Yesterday I pit a 3" mandrel straight pipe and an absorption muffler on my 3.0 VETEC 6sp Accord coupe. Love the power band shift, not so much the increased sound but then I did acquire this car to modify. Always have removed as much back pressure as possible from the cars I've built from mild to wild & had to laugh at the "Old man" remarks concering back pressure a staple of the Mom & Pop muffler shops, was even told once the car will never make enough power to get off the rack with the size pipe you want installed ha ha ha

[@monkeyportions1722]

Good to hear this after gutting the dpf on my van, was worried it had lost boost as it felt less punchy but it seems to just spool earlier and smoother so feels less aggressive but more efficient, great explanation!

[@Patrick-857]

Hope you got a tune afterwards. Any change to a modern engine setup needs to be compensated for in the ECU, otherwise it screws up mixtures ect.

[@monkeyportions1722]

@@Patrick-857 yeah first test drive was to be reprogrammed. The dpf done a regen on the way and got smokey but it was alright to drive there.

[@Sackmatters]

Finally a well thought out and explained video. So tired of people saying they need back pressure.

[@Widowmaker2828]

A lot of guys like me say that it is true, you do in fact need a certain amount of back pressure. The guy actually states this in the video. You need the exact amount of back pressure to maximize velocity through the motor of any given aspiration. 🥂

[@mrman1536]

@@Widowmaker2828 true that. It also reduces burnt valves.

[@robrichert8830]

I could listen to this man for days. He's a definite virtuoso of the auto engine.

[@rockman4699]

The torque factor needs to enter the equation. Depending on the application back pressure can help with mid range torque. Drag racing and road racing applications don’t require mid range to work but motocross and Enduro application certainly do

[@SemiglossOyster]

This man's cadence and demeanor do not seem naturally aspirated and I'm all here for it. Thanks for dropping this one I've been curious about this for years!

[@FragEightyfive]

Interesting about turbo exhausts. On my VW 1.8t you could change shift the power up and down with exhaust size. a 2 to 2.5" had more low-mid torque, and a 3" had more up top. For a street driven car I preferred the middle sized options. Not sure why the turbo spooled faster with the smaller exhaust. I didn't notice much of a difference in fuel efficiency, maybe slightly more on the smaller stock exhaust.

[@nicksmith4808]

It spooled faster because smaller exhaust same volume of air means more pressure making the turbo to have more pressure to spool it faster. Imagine blowing on a fan and then doing it with a straw, which one makes it spin up faster. And for the torque and HP you mentioned. It's a turbo engine it's not gonna make power unless the turbo is spooling. The larger exhaust just caused it to spool a little slower which could result in less bottom end torque but is less restrictive for better turbo efficiency on higher rpm ranges

[@zzzzzsleeping]

likewise, I have a 2020 Acura NSX with just JB4 piggy back ecu (nothing else). My friend has JB4 too, cat deleted and muffler deleted (after market exhaust system). At the drag strip, I constantly getting 10.8 and a few times I got 10.6 where as he get 10.9. Seem my turbo like the way it is while making exhaust bigger piping gain horsepower but lost turbo spool time. My turbo spool faster.

[@midnight347]

​@Nick Smith that isn't how it works. A larger exhaust actually make the turbo spool quicker. I noticed this on my fbo hybrid turbo fiesta st which is my daily. I had a 3 inch free flow downpipe and stock 2 1/4 catback with resonator delete and stock restrictive muffler. I went to a full 3 inch cpe catback with no neck down and it spools up quicker now and the guy in this video even backed this up saying a largere exhaust will spool a turbo quicker. It isn't the same as an na setup

[@mjc1177]

Man learned so much in just a few minutes from this guy. Love how he explains everything!

[@Dr_Reason]

Agreed, with one slight exception. Straight Piped Harles will triple carbureted due to the revision of sound wave in the open pipe. This only happens at low rpm but it kills low-end torque. This is why they invented torque cones which hurt power a bit up top but fix the low end.

[@gordowg1wg145]

Reversion (yeah, I know it's a typo' 😎👍), sometimes called fuel stand-off. This is a very common problem with IR setups running significant overlap camshafts. It isn't down to the sound (pressure) waves, but the air column in the port being physically pushed back through the carbie's venturi - which works almost as well in reverse - before passing back through in the normal direction on the next intake stroke. As rpm increases, the air column inertia from it's higher velocity resists the reversion to the point where it stops and the engine will start to run properly - sometimes this is referred to "coming on the pipe". As you will gather, that reversion also compromises the initial cylinder filling, which is more likely, IMO, to explain the "pipe" phenomena 😎

[@lcameeno1]

@@gordowg1wg145 😎👍🏼

[@gordowg1wg145]

@Jasen Vernor You may have posted this in the wrong comment' thread?

[@dormantmenace]

Yes properly tuned Duals will never have as nice of a torque curve as properly tuned 2 into 1 on a big twin.. torque cones and torque inserts help!! But a good quality 2 into 1 with a tune will crush them performance wise

[@ravenquinn7774]

This is awesome. One thing to note though is that this is applicable to 4strokes only. 2strokes use an expansion chamber to create a reverse pulse (causing back pressure) in order to efficiently scavenge due to the nature of a 2stroke (as a 2stroke runs, some fresh charge is moved out of the transfer directly into the exhaust. The expansion chamber uses exhaust pulses to capture this lost charge and force it back into the cylinder, increasing dynamic pressure and power/efficiency)

[@imperialenforcer2271]

Truth. Back pressure on a two stroke is essential for low end power. That's why power valves exist. They open up at high rpm and close at low rpm, creating some back pressure. I had done a cat delete on a two stroke a while back and I immediately noticed how my low rpm power had gone to shit.

[@motoiq]

No it doesnt, pulse tuning isnt adding backpressure. We did a video on it.

[@motoiq]

deleting the restriction on your expansion chamber slowed the reflected wave speed and threw your pipe out of tune, that caused your loss of bottom end not reducing the back pressure.

[@ravenquinn7774]

yes, because, as I explained, the pulse wave is tuned to use back-pressure to force charged air back into the cylinder. the pulse wave is by definition backpressure. Backpressure- the pressure within a system caused by fluid friction or an induced resistance to flow through the system. An expansion chamber uses fluid resistance caused by variable pressure (done by opening or expanding the pipe diameter and then narrowing it) to create the pulse. The pulse itself is timed by the specific angle, distance, and geometry of the expansion chamber and header. When these synchronize with the rpm of the engine, (known as "coming on the pipe") is when the effect takes place. A shorter expansion chamber or header throws off this band, which is usually a 2k rpm band (depending on the expansion chamber type) 2 strokes depend on the resonance of the exhaust pulse to push against the motor(backpressure) and force the expelled fresh charge back into the motor(scavenging). It is also the reason why a 125cc 2stroke motor can combust 180cc of fuel/air charge at a time. Pressure waves reflected back to the motor forcing the charge back into the cylinder. A pressure wave...flowing in the reverse direction...a back-pressure. @@motoiq

[@motoiq]

I am talking about mean backpressure from a thermodynamics sense not periodic pulsations in the pipe.

[@blutorlz3]

I think too many people equate back pressure with scavenging. You do not need to add back pressure for scavenging. Excessive back pressure will actually harm scavenging...

[@duanebailey6253]

It's more for low end power. The higher the power band the less you want. Also turbos don't need any scavenging making this more complex. Might car mods did a dis service with that potato years back.

[@bonifacijus11]

and excessive pipe diameter will also harm scavenging due to gas velocity loss which will give the same negative effect as too much of backpressure?

[@justmemyselfandi5532]

This is calculated based on revs at which you want to have proper scavenging effect. For these having minor problem to visualise, imagine syringe that you closed of with your tip and you are pulling out the syringe rod. Syringe is trying to suck in the air but it can not because you blocked the tip so you are making under pressure. Exactly same thing is happening if you have right amount of (small) back pressure. When exhaust valves are closed moving mass of air inside the exhaust pipes, continues to act same like when you are pulling the rod of the syringe. This is making small under pressure and helps to evacuate gases from cylinders once when exhaust valve opens fully. Since this is mechanically limited action, based on mass of air, velocity, size of engine and size of exhaust pipes, it is tricky to tune and can be tuned for narrow range of RPMs. But thanks to turbos, this "black magic" is not needed so much anymore. In order to add to this already good video I can share this old one that also brings good information on exhaust kzbin.info/www/bejne/iIi3coWgrc6kotE

[@beepeesoup]

What an amazingly well done explanation. Probably the best I've seen.

[@Bane196Ci]

I know this to be absolutely 100 percent true as I would have weird stumble in acceleration with my exhaust setup that is free flowing on OBX Long Tubes , Y pipe and test pipe but kept the OEM muffler on my 93 6 Speed Coupe Acura Legend. Until I finally change that huge OEM muffler out for a Free flowing but chambered Super HK5 Dual 3 inch Burned tips muffler that definitely took care of that stumble in acceleration at the cost of being just a little louder then stock. I love your videos and awesome explanations for your content, thank you for making super informative vids!!

[@huckwild6464]

MAKE OPEN HEADERS GREAT AGAIN

[@bend185ont]

Love this one Mike, thank you.

[@kirbizle]

I get so many customers that want to stick massive tubing on stock engines, because the opposite is also true. Some people take it too far with low backpressure. Nobody knows they should care about VELOCITY also!!!

[@NXT_LVL]

Thats why I kept my 2.5" diameter to the OEM size, and single layout. Of course everyone who doesn't know better says, "You should have went duals" yeah and I'm not having a 20 year old 240hp V8 truck with two tiny pea shooters out the back.

[@BMWCEO]

Just found this channel and you’re honestly one of the best car channels on KZbin thank you for your content

[@Elaba_]

He and Daniel Ricciardo would be great presenters.

[@johnbourdeau9]

Should do another one on Running Header only, have heard people in the past say it can cause bent Valves. My car has had Header only(with about 1-2ft of straight pipe) for years and never had a issue with Valves or Valve sealing

[@RadDadisRad]

Or open port. They say the oxygen shocks the hot exhaust valves.

[@ALMX5DP]

Heard about people saying it is detrimental to valves so that would be kinda cool. But sounds like not a great idea to run open headers based on this video too, due to trying to keep exhaust gas velocity up to promote scavenging.

[@aDaWaN]

The downside with running headers only is the lack of scavenging, you need the inertia of the gascolumn after the header to get good scavenging.

[@imakedookie]

@@aDaWaN but if its boosted in any way there cant be a downside other than heat dissipation?

[@motoiq]

@@2001chevycamaross that's about right

[@ricepony33]

We are all becoming the new old farts…

[@alierrtrillo9368]

The misconception comes from two stroke gasser engines which use the scavenging of the fuel air mixture coming into the cylinder to push out the exhaust gases. Since the is no valve to close the exhaust port some mixture leaves to the exhaust pipe without combusting and you use the resonator to redirect it back inside the cylinder to ignite it. But that process uses the sound shockwave in front of the exhaust gases not the actual back pressure.

[@littlefellahardcore]

This one video has answered all my questions about exhaust systems in one go, thank you.

[@Nuthing_but_the_Truth]

I'm pretty sure the argument is not lack of back pressure per se, but the affect of the slower velocity on scavenging. Loss of the scavenging will result in a loss of power.

[@chumly8596]

Correct. Properly tuning back pressure waves is what helps draw more exhaust out, and more intake during the overlap. This is why there is a difference between short fat heads, and long narrow headers. Long narrow tubes will be more efficient at lower RPM. but kill upper power, while short fat headers will kill low end power but make more upper rpm power. Properly tuning the intake, cam, and exhaust on a naturally asperated engine is important. Slapping parts on because they say it makes 50hp is a quick way to kill your performance. All this goes out the window the minute you add a turbo... the turbo is now the limit, so big fat exhaust can't hurt it any more.

[@trailerparkcryptoking5213]

Learn about pressure transient. In this case the low pressure wave racing back to the exhaust valve.

[@1darryloflife]

Well we had the best results on the dyno with a minimal overlap cam using a 2 stroke type muffler made from quarter inch thick steel. We also had a valve in the end of the exhaust with vacuum control running a 1/4 inch diameter hole and we found that the valve only opened at WOT.

[@motoiq]

That is because of the minimal overlap in the cam timing.

[@1darryloflife]

@@motoiq Well our dyno results showed a different story as we ran a phase iii cam on a 351 restricted the exhaust flow via the 2 stroke muffler and the resonator at the end of the exhaust with the block off valve with the 1/4 inch hole and vacuum control. The smallest cam was based on a Holden 179 six cylinder stock engine .

[@mjhmech4903]

Do you mean 2 stroke muffler or a 2-stroke style expansion pipe? Muffler construction does not actually change between 2 & 4 stroke engines as such. Muffler wall thickness also has zero bearing on power output. Too many variables to your comment including 2V vs 4V, open vs closed chamber heads, headers vs manifolds, 2bbl vs 4bbl, diff ratio + actual desired operating rpm etc etc. Combination is the key.

[@1darryloflife]

@@mjhmech4903 I must admit we are about 30 years ahead in our technology concerning Otto 4 cycle internal combustion engines. There is a whole science starting with temperature control which then brings in proper tune spec as the engine is then better equipped to handle a proper constant A/F ratio , then proper exhaust design measured from the back of the exhaust valve to the collector then proper exhaust size according to the engine demands. Check us out at www.bennettclayton.com.au Cheers

[@chriswalker4272]

Wonderful explanations making complex stuff seem obvious with a smile. I will never get exhausted listening to somebody at the top of their game who obviously has such a passion for what they do. Thank you for such a to the point explanation, politicians need to more like you sir 👍

[@needmoreboost6369]

This people need to hear more often! Nice vid! I think what gives the misconception about back pressure is often increasing flow will also often lean out afrs and on a already lean tune this does not improve torque! also removing an anti reverberation tuned exhaust for a big cheap loud cannon don’t make it a race car

[@P71ScrewHead]

I agree loud doesn't make it fast, but for some reason ppl think my P71 is one when I drive it..lol I drive it open cutouts for that zero restriction/back pressure which makes my car a few tenths quicker n frees up a few hp (12 to be exact) but that's the only reason, not bcuz I want to sound fast..lol

[@needmoreboost6369]

@@P71ScrewHead cool Sounding I bet! I use a cutout too the first one I had Was home made mild steel butterfly and 3” pipe and a throttle shaft from an old throttle body operated by cable mounted at the end of the turbo dump and “t” off the dump to flow to the rear muffler when closed, and it was a packaging nightmare to make it work with a wideband accurately! I’m using a normally open vac operated ebay one now and a mower fuel tap in cabin to fix it open but it’s still got the factory 90s exhaust below the dump pipe and it literally would loose half it’s power on quiet mode due to the ridiculous back pressure so the vac system was a safeguard in a way

[@needmoreboost6369]

@@P71ScrewHead I also have a Nissan Pathfinder I hacked the rear 2 to 1 muffler out and replaced with a y and damn thing sounds like a methanol stock car

[@ryurc3033]

Your right, but at the same time how many guys with stock k series in a Honda Accord just heard they need 3 inch straight pipe?

[@needmoreboost6369]

@@ryurc3033 probably 85% but with enough coins spent even 3” could be tuned pretty quiet, lol that’s coming from a bogan with an annoyingly loud exhaust but with a cheap vac cutout and factory muffler’s

[@automotivelove]

U need exhaust, the header continues to work up to 12 inches after the last merge. If you cut the header off you won't get the "extractor" to work.

[@motoiq]

That is not true. Cars go fast with no exhaust and just collector extensions

[@automotivelove]

@@motoiq Are you just replying to reply or did you read what I wrote? The HEADER continues to work up to 12 inches AFTER the last MERGE. For example a 4.2.1 the 1 is where the last merge is. If you cut it off 12 inches from there the car will be fast, it don't need the rest of the exhaust but you need the header.

[@automotivelove]

Also some of your other videos are very misleading. There was one about the 4AGE crankshaft not making more than a 200hp, when we have taken standard 4AGE block with silver top head to near 700 and climbing, meet after meet, back to back 10 second passes. No issues with the crank. What did he mean by that?

[@motoiq]

I was talking about NA high-revving engines. Turbo engines with boost turning lower RPM can tolerate a lot more. Compression by boost is tolerated a lot better than spinning high revs. With a little 1.6 you got to spin fast to make NA power. Also drag racing and street use are much less demanding than road racing, drift or rally. Running 20 minutes is a lot harder than 10 seconds. If you are only at full throttle half the time, one track session is like 600 10-second passes even if you consider that you might not be at full throttle then let's say it's 300 passes, let's say you are a lame driver so it's 150 passes. There are usually 3-4 sessions per day at a road racing event. So that's like 450 passes being super conservative. So one track day is like a few years of drag racing.

[@motoiq]

It's not always an arbitrary 12 inches. I would say 18-24 is safer to generalize without dyno testing. A merged collector's extension can be shorter than one on a box collector.

[@felixwankel3989]

I literally had a mechanic tell me "your exhaust needs backpressure". He also told me "your car is perfect from the factory, dont modify it or it will be ruined". As if oem engineers know more about performance than aftermarket.

[@MrTheHillfolk]

Sometimes techs like that don't think the aftermarket can make an improvement. Well in that world of Volkswagens where I'm at ,there's a company who makes cast aluminum water necks to replace the crappy factory plastic ones. That's just 1 example of better aftermarket stuff. As far as backpressure goes ,less is better ,but it might be an issue of recalibrating the fuel flow now required,some carbs or fuel injection or engine management systems are easier to retune than others.

[@javiervelasquez8002]

The factory engineers might actually know better, but the have to make compromises that the aftermarket doesn't have to consider. For example when it comes to choosing tires. They have to go with an all season or summer tire even if they're selling the car in Canada during the winter

[@thatslegit]

@@javiervelasquez8002 no modern engineers do it by "how much can we shave off the cost" and "just make it last until the warranty expires" they don't care it's not the best, as long it doesn't fall apart during the first ownership it succeeded. Aftermarket knows best because their goal is to "Keep it alive" fixing design failures and optimize the car overall. car engineers haven't peaked since the 90s and that's because they had something to prove, and that was pride, not greed..

[@joeangell5652]

Actually that’s excellent advice when it’s being given to someone that doesn’t have a clue what they’re doing. Maybe he had a good read on his customer?

[@wyslink9354]

bro that last part is gold lmao. if they knew more then theyd be selling cars not just exhaust pipes, and other parts that dont even fit half the time.. FFS man that is a good one

[@geoffreyfox7618]

When I get people asking me about back pressure, I just ask , do you see a restriction on drag cars exhaust?, even a small amount of back pressure takes power from a turbo

[@dostuffchannel]

The knowledge he's dropping is college level!

[@chestrockwell8328]

That's sad you think this is college level.

[@dostuffchannel]

@@chestrockwell8328 from my perspective as a peasant one of two things is happening here: 1. You are also a peasant but hate your life so you hate on others or 2: You are college educated with a successful career, likely as an engineer, but still hate your life so you hate on others. Either way you are a hater and my man at MotoIQ is college level!

[@chestrockwell8328]

@@dostuffchannel Thanks for providing a glimpse into your limited perspective. Now go ask your mom to make you a sandwich, crusts cut off I'm certain.

[@motoiq]

@@dostuffchannel 😀

[@m4rvinmartian]

@@dostuffchannel Neither Chest nor I hate our lives. In fact, mine's a peach. And believe it or not, that doesn't mean that we can't be mean to your pathetic ass. Btw, I'm calling you pathetic, because you took great exception to Chest's comment. When you could have just ignored it. Also, I agree with Chest. This barely counts as high school stuff bro. So... you played yourself.

[@Chevyholicsanonymous]

This guy speaks so fluently and makes everything very digestable. All excellent information. Cheers!

[@tbas8741]

This is why i Made a Custom Exhaust for my 99 WRX EJ205. Firstly All made from Mild Steel 2mm Thick for much deeper note. 3" Bellmouth Turbo back W/ 2 x 300 cell racing cats (1 in down pipe, 2nd 2ft further on downpipe) Into 3" Main Pipe with Resonator Into 2.5" Diff Back Pipe with 2 x 90* Bends Into Open Twin 3.5" Tipped Stainless Muffler. The 2.5" diff back increases the speed of the gases as they have lost lots of heat energy and on EJ205 without AVCS Majorly improves Low end Torque by upto 25-30nm below 4000rpm

[@jasengibson6619]

"Back-pressure" as a thing to discuss anymore should be a myth. There is no such thing, unless your exhaust is plugged. The goal in designing the exhaust system is to achieve a laminar flow, at a velocity sufficient to move the volume of exhaust gas the engine is outputting as quickly as possible without moving so fast as to create turbulence from the drag or viscous forces the pipe walls generate, or so slow as to stagnate and create turbulence just from faster flowing gases bouncing off of slower. When you consider the fact that gas output from the engine is constantly changing with engine speed, and the possible variance in output can literally be 10x higher or lower (think 800 rpm vs 8000 rpm--those cylinders are pumping out the same volume per revolution, but 10 times as many revolutions in a second), and that the volume and viscosity of a gas changes with temperature, its an engineering nightmare to create an exhaust that is optimal over any wide range of engine speeds. At least until someone invents variable diameter exhaust, and I expect we'll have flying cars before Honda announces VDEC. ok, so what's this all mean then? Choose an exhaust setup that maximizes laminar flow at the engine speed you'll spend the most time driving in. Live downtown in stop and go traffic that rarely lets you get up to speed? Smaller diameter pipes would actually benefit you. Building a track car for events or fun that you dont intend to be a daily driver? You can use some much bigger pipes. Drag car? Fuck the exhaust, find some side pipes and vent that bitch right from the headers. :)

[@charlesmarshall2697]

I've been building high performance exhaust systems for 40 years using 3 inch stainless mandrel bent tubing. Fortunately I live in a state where I don't need a catted system. Normally I mandate 3 inch tubing from the header collectors through the X pipe and into high flow non S type mufflers. I do switch to 2 and 1/2 inch tailpipes to reduce noise as there is little restriction on the open ends. This is the system I currently have on my 2015 Ram R/T fitted with a 445 cubic inch NA gen 3 Hemi. It is nearly as quiet as stock and is a bear at any RPM. Back pressure my ass! Let it breathe and don't plug it up with Cats if the regulations where you live don't have an issue! Use longer mufflers if you really want to cut down on noise!

[@dicktomasko1553]

So my 01 subaru 6 cyl had a muffler with a restrictor valve in it from factory. Was spring loaded to blow off at a certain pressure...I'm guessing it was for good low end performance ...Only one company sells the muffler now. Everyone uses the muffler for the 4 cyl 2.5l...The intake has a valve in it that is supposed to work with the muffler...I don't need a rocket ship just wanted it to run the best it could...

[@Myvintageiron7512]

You nailed it the reverberating exhaust compression wave really helps scavenging during over lap we tested a bunch of exhaust and pipe dia combo's on our dyno with a healthy LS1 and the single exhaust wit 3" diameter made the most power with the correct mufflers of course we used a Streight thru Bullet muffler and got more HP and TQ than duel 2.5 ex however when we put the turbo type muffler on the single 3" we lost almost 80 HP wow what a difference. it's like you said back pressure is bad I've been teaching that for 20 years you need really good flow but you also need the correct resistance to flow in the form of an ex pipe to keep the velocity of that gas high and hot

[@motoiq]

It's amazing how much a Turbo muffler or a chambered Flowmaster can kill the power. No one belies me until they actually do some dyno testing on a healthy engine.

[@funcool32]

Volumetric flow (m3/s) is impacted by back pressure. By increasing pipe size, you might gain some volume flow, but when you divide the increased cross sectional area of your pipe, your velocity will most likely decrease. Great video.

[@motoiq]

Someone who gets it!

[@n5sdm]

A little back pressure meant that in the smaller pipes, you had velocity, the engines back then did not Rev as high and if they did they were not happy about it. low end torque was king. Longer stroke lower compression, means that a long pulse and fast velocity meant smaller tubes. This meant backpressure was going to happen. With shorter strokes, better intakes, ect, being able to dump the exaughst in a large pipe is better

[@thehouseofhorsepowerautobo4506]

Thanks for sharing! Everyone argued with me for putting 3-in exhaust on my old fox body drag car. It ran great sounded amazing even set off people's car alarms cruzing through town haha!

[@frankwren8215]

"backpressure" is the hot rodders' "blinker fluid". Also nothing quite helps velocity like straight pipes!

[@MorneBooysen]

I remember doing this as a plug flow simulation as chemical engineer degree 2nd year simulation…back pressure was to allow residence time to allow optimal catalyst converter to scavenge or clean exhaust…but I thought complete free flow would be best for performance when I removed catalyst but it wasn’t…it depended on the setup and could be tuned to range depending on flow and pipe size and the variables you did mention

[@aashishpatel3969]

well said, i'm more a motorcycle guy, but learnt more on this video re exhaust/power then all the other videos put together. i bow to you sensei.