Stop Saying Car Exhausts Need Back Pressure

[@TheBobes]

Oh yeah, I get it! *Mounts 10" straight pipe into his '90 Honda*

[@EngineeringExplained]

Praise be.

[@SliKdaASS]

now I want to know how to calculate length/diameter for my project honda.... :/

[@TheBobes]

How much you paid for it on craigslist (a) Amount of monster stickers on the car (c) Desired ricer factor (1-10) (b) a / c + b = diameter in mm

[@incredijax]

StanceArmy Jimmy same here

[@tjustice09]

Yeah because s2ks are from the 90s 🙄

[@rocketkinger2506]

As an engineer I’ve always been irritated by people saying cars need back pressure. Drove me nuts

[@ralph02136]

Isn't it necessary to help boost mpg?

[@Febreeze_Odor_Eliminator]

The guy who did my muffler delete on I4 said these engines need a bit of back pressure. I definitely felt a loss of torque at lower rpm. Also I’m surprised that so many mechanics think no tuning is needed after a muffler delete. My area doesn’t have that so yeah car sounds nice and loud with lesser torque which means loss in performance. ☹️ I do have a k tuner. Do you know if I can tune muffler delete by myself? And if so how? @Engineering Explained

[@rocketkinger2506]

@@Febreeze_Odor_Eliminator you believe that coming from a guy who chops mufflers off for a living? The only reason you “need” back pressure, is because it was tuned for it, from factory. So of course it “needs” it, but engines are just air pumps, why have a higher differential in pressure, when could just NOT

[@carter.coleman]

@@rocketkinger2506 so if I straight pipe my 4 wheeler, I just need to rejet it and it's good? After I upgrade the intake

[@wildestcowboy2668]

@@rocketkinger2506 That's a lie

[@Texas_krazy]

So I dont need anything after my headers. Gotcha

[@Snow-vo1yi]

Who needs headers? I use those save the turtles metal straws instead, cheaper that way

[@BackstageChief]

@@Snow-vo1yi gives you that perfect exhaust note too 👌

[@chinchy111]

Unequal length straws

[@kumo9993]

@@Snow-vo1yi Headers are just really big metal straws

[@billbergen9169]

I would love to have side pipes for each cylinder out of the fenders.

[@ryanaponte8115]

I stopped doing absolutely nothing to watch this, it better be good

[@EngineeringExplained]

Hahaha, I'll do my best to entertain you for the next 9 minutes. :)

[@ryanaponte8115]

Engineering Explained update: it was good

[@superchargedmustanggt9689]

Ryan Aponte Hey I just saw that you left this exact comment on ss717's vid :)

[@ryanaponte8115]

Supercharged Mustang GT yes I did :)

[@dougefresh123]

it was good. I learned that I missed my chance a few months ago to see a HP increase by driving around when the eye of Hurricane Irma passed by, due to lowered atmospheric pressure.

[@si3500]

me: gets pulled over cop: your vroom too loud! me: "my vehicle is operating at maximum efficiency, and reducing pollutants while increasing performance. *shows cop video* cop: understandable, have a nice day

[@netprowlerp]

Ya mate, that's not going to happen. Play this youtube video for the cop, you may go to jail.

[@ljgarrison6910]

@@netprowlerp you must be so much fun at parties, I bet you're idea of "rebellious" is missionary with the lights off.

[@SpadeNya]

@@ljgarrison6910 😂😂😂😂

[@netprowlerp]

@@ljgarrison6910 Hahaha. have to tell that to my wife. She would beg to differ.. Ya, not everyone here is 12 years old.

[@MJT-DA]

@@netprowlerp he was making a joke. And you took it too seriously finally calling his audience 12 year olds without knowing the real ages. The only thing you should be telling your wife is organizing counseling cause you're way to serious.

[@zeusm8498]

Thank you I'm so tired of people saying ”back pressure " !

[@Coyote.five.0]

back pressure.🤭🤭🤭🤭🤭

[@80s_Boombox_Collector]

Whatever you want to call it, if you have no back pressure at all, it sounds like ass. You need some kind of baffling and resonance to tune the sound and make it sound good.

[@tejaspadhye]

@@80s_Boombox_Collector so engines don't get damaged with free flow exhausts? Dragracers rebuild after every race so yeah

[@richardbossman9875]

Them rebuilding has nothing to do with “back pressure “ or lack there of. Correlation =\= causation.

[@itptires]

@@tejaspadhye I thought the lacl of tread on the tires caused them to need to rebuild

[@johnny-bracer6515]

One of the biggest arguments among builders & tuners ever. thanks E.E. for breaking it down!

[@johnny-bracer6515]

+TheGhost ....lol, you're absolutely right!

[@mathieulevasseur4082]

More like among wannabe tuners and builders.

[@johnny-bracer6515]

+Mathieu Levasseur .....lol, also true

[@tomr1107]

wish this video was available before i wasted so much time explaining this fairly simple concept to people. could have just dropped the link and walked away lol

[@TheDealinDave]

engine masters on the motortrend channel did dyno tests with different full exhaust systems and pretty much theres no such thing as too big of exhaust but you do lose a some power and torque at the very start like below 2.5k rpm. what people don't really look at though is partial throttle tuning and just going bigger everywhere can hurt that response and drive-ability.

[@balthasarlumbantobing4112]

the best thing to do in the last hours of 2017: watching Jason's explanation with his whiteboard. Nice as always, man!

[@balthasarlumbantobing4112]

thanks for the love Jason!

[@Kevinb1821]

5 4 3 2 1 happy white board!

[@Valdismith]

That is what I am doing, I even wonder if I will be going to celebrate at my cousin's in an hour or 2:)

[@irisgardener4141]

Or last 60 minutes of 2017..!

[@ViewThis.]

No Roadkill #72....It didn't come out on time. So end the new year learning valuable stuff instead of watching Junk Food.

[@meifert2]

It's amazing that people will not listen to a word you said because Cleetus at Smithys Muffler Shop tells them the backpressure of a Flowmaster will give them more power

[@CalebDiT]

Interesting comment. I think it was in the first minute or two of the video that the feller explains why some back pressure is good.

[@zeallust8542]

@@CalebDiT Did you watch the same video?

[@netprowlerp]

@@CalebDiT No, he mentioned it and shrugged it off as if it was to complicated for us poor folk to understand. He never explained it. Also the title of the video is "Back Pressure is Bad!".

[@Talos_Valcoran87]

@@CalebDiT What he said was having a balance of velocity and restriction is ideal, not that having some back pressure is good. More restriction increases exhaust velocity but inhibits overall flow of exhaust, less restriction allows more exhaust flow but lowers velocity.

[@CalebDiT]

@@Talos_Valcoran87 You and the video maker seem to understand what is necessary, but you don't like the terms. "Back pressure is bad, but some calculated restriction is ideal." It's a contradiction to anybody truly listening.

[@turtlefights4194]

He seems almost agitated making this one ill bet somebody ticked him off

[@owensharp8568]

And I understand his frustration so well.

[@JasonM69]

The argument of what back pressure is, is an argument as old as hot rodding.

[@n.o.b.s.8458]

*some guy at the auto part store says something about back pressure* EE: races home and starts writing furiously on a whiteboard while collecting valid explanations.

[@natereinhold6180]

I get aggravated when most people open their mouth, I cannot believe he is as cool headed as he is!

[@Strider9655]

I think he knows his team have let him down, some of the stuff at the beginning of the video is very messy, not false, but over simplified and a a result not accurate. Back pressure is not air trying push back into the exhaust (and I think he knows this), it's the positive pressure built up by exhaust gases inside the exhaust system, the higher the engine rpm, the higher this pressure, which is why some high performance vehicles switch between 1 or 2 exhaust backboxes at specific rpms to alter the "back pressure" in the system to maintain higher performance and fuel economy throughout the rev range.

[@squidwardshouse67]

Engineering Explained coming in clutch with the last vid of 2017

[@EngineeringExplained]

Happy New Year!!

[@squidwardshouse67]

And a happy new year to you too bro!

[@glentight]

First of 2018 here😂

[@romoalex]

no one says clutch

[@joeKisonue]

Squidwards crib does buster

[@spartanboss4189]

some Guy: you´re losing power, you need back pressure. Me: (inserts banana in tailpipe)

[@netprowlerp]

Guys, I keep blowing engines. What happened. I listened to a guy on youtube. Do you have bananas for brains?

[@thatvolvoguy6565]

Lmfao

[@trunkbangking]

*losing

[@oxaile4021]

MCM put a potato in the exhaust of an old Corolla and it actually made more power.

[@JohnLGalt_1]

"Yeah, we're not gonna fall for a banana in the tailpipe."

[@meanman6992]

Can't tell you how many old guys I have argued with about this. I quit years ago. Velocity they confuse it with back pressure. Yeah smaller pipes have high low rpm velocity, so you get better low rpm scavenging and better low end power. But then at some point they start choking the engine.

[@kevinnorris6558]

Mean Man You're incorrect

[@TheMick26]

Kevin Norris ... said the old guy.😜

[@dsmmike95]

I thought the high and low end power came from the length of the header/manifold tubes?

[@droyce4596]

i had a 150cc scooter, and the studs that hold the exhaust to the exhaust port broke one time, and made the scooter so slow, it was normally able to go 60ish mph, and with the open exhaust port, it could barely go 15mph

[@danesebruno]

Devin Royce was it 2 stroke?

[@Sold_a_dummy]

Thank you, thank you, thank you! I'm glad you made this. Please, make sure this goes on Wiki-everything! _My man_

[@VikingRul3s]

Nice work!!! And you are absolutely right, this is GOLD for all amateur tuners! My advise to budget track cars; Keep stock exhaust diameter, remove cat, get "straight through" resonator and muffler (with a tail/end dia' similar to exhaust dia' = no deep tone, pure race sound), this will roughly give you same performance boost as a expensive $1500, from headers and back, race exhaust @EE: if you don't take into account the Euro and newly establish similar US emission restrictions (completely disregarding the cat here) the backpressure is needed to recirculate some of the exhaust back into the intake, to burn unburned fuel, at given loads at specific rpms. So in modern cars it's needed to fulfill legal requirements ;) PS not all engines need this system, but if you increase 1st weight in your search terms and 2nd search increase engine volume/capacity and cross those you'll find it being implicated more and more

[@bkretschmann90]

YES!! Thank god! So sick of hearing the crap about "needing" back pressure.

[@jasonjackson3114]

Yep. Beat me to it.

[@JrSpitty]

you cant have scavenging without backpressure. you don't need it... drag cars don't have scavenging. lmao

[@80s_Boombox_Collector]

You DO need at least some back pressure to get good sound. Try taking off everything after the headers. It will sound like ass.

[@Jupiter__001_]

@@80s_Boombox_Collector Sound doesn't help it go, contrary to what every Honda driver around here seems to believe.

[@80s_Boombox_Collector]

@@Jupiter__001_ No, but good sound makes it more enjoyable.

[@jmh1189]

You have absolutely no idea how helpful your videos are. I just recently started trying to learn about my car and attempt to fix it myself. I normally troubleshoot radar equipment but when it came to my car, I was lost. No one could explain the theory to me very well so I found it very difficult to troubleshoot the symptoms I had. Your videos changed everything. Thank you.

[@EngineeringExplained]

Happy to hear it, thanks for watching!

[@youdontevenknowwhatyoudont7657]

Yea he's spot on, think of your exhaust and intake for that matter like sucking a milkshake through a small diameter or larger diameter straw. Or sticking your thumb over the end of a garden hose.

[@BiologicalEngineer]

I thought you owned a toy company? You changed to radar equipment after the war?

[@TheMick26]

Hey Jason.👋 While some have mentioned the whiteboard looks a bit small and restrictive at times, my guess is that it has been engineered to just the right size for this application.👌 Much like your explanation in this video, everything on the board usually has a good, positive flow without feeling restrictive. We all know, knowledge is power and the power to weight ratio of the whiteboard is immense!💪 Great flow and power without undesirable drone... what more could we ask for? Thanks for a fantastic run in 2017 and all the best to you in 2018, sir. ✌😉

[@EngineeringExplained]

Ha, thanks for the fun comment, happy new year!

[@jason454ss]

Should get an award for most efficient KZbin creator.

[@LanguagesWithAndrew]

If you ever want to do a charity fund-raiser, take the current whiteboard that's been used in so many EE episodes, sign it in permanent marker, and stick it on ebay.

[@gooz0mbie]

Good comment but get rid of the freaking emojis lol

[@TheMick26]

gooz0mbie Thanks, but that's what happens when you give old people "smart" phones... Dad jokes and emojis.😜

[@lancelot1953]

Thank you Jason for your great videos - I graduated in 1977 then went post-grad but I am still learning, your delivery style is enlightening and your enthusiasm teaching is contagious. You may mention the component to the scavenging negative (relative) pressure wave contribution from other cylinders connected to the same collector, what we used in "tuned header exhaust". As I read comments, I think that some people misunderstand the "negative" pressure wave (or interface) you are mentioning possibly because they consider the word "pressure" as necessarily positive not considering that this "pressure wave" is negative relative to the pressure in the rest of the system. Great video - makes me feel young again. PS: Born in '53! What is your secret, are you wearing a wig, genetics, special engine oil formulation, ;-), take care Ciao, L (FoMoCo engineering retired)

[@n7565j]

I live in the heart of NASCAR country, and the old time engine builders would bolt a straight pipe on the headers and run the engine up to 7k rpm, and where the pipe glowed cherry red, he'd cut the pipe and weld on a turn out. Said that little trick would add 5hp... This was back in the early 80's, so don't take any of this for gospel ;-) Excellent explanation Mr EE :-)

[@RoboticsNShenanigans]

Painting flames on it adds another 5 hp.

[@arealassassin]

That is exactly what they did- old drag racers know this.

[@FireAndEMS]

He lost me at hello

[@pnwoffroader331]

@ 😶😶

[@moto6ixmoto83]

It’s not that hard of a concept

[@glenwaldrop8166]

"and welcome" It was just too much for the poor guy to handle. He must have been on one of those "three cups of coffee" days.

[@jasonjackson3114]

Did he say hello? I noticed some pictures. Then something about "scavengers"

[@Outdoorsman_Z71]

You lost all credibility at this comment

[@codywillimas328]

Damn bro you are smart af!! I’ve been following you for a couple years now and bro I have learned a lot from you

[@jasongillean44]

Great video and very easy to follow explanation. I work in automotive parts and it is constantly preached by many that "you have to have plenty of back pressure". No one knows why but they are certain it's true. Thank you for debunking this ancient rumor.

[@antoniovillanueva308]

I will explain to you the source of the backpressure myth. In the late 1970's carbureted cars were forced to run an extremely lean mixture. The older catalytic converters were quite restrictive. These cars were able to run so lean (without melting pistons) because the old catalytic converters created a form of passive EGR. If the exhaust on these cars is replaced with a less restrictive system the passive EGR was no longer present. These cars would run VERY lean, melt pistons and overheat UNLESS the carbs were replaced with pre-emissions carbs.Timing was not computer controlled and knock sensors did not exist. Hot engine, lean mixture, detonation... BOOM! It was a crappy system, there were 350 cu. in. V8's with less than 150 hp! So many vacuum hoses were required that underhood vacuum maps were quite detailed. Bad days for automotive tech.

[@BashMonkeyRC]

So basically, early emissions systems were crappy and complicated, and were doomed to kill power unless there was an electronic babysitter to reign everything in efficiently? Big shocker.......

[@d.e.b.b5788]

The 70's???? You've got to be kidding me. Or is that as far back as you remember? Because exhaust tech as you explained goes way back beyond the 70's vehicles.

[@BashMonkeyRC]

D.E.B. B what he's saying is it really originated in the emissions days, which started very late 60s and came full swing in the 70s, that's when it really became an issue and the backpressure idea came to light

[@antoniovillanueva308]

D.E.B. B - Catalytic converters became mandatory in the early 1970's. The tech was not the same in the 1960's. My first car was a 1967 Camaro one owner, low miles PERFECT...$1200; it was very simple (points, carb, drum brakes (all of them), and it had about 3 vacuum lines under the hood). My second car was a 1975 Monte Carlo. The Monte Carlo had dozens of vacuum lines, a cat, front discs, HEI ignition, AND IT WAS SLOW AS MUD. *Car tech changed drastically between 67 and 75.*

[@d.e.b.b5788]

Catalytic converters did not become mandatory, it only seemed that way. Exhaust emission limit levels kept being reduced, and because of that, MOST auto manufacturers began using a catalytic converter in 1975 because they couldn't figure out any other way to meet the emission standards. That was the beginning of no lead gas,because the lead would supposedly coat the tiny platinum catalyst pellets inside the converter (I don't remember ANY cars using a cat con before 1975, and then all of a sudden, it seemed like all of them did). However Honda's CVCC engine (in Civics and Accords) was able to meet the emission standards WITHOUT a catalytic converter all the way until 1980 (except in California, I think that Honda needed cat cons there in '78). Exhaust tech did not change; as he mentioned in the article above, was just part of the science involved to get the engines to make certain amounts of power at selected points in the torque curve, while keeping the noise to a minimum. Short open pipes were usually used on vehicles that needed to make maximum power at high rpm's. Longer, smaller, more restrictive exhausts were used on street vehicles that had to be quiet. Somewhere in the middle were exhausts which were a compromise between the two. And for some bizarre reason, the aftermarket often did better than the OEM's; my Honda 750 bike with a Supertrapp exhaust made much better power than the stock pipe, yet was also relatively quiet (that was the exhaust where you could add or delete some discs to tune the exhaust). And yes, the vacuum line rats nest of some of those smog engines was enough to drive us nuts! One little crack in those rubber lines would make the car run terribly! I vividly remember taking polaroid pictures of the top of my engine, pulling off the Autolite carb and replacing it with a pre-smog Holley, plugging up all those little holes, and running it like that until the next emissions inspection when I'd reverse the whole shebang! Ahhh, the good old days!

[@user-qm9oo2fd2o]

This is why great aftermarket exhaust systems and great tuning go hand in hand. Yes they are expensive due to all the R&D that must go into the correct design specifications.

[@leokemsley5086]

I think the myth comes largely from very old cars, with extremely inefficient high back pressure exhaust systems -like original VW Beetles -if you fitted a free-flow straight-through exhaust to them, it would make such a dramatic change to flow / air:fuel ratio, that if you didn't change the carburettor settings at the same time, they often wouldn't start or would run very badly, historically motor manufacturers would tell mechanics and anybody that asked, that this is because you "needed back pressure" because they didn't want to encourage people to modify their engines, as they had other concerns like reliability, longevity and low noise.

[@libtrs838]

I think it more comes from people putting oversized diameter exhaust on their vehicle and losing power/torque because the exhaust velocity has slowed down too much. They don't realize there is a difference between free-flowing and oversized.

[@michaelszczys8316]

Their carburetor was set up fora high back pressure exhaust and they put on a low back pressure exhaust and now it don’t run the same. That’s when they say they “ need back pressure”

[@trollmcclure1884]

that's what I thought when I had a rusty hole in the exhaust but my emissions were bad untill I fixed it. And it's not an old car.

[@sethmanukula176]

Loss of power due to oversized piping is a bit more complex If the working routine of the engine is not modified, it will keep on kicking the same quantity of gas out for a given load, no matter what pipe size is used The loss of power is not due to the fact it looses velocity on getting out of the engine but on the pressure in the cylinders A bigger pipe allows more gas out so the cylinders will be emptied easier but If the cylinders feeding/emptying scheme is not modified, some of the mix supposed to ignite wil slip through the exhaust system before it is consumed, which turns out to leave less mix in the cylinders, giving that loss of power It's pretty simple Bigger exhaust implies better ignition timing

[@MikeSmith-vb8ul]

@@sethmanukula176 Yeah exhaust valves usually open before BDC

[@ViolentKisses87]

Vent at the headers as god intended.

[@mcf3778]

Nice to know your god created the internal combustion engine... Carry on

[@glentight]

Zoomies😍

[@thomgt4]

mcf3778 I think/hope it was a joke (ish)

[@THECobrabitch]

No!!

[@ViolentKisses87]

Yeah it was a terrible joke and some people failed to see it.

[@RipRoaringGarage]

Very well said. Its a pet peeve of mine about back pressure. Its as if piston fighter aircraft arent a thing. No backpressure and scavenging is done via Bernoulli from airflow passing over the end of the pipe.

[@RacingRejects]

Thanks for clearing this up. I have argued with people for years about this but now I can just tell them to watch your video. Great explanation man

[@nitrousshovelhead]

I have been applying this for years and it is nothing short of incredible how much torque can be found by building a proper exhaust system. not only length and diameter but collector and muffler design.

[@shaunjohnson9827]

Back pressure is helpful in a 2 stroke engine, hence “tuned pipes”

[@gatorage850]

kinda... how an expansion chamber (tuned pipe) works in stages. when the pressure is moving back to the piston from the exhaust it's in the compression/intake stage it's good (helpful). any pressure that moves back during the power/exhaust stage (back-pressure) is bad. the reason the pipe is so fat in the middle is to reduce back-pressure in the power/exhaust stage.

[@niftyschnifty813]

thats why 2 stroke enduros have those resonators.

[@kaisersose5549]

@@gatorage850 A clearer explanation is that a tuned pipe is designed to allow sound waves to reverberate back to the exhaust port when the air/fuel mix is compressing. This allows a sort of pressurized charging by pushing the air/fuel mix that would otherwise be wasted back into the combustion chamber. Tuned pipes are only effective at a range of RPMs that time the reverberation of the sound waves with the compression of the air/fuel mix. The fat part of a tuned pipe is at a distance from the exhaust port mathematically calculated by the exhaust gasses moving through the given diameter of the exhaust pipe at the speed of sound, usually 1/3 wave. This is why a power band fades away as the exhaust becomes overheated as well.

[@gatorage850]

@@kaisersose5549 I was trying to keep it short/simple and just showing the pressure on. mostly the end result of the wave affecting gasses near the piston. not trying to explain the whole wave process for how technical and confusing it could be for someone new to understand/picture clearly or want to read.

[@williamcharles9480]

As always you deliver the most interesting and understandable discussion. Thanks for sharing your extensive knowledge.

[@earlgrey1492]

I think people used the term "back pressure" without fully understanding what it is. Thanks for clearing that up.

[@Cryostal]

Does my butt have back pressure when I fart?

[@mbmarsh47]

Yes, the sphincter has a smaller diameter than the colon. This could be the reason for two toots sometimes.

[@netprowlerp]

In your case yes. It causes brain damage.

[@fullmetaljacket7]

No. That's scavenging.

[@OhioFootHills]

Are you wearing spandex or cotton?

[@120rain4]

If the smell gets passed your underwear and jeans to people's noses than you don't have enough back pressure !

[@jenniferwhitewolf3784]

Decades ago when I was helping out a SCCA team, our engine guy made 2 sets of headers. One set was equal length and rather short to maximize peak output at higher RPM. For some tracks needing a bit more midrange torque he had another set with a short and long tube set mixed. This was a better compromise than all equal longer tubes that gave more low torque, but really dropped off the high. Half of engine building is exhaust design. One engine guy I knew even made internally tapered steps hidden inside the constant diameter OD of the header pipe ( to hide the steps) in order to better control the back wave to his desired flow characteristic. Interesting combination of fluid dynamics and acoustic pressure waves in exhaust systems...

[@jockellis]

Jennifer WhiteWolf Multiple EP national champ Joe Cogbill had engines for his 96Rock Speedster specific for fast and slow tracks. IIRC the fast track engine had a 1.75” dia. exhaust and put out 172 HP. The slow track motor utilized an exhaust of 1.625” dia. exhaust and put out a max of 158 horses but more in the lower rpm.

[@smiley800]

jockellis; do you race too? You comment about 96rock makes me think you're from the Atlanta area.

[@jockellis]

Ornery Hermit Well, I went to the Skip Barber 3-Day school at Roebling Road and raced Karts on ovals but no other wheel to wheel racing. But if I can ever get my 914 project car finished I want to go hill climbing. I interviewed Joe for a story for VW & Porsche Magazine but could never figure how to save it to my Radio Shack Model 4 and it kept disappearing.

[@smiley800]

Wow. Talk about a small world. My first racing school was also with Skip Barber at Roebling. We ran Formula Dodge open wheelers (no wings). Then I did the SCCA school in a rented Miata at VIR. Been racing spec miata and Spec Racer Ford's since then. Will actually be at Roebling Road again later this month for a SCCA event.

[@jockellis]

Ornery Hermit How fast could the FDodges go around that last turn onto she straight? Did Bruce McInnis run the school?

[@skippy2987]

David Vizard wrote about this (World Horsepower Guru). He commented on some brilliant testing that drew the wrong conclusion. The test with more backpressure did gain power because the muffler designs were different and changed (inverted or moved) the pressure wave tuning point at the end of the pipe. It happened that the more restrictive muffler had the better resonance tuning. More or less what you say here, but it should have been more widespread knowledge

[@skippy2987]

Have you heard of "pressure wave termination boxes"? It's a trick used to simulate having open headers (no exhaust) while actually having a full system The inverse trick exists too. Headers designed for use with cat converters immediately after them (but in use on a non-emission car) can have a temporary (and hopefully smooth) decrease in pipe size to simulate the wave reversal characteristics of a cat converter. Wave tuning is cool. I play with it on Engine Analyzer. I've seen (simulated) volumetric efficiency of 112% at times

[@jacobf.8992]

I didn't think that exhausts had to manage pressure to that sort of precision! Details like these are interesting to see manufacturers get into. Thanks EE!

[@TORqueTV]

I’m so glad you made this, too many people think back pressure is a must to have on a car!!! I’ve been arguing this for years!!!

[@sebschimeck1273]

Haha yup, I don't think I could find a better example of an engine designed with this goal in mind, than the Porsche 918's. Reverse-flow heads, and the entire exhaust system is like 6 or 7 inches long

[@partymanau]

Talk to the racers if u think back pressure is bad.

[@tomedgar4375]

Its about the pressure wave, a completely open exhaust won’t scavenge and mid range torque is lost

[@raregarage]

I thought that a bit of back pressure was good from what I've heard from people but I guess not. Thanks for making this video, this clears up a lot!

[@daleinaz1]

Many years ago, engines ran a fairly large amount of overlap (intake valve opened well before the exhaust valve closed) in order to scavenge all the old burned fuel out, with a large amount of muffler backpressure. If you ran straight pipes with no backpressure, the incoming (cold) air/fuel mixture would flow out the (hot) exhaust valve, and could cause the exhaust valve to warp. As muffler design has improved, we can get lower backpressure with similar noise dampening, so I believe the overlap is now shorter. So maybe valve warp would not be a problem anymore, but I don't know.

[@Psycandy]

maybe it's no longer the case, but i was told a long time ago that back pressure protected the valves from burning. i took the exhaust off my bandit 400 16V and it was loud but all the way to 12K RPM just bonus grunt i loved it

[@bushcraftnorthof6012]

Great explanation. I'm learning the older I get, the more I trust OE engineering, and I just leave my vehicles alone. Big automakers probably have more R&D money than a backyard fabbing company. Take care! Awesome channel.

[@turbodiesel4709]

+ Bushcraft North of 60 : Well, it's not so much that they do more R&D per se... It's more that as you age, you realize that daily driver vehicle functionality and reliability are far more important than a little extra horsepower or torque. Typically, when a person starts "screwing" with any engine outside of the OEM settings, you will find more power, IF and ONLY IF you have a clue what you are doing. That said, typically the engine becomes less reliable and more expensive to operate. It's really just common sense. (Sadly, something that many folks lack.) That said, there are always some exceptions to the rule. For example, most modern emissions controls are literally "choking" the longevity, economy, and power outta the modern ICE's, both gas and diesel. That too is a proven fact.

[@thundercactus]

You'd be amazed how many people I've come across that think they are smarter than a department of German engineers.

[@Corsairforu]

I quickly swapped my Urethane Transmission Mount back to a stock mount after being vibrated to death in my LT1 Pontiac Formula, the OEM people really do their homework/testing about components & conditions.

[@Johnnycdrums]

After a HiPo build they recommend running the pipes off the headers right out the side, wind up the engine until white hot, cut pipes where they change to red and you have a race car exhaust.

[@chadportenga7858]

But, as TurboDiesel said, a daily driver is usually better off without screwing with it. The tuned exhaust (and I don't mean "tuner exhaust" on ricer cars) is tuned for a specific purpose - whether it's drag racing, circle track, or whatever, but it's not ideal for daily driving - unless you tend to drive stoplight to stoplight in 12.0 seconds or less.

[@ScottStonefield]

Jason, keep up the good videos. Loved this one! Happy New Year from Down east.

[@990diego1]

Scott Stonefield I’ve been watching this dude for 4 years and just today learned his name is Jason...

[@brandonbowman1035]

Learned the principles of exhaust tuning via the reflective and transmitted waves in vibroacoustics and have always jut shook my head when people talk about back pressure and why its needed in an engine (aka air pump).

[@nferraro222]

I'm a little suprised that anyone has even heard of this old piece of advice. Doesn't have anything to do with scavanging. Dates back to when vehicles were still carbureted. If you significantly changed a cars' breathing capabilities, you screwed up the carb tuning, and there was no computer to mod the A/F ratios. Every kid in high school would slap on open-element air cleaner and headers with straight pipes, but no one would bother to dig into the carb and change the jets and rods. The result would be that it would start eating gas and wouldn't really idle right, and you'd be throwing HP right out the tailpipe.

[@karllued]

Stock 289 High Performance Mustang from Ford, 271 HP. Shelby Modified High Performance Mustang 306 HP?

[@inox1ck]

Why not? The pressure waves make sense as well.

[@Cherokee93]

Also some eary fuel injection motors computers are too simple and dont know how to do anything but stock

[@kennethstegall1075]

It's the same concept be it ecm or carb. If you change something to the car, motor intake exhaust etc, you have to tell the computer you made those changes otherwise it will keep doing what it was programmed too. Same with carb if you dont adjust or change the jets it will keep doing the same thing. Causing the idle issues, spitting unburned fuel air out the back, and other things

[@kootsmike5833]

I have been trying to teach this to people for decades. They don’t learn. Please everybody, spread the word!

[@m4xwellmurd3r]

Michael DeJong seriously. Ive been trying to get this misconception out of peoples heads for years now.

[@FerralVideo]

Exhaust scavenging is even more important in small two-stroke engines, due to not having actual valves. In fact, when your two-stroke lawn equipment is "sputtering" and seemingly running badly at idle or under low power, that's exactly what's happening - there's not enough exhaust velocity to scavenge the cylinder effectively before the exhaust port closes and it tries to fire the next cycle. There's too much exhaust gas and not enough clean gas, and that cycle fails to ignite.

[@ELEISONCARABALLO]

Very good video. Would you try please to explain better about the sweet spot, where we get the optimum flow and equilibrium between back pressure, inertia scavenging and velocity. How can we calculate it.

[@miguelruiz2288]

The Dislikes are the people that said the car exhaust needs BackPressure.

[@zippyoya]

Miguel Ruiz the only type of engines that needs back pressure are 2-strokes I think that’s where they got that from

[@wannabecarguy]

@@zippyoya typically the flow should be balanced. If your exhaust is sucking raw fuel out of the cylinder before the valve closes you might consider changing the flow of the exhaust. Valve timing sometimes overlap incorrectly. Also the expansion chamber of the 2 stroke doesn't create back pressure.

[@r2dxhate]

I disconnected my cat from my old 90 4runner once and it felt 10-20% slower.

[@zippyoya]

r2dxhate that makes no sense a high flow exhaust wouldn’t change that much you might get a gain of like 1.5 to 3 horse power not much

[@r2dxhate]

@@zippyoya I'm not talking about a hp gain, I said my 4runner got slower. Someone suggest my cat might be clogged, so I disconnected my bolt-on cat to see if it ran better with no exhaust at all besides headers, and it was way slower instead. I'm not saying that proves engines need back pressure, but for some reason it wasn't running as good when it was allowed to breath better.

[@steveg2277]

Lol you demolished the backpressure argument in 1 minute 58 seconds 😅

[@mustangecoboosthpp3869]

He did not demolish anything, to much back pressure in an exhaust system and exhaust gas velocity is killed, to little back pressure in an exhaust system and velocity is killed, all these videos do is play semantics trying to say back pressure is a myth yet all properly designed exhaust systems will have back pressure. It is about exhaust gas velocity. Borla worked with John Lingenfelter and he built 600 horse power corvettes using 2 1/2 inch exhaust systems because that is what gave him the maximum exhaust gas velocity if a bigger exhaust pipe would have made Lingenfelter go faster he would have used it but a bigger exhaust pipe removed more restriction i.e. back pressure and killed the exhaust gas velocity. A 2 inch exhaust will only flow so much volume, a 2 1/2 exhaust will flow more volume than a 2 inch system and a 3 inch exhaust system will flow more volume than either a 2 inch or 2 1/2 inch exhaust system. What you are looking for is exhaust gas velocity if the 2 inch is to restrictive you kill exhaust gas velocity what is restriction, it is back pressure. If the 3 inch exhaust is to free flowing it kills exhaust gas velocity because it does not have enough back pressure. The 2 1/2 inch exhaust in this case has less restriction than the 2 inch system and has more restriction than the 3 inch system yet the 2 1/2 inch exhaust system has the highest exhaust gas velocity. Having said this what you have to do is design the entire exhaust system with the optimal size pipes to get the maximum exhaust gas velocity so it takes proper design to get maximum results, which is why Borla is the leader in aftermarket exhaust systems and they have won more races than all other aftermarket exhaust manufactures combined. All these videos are doing is using mental gymnastics trying to eliminate the word back pressure.

[@kentvandervelden]

The physics involved here are fascinating and is just a single area of improvement with engineering. It's sad to think of how all the really cool physics involved in ICEs may be displaced with electric and software. There's a lot of skills and knowledge that is potentially at risk. (Said by a father who will teach his son how to overhaul a small engine as soon has he is old enough to use a wrench.) Happy New Year!

[@djsjjd15]

Sounds like a fun activity - for you. Unfortunately, it is a skill that is becoming less and less valuable each day and he might not even have the opportunity to use it much if our roads go the way of autonomously driven electric cars when he's an adult. I think your fears are correct. Maybe learning to make his own PCBs to hack a Tesla would be more useful . . . I grew up in the '70s & '80s and I remember when dad got an electric weed-eater, then an electric leaf-blower and slowly, our garage went from lots of small ICEs to extension cord racks.

[@kentvandervelden]

Less so than ICEs disappearing, I fear loosing a more accessible and tangible technology to learn. A young child getting hooked on ICEs might later realize them as a gateway to Physics, ME, or AeroE (or environmental conservation.) Who knows their trajectory. Modern electronics are pretty highly integrated, so mostly serial protocols, software, data sheets and integration. Early on it's more about creating experiences, especially sensory, and the ability to adapt. I spent a lot of time at university and every time a topic was completely dropped the students suffered. The argument was always the same... "computers can do this for them, they will not need this in 10 years." BTW: You guys were way ahead of everyone else. Today, I only know one person with an electric mower and two (high income) people with electric cars. Might be regional thing. Have a good one!

[@pizzaki582]

I don't understand older generations sometimes, look at car engine progression over the last 20-30yrs gone from a mechanically operated engine that was time consuming to diagnose or fix. now to having inputs that a very impressive that are processed at the speed of light, even looking at fuel trims can give easy idea of where to diagnose the problem, it was only easier back the ole days cause u did'nt have to worry about catalyict convertors or evap codes u could vent the engine crankcase straight into the atomsphere( a no no in todays world) and the fact its easier for the modern joe to keep his car serviced, or u gotta do change the oil,no more replacing contacts on dizzy. if u know how to read a relay diagram u know to diagnose problems for electrical theres alot of good channels out there. engine still use the same princepal air/fuel compression spark, just different way to operate.

[@JodyBruchon]

We don't need no stinking headers! The bottom line is that "back pressure" is really just "choking exhaust gases trying to leave the motor." Most cars seem to benefit from small increases in exhaust diameter.

[@mothman.industries]

So what happens if I steal a tuba from the local high school and duct tape it to my headers?

[@EngineeringExplained]

With great horsepower comes great responsibility.

[@JakobusVdL]

Engineering Explained But the tuba / trombone idea would actually be a way to tune an exhaust system to get the -ve pressure at the exhaust valve at the right time throughout the rev range. Does anyone do that?

[@buddylee8580]

Nothing really except restrict the exhaust gasses as Tuba's have very small pipes. It is the mouthpiece and the way the lips are pursed that actually make the tuba sound. No lips, no tuba sound :) Same with the trombone.

[@Milesco]

-- "So what happens if I steal a tuba from the local high school and duct tape it to my headers?" Depending on the number of cylinders and the rpm range, your car will either play "Smoke Gets in Your Eyes" or "The Piña Colada Song" as it glides down the highway.

[@Acroposthion]

....the cop car taking your ass to county lockup, will sound a whole lot more the same.

[@thenigletcyborg]

It is so damn cold here, my car has been keeping the exhaust for heat...

[@ghostassassin1107]

Same here. I'm staying inside until cabin fever drives me mad. :)

[@7sins979]

finally someone that get that the smaller factory exhaust exists to increase exhaust velocity. one thing though, for the scavenging section the intake valve opens at the end of the exhaust stroke while the exhaust valve is still open so that instead of creating a lower pressure in the cylinder, the cylinder is swept with fresh air removing the last bits of the spent air fuel mixture, this is called valve overlap. the factory exhaust system is designed to provide good drivability at lower engine rpms while providing reasonable performance at higher rpms. the reason to change to a larger free flowing system is to provide more performance in higher rpm ranges, sacrificing low rpm drivability.

[@kensmith8832]

Thanks for the laugh! I did a test on this on my car during the Spring of 1990. With no back pressure, the car had no bottom end power, meaning power picked up around 3500 RPM. But with back pressure the car could do burnouts. This car didn't have turbo. This was before EFI. A class to take is fluid mechanics in a pipe. Here again, no two engineers have the same solution.

[@mustangecoboosthpp3869]

Thank you, what you have are a bunch of people trying to eliminate the word back pressure when if you have to much pressure in the exhaust system you kill exhaust gas velocity and it you have to little pressure you kill exhaust gas velocity. It is all about finding the right sized pipe for the highest exhaust gas velocity. Where all the misinformation and confusion comes in is an exhaust system is not designed with back pressure, an exhaust system is designed by maximizing exhaust gas velocity. John Lingenfelter built 600 horse power Corvettes and Borla worked with him on the exhaust systems, John Lingenfelter used 2 1/2 inch exhaust systems because that is what provided the best exhaust gas velocity to make the most power, as Borla says if a bigger pipe would have made John Lingenfelter go faster he would have used it.

[@collarbonecam]

I never believed in backpressure, but I wanted to watch this anyway. Care to elaborate on what methods manufacturers use for turbo applications? It's going to act as a restriction for sure but are there any tricks manufacturers use to mitigate this?

[@EngineeringExplained]

Generally for turbocharged applications, you simply want low restriction after the turbo, because you're no longer going to gain the benefits of scavenging from exhaust pulses. Leading up to the turbo, you want to keep routing short, and hot, so that little energy is lost from the exhaust valve to the exhaust turbine.

[@4G12]

Engineering Explained To me that sounds like a short and fat turbo manifold leading into the turbine side of the turbo with a high flow cat right after it, and the absolute lowest possible restriction muffler after the cat to keep noise humanly acceptable. I am simply not psychopathic enough to needlessly pollute. Droning exhaust notes get old real quick.

[@MrHarey31]

Great video, this is for non-turbocharged vehicles though, with a turbocharged vehicle you just want the highest flowing exhaust after the turbo as possible. All the scavenging etc happens in the headers before the turbo

[@davidpowell3347]

I think a turbo dampens out some of the pulsations that are important in a non turbo system making post-turbo more about big cross section and less about "scavenging"

[@lazomaniac]

this makes me scared of making any modification to my car exhaust now

[@connivingkhajiit]

Just use parts engineered by well known manufacturers like Borla

[@ianroz1359]

or just use what he just said and if you use bigger diameter piping, tune it correctly. most cars come stock with 2-2.5" exhaust piping. you are gonna have some back pressure at the header collector, downpipe/cat. use the available bp to balance it out and a tune to dial it in.

[@jacknickolstine3355]

It's just pressure vrs flow

[@MrHaggyy]

you can't really damage anything with the parts that you can mount on a road car. But something similar to the stock exhaust or well-engineered aftermarket is better for sure.

[@shawndubay4050]

@@MrHaggyy like a Flowmaster hush power 2 muffler and two or two and a half inch pipe to the rear bumper?? Or is this too free flowing?? It's a V.6 tiburon.

[@Javierm0n0]

So basically everyone needs at least one turbo 😆 (it's a joke)

[@MrtinVarela]

*one turbo per cylinder

[@seanwatts8342]

...you'll blow the welds on your intake ...

[@NousagiMechanic]

Sean Watts Joke's on you, my intake manifold is cast. XD

[@xyrex5243]

Martin Varela Unless you're a diesel guy in which case you gotta run a compounding turbo setup for each cylinder just to be safe. So one turbo per turbo per cylinder.

[@huntguy3831]

Sean Watts If your welds break that means more air will get in so that means more power! What’s the problem? Lol

[@firstielasty1162]

I have had this debate with a few friends. Something that helps to (erroneously) 'confirm' that backpressure is beneficial is that people sometimes do feel a power loss with the installation of a better exhaust, due to failure to finish the job. That is, proper jetting or EFI tuning. Most shadetree mechanics can change exhaust parts, many cannot accomplish the latter. Exhaust improvements are available for many engines, for which no carburetor jets are available, or which have virtually untunable efi. Many tolerate or are even unaware of their power loss, and enjoy the loud noise regardless.

[@themotofixery]

So all those coffee can mufflers on the rice burners really do suck? So fitting!

[@danfenner8278]

I got lucky with my 77 1/2 ton 4 bolt 350. yanked the fucktory choking manifolds, slapped a pair of headers on, cut a drive shaft in half that was thee same size as the collectors then chopped and gutted a pair of old glass pacs, took 2 1/2" cyclone fence posts and cut slots in a spiral pattern from end to end, used my air hammer with point bit and drove into each slot causing it to swirl the exhaust as it passed through, welded it all together and damn what a sweet sound not to mention large improvement over the entire rpm range. couldn't believe how much it did for it

[@nounours2627]

It would be great to talk about turbos and whether or not all these effects does still occur behind it. :)

[@devinbender8428]

Nounours2627 you want that pressure wave to be as consistent as possible drawing maximum air out of the engine as possible. Ie maximum velocity with minimal resistance to keep the turbo spooled higher.

[@waltciii3]

What about stock cats vs high flow cats or no cats? I heard that you could lose torque with test pipes. (It's been two years but any response is good)

[@Big1_]

No back pressure you lose power because fuel is going out of the chamber... What does the backpressure is to keep the fuel inside the chamber before the piston goes up.

[@victorjohnson7512]

Only 2-stroke engines need tuned back-wave pressure to cycle properly. That's why they have those expansion pipes.

[@philtripe]

Now prove the theory by removing the entire exhaust from your Honda and showing the dyno results

[@arealassassin]

Your comment proves that you didn't learn anything...

[@yurik1994]

That’s kinda too hard to understand on a new years eve 😂 but thanks for video anyway!

[@EngineeringExplained]

Haha, no time for vacation! But nah vacation is important.

[@Pork-Chopper]

You forgot to mention firing orders where the Pistons are in varying states of the 4 cycle process, most firing orders keep one piston exhausting one after another, continuously pushing the exhaust flow out. Reversion waves are kept at bay, sometimes they never reach the cylinder. The other is, for those old timers who remember, cast iron log type exhaust manifolds, were also called "heat stoves" sometimes had a spring loaded gate the would close the exhaust flow n would open up when the motor was accelerated more exhaust. This device to an extent would increase back pressure, n therefore also increase "heat" which is energy. Keeping the exhaust manifold, "heat stove" hot or hotter would increase "energy" and also exhaust "scavenging", one of the reasons hot rodders "wrap" their header pipes with exhaust wrap, which is not only to keep from burning yourself, which most people think, but it's to retain "heat" which is energy... So, a little bit of back pressure is beneficial, as you say, you have to find a happy medium in exhaust pipe diameter to get the best velocity, and in doing so, you are introducing a bit of back pressure. Also depends on engine size, stroke, n displacement...

[@cubey]

They do need back pressure, at least on old engines. I had a 1978 carbureted Dodge 360 that flooded and blew out muffler during cranking. While driving to a muffler shop, it was weak, lacking power. After the new muffler was installed, it was back to normal.

[@EricErnst]

I've been saying this for years. Old timers tell me you 'need' backpressure. You do your thing, let me do mine. Flowtech long tube headers and y-pipe to single open 3" on my old 350. Single 3" is restrictive at my power level but it was fine when I designed it (before heads and roller cam swap). Now I'm looking at building my own y pipe and running 4-4.5" open. I think I've heard 2.2psi or 2.2" of water is the ideal backpressure. You won't lose any top end power but your velocity should be pretty high through the powerband. I'd have to go back and check the unit.

[@black_yeti]

They should teach this in school

[@brucepeebles4939]

I learned this stuff from a book written in the 1960s.

[@0swaldpwns]

They do, in college

[@danielmikels]

I learned this at UTI, but I think I'm the only one who listened.

[@midnightdarkchocolate]

They do at least the basic principles in AP Physics at least they did when I was in HighSchool

[@Michael_00001]

They do once you get passed the 4th grade. Be patient.

[@D.SN0W]

I swear, I feel like if you really wanted to and actually paid attention ...all you have to do is watch all of his videos and you’d be able to tune your car at home without the help of these expensive shops.

[@DukeGaGa]

Ahab The Crusader true but you still need the tools to do that, and those tools ain't cheap

[@D.SN0W]

Yeah, I know that. But with the knowledge provided, it’d be cheaper to do it yourself than pay someone else to do it (in the long run and depending on what vehicles you prefer) if cars are that big of a part of your life.

[@DukeGaGa]

That's why I said true.... I'm agreeing with you not disagreeing.

[@RDSAlphard]

lol, this guy speak like tools are $0.5 each

[@jessefranco8987]

I could create an entire car from scratch lol

[@pjay3028]

Hi, I've just read this about the launch of the revised Audi TT: (see 2nd paragraph of quote) "For the 2.0 TSI, three states of tune will be offered, with the first, the 40 TFSI, producing 194bhp, 16bhp more than outgoing 1.8-litre TFSI that features at the bottom of the current TT range. There will also be a 242bhp 45 TFSi, while the TTS (pictured above and below) gets 302bhp, a decrease of 4bhp thanks to the fitment of a new particulate filter. An Audi spokesman said that the resulting increase in back pressure has helped to boost torque, helping to shave a tenth from the pre-facelift car’s 4.6sec 0-62mph time."

[@rotring8581]

Im more disturbed by the fact that your pistons are never seated fully in the cylinders, you wont be driving anywhere with a car that has that much of a gap between cylinder walls and the piston

[@EngineeringExplained]

Are you okay with the flat cylinder heads? Pretty modern stuff in those drawings. :D

[@darthhomie]

I mean if you want to look at it that way, the valves are just going to fall into the cylinder anyway.

[@tonyppe]

In my experience of fabricating stainless exhausts for hundreds of cars, one car stuck out. I believe it was a Citroen C2 with a 1200cc engine. It had a rather large bore exhaust from the factory (2just over 2" but not quite 2.25") with restrictive silencers. When I made the exhaust for this car, the customer mandated that I kept the bore the same size for various reasons that werent valid. Anyway, after fitting a system with free-flowing-silencers and that 2.1" bore, the customer came back and said he had lost power. To fix it, I removed the back-box section and rebuilt it in 1.75" bore with a free-flowing silencer with the same internal bore. Problem fixed. When full systems are built, we went either from the downpipe or after the original cat section. This is so that forwards of the exhaust system is kept original. Often if the customer requested too large diameter, they would lose power.

[@Mallikii27]

I’ve always heard that exhaust valves could deform due to heat if you went with too low of a back pressure such as straight pipes or just headers. Your explanation was great. Lower velocity due to less restriction means the hot exhaust gases are lingering causing heat issues. I understand velocity and pressure (PV/nrT) but it still seems counter intuitive sometimes. That’s why I’m an Electrical Engineer ;)

[@jcjcjcjcjcjcful]

lol the V in your equation is volume fyi. I hope you really understand velocity and pressure mr electrical engineer

[@dollydehart4813]

I was taught in automotive school too low of back pressure would burn up exhaust valves

[@sirfrancischl]

Dolly DeHart can somebody confirm this? I run my 05 wrx with a 3" straight pipe after the downpipe which is 2.5" (stock) and I was told I would deform the valves sooner or later....is this actually true?

[@littlerhino2006]

Just theory to consider here. Assume the header was tuned to perfectly scavenge the remaining hot gasses from the cylinder at peak torque as demonstrated in the video. It's pretty easy to see why this ideal, you would get a boost in peak torque. So imagine the result if you removed what was scavenging the last bit of hot gasses from a combustion chamber. A little bit of that hot mass lingers there when chamber pressure reaches equilibrium with atmospheric. I think this would create a hot spot on the head that the cooling system wouldn't be able to deal with. After the energy (heat) has been extracted and turned into mechanical work via spinning a crankshaft or driving a turbine, it's just causing issues by hanging around. Once it's useful life has ended inside of an engine it needs to be returned back to mother nature as quickly and efficiently as possible so the cycle can repeat.

[@Broadpaw_Fox]

I+Dolly DeHart - it's not the pressure, but the slow flow. Low velocity is what helps burn valves - it allows the hot gasses to sit around the valve longer. That is the exact point of this video - the common misconception is that people use 'backpressure' when they mean 'low velocity'. Look closer at the top right of the whiteboard in the video - that's talking about pipe diameters. Too small gives you high velocity, but high pressure. Too large a pipe does the opposite - low velocity (cook valves) but low pressure. There is a sweet spot for each engine, but it's more about power generated percylinder. Power = airflow, and it's true in the exhaust as well. You can calculate how much exhaust gas a cylinder will produce for a given power level - then you need to find the sweet spot for that flow in pipe diameter.

[@ShakaAantoine]

Excellent video! You've answered many of my questions except one. How do I figure out what is the correct tubing length and diameter if I was to design an exhaust system so that it makes power instead of losing power?

[@billbergen9169]

No exhaust! JK

[@endurofan9854]

i think thats the hardest part... i'm pretty curious too

[@samfixitguy1661]

@@billbergen9169 No exhaust usually is not the answer. There is no "tuning, no scavenging effect.

[@AkbarNurPribadi]

This is so old, anyone figure it out how to calculate those?

[@Samael_Monasteriis]

2.5" for motors up to 2L 3" between 2L & 4L 4" for 4L to 6L Dual pipes on the V motors & use a H pipe for the low end and better sound.

[@89psychobillyford38]

Here's my personal hands on experience by feel not by dyno. Vehicle: 89 f150 2wd carbureted 351w 4spd manual. Engine mostly stock with long tube headers, electric cutouts in front of 10 series single chamber mufflers. With cutouts closed and running exhaust through mufflers, strong pull, but hesitates at first. With cutouts open, smooth response but feels like it isn't picking up speed. As said before, this was all based on the feel not the dyno sheet. Never went to dyno because it was a beater with enough pep to have fun on your drive to work. Also had a passenger that confirmed the feel.

[@be4tnl]

next question, how do you calculate the right diameter and length of the header?

[@EngineeringExplained]

It's dependent on many factors, and would probably come down to simulations rather than exact calculations. Dependent on how much airflow you have (power), and then calculating (deciding) what RPM you want it to be effective for, by measuring the time it takes the pressure wave to reach the exhaust collector and then return to the exhaust valve.

[@be4tnl]

Engineering Explained I'll stick to the original exhaust than, instead of welding a stainless one myself. 👍 happy 2018 btw.

[@mustangecoboosthpp3869]

@@be4tnl This is where you go to a company like Borla because they have the equipment to measure exhaust gas velocity and they do the R&D to make sure their product does what they say it will. To small of a pipe results in exhaust flow restriction to big of a pipe and you do not have enough restriction, both of these kill exhaust gas velocity. John Lingenfelter built 600 horse power Corvettes using 2 1/2 inch exhaust system designed with Borla, if a bigger pipe/exhaust system would have made John go faster he would have used it.

[@coscorrodrift]

Great video Jason, and really interesting topic! One question, did you learn all of this knowledge in your degree, or did you have to research it for this video, because it seems like they are some really specific knowledge and phenomena to have learned at school.

[@JDMJACOB1]

coscorrodrift I believe he learned some of the basics through classes-thermodynamics, design, etc and then did self study outside the curriculum

[@EngineeringExplained]

Most of this is from online research - there were a few well researched papers and articles I found on the subject for bits and pieces. As is generally the case, there's a lot of good (and bad) information floating around on the internet, just needs to be sorted down to the facts. That's my goal! As far as my degree, I just did a bunch of challenging math problems for four years haha. Not a huge amount of practical knowledge, but definitely problem solving skills.

[@martytaylor1478]

The way it was explained to me (by aeronautical engineering school) was back pressure was needed to help clear the space between the piston and top of the cylinder when the piston was at the top of the stroke and both the intake valve and exhaust valve was open. The air/fuel mixture would push out the remaining exhaust whilst the back pressure would restrict the amount of air/fuel mixture from entering the exhaust pipe. Not enough pressure could cause ignition in your exhaust pipe. However the engines we worked on were old VW boxer engines which surprisingly made pretty good aeroplane engines

[@elsanjopucela]

Great videos! Thank you so much from a fellow engineer. Sometimes it's difficult to see concepts of theory applied, but you made a brilliant job ;)

[@bjorn1583]

remove the back pressure on any non racing vehicle and you will loss low to mid torque and your fuel usage will go up (race cars run at high rpm and fuel use is not an issue). never listen to people that say back pressure is not needed on street vehicles and that info comes from a transport engineer with over 30 years experience. it doesnt mater what type of engine you have you MUST get a tuned exhaust to suit or it WILL cost you money in the long run. ps. unless you are a lead foot putting a tuned exhaust on can save fuel and give you a nicer sounding exhaust

[@MichelLinschoten]

You clearly have no idea lol

[@josephbaker6083]

Even on race vehicles, they have some exhaust. Even dragsters don't just have bare exhaust ports at the heads, they have exhaust tubes, and even though they are only 30" long and no muffler, those tubes are carefully selected to control the velocity, even though they have a big blower on top.

[@pauldalby5413]

You can't have scavenging without backpressure! One way to think about this: The column of gasses in the exhaust is a football. Attached to the football on a string is a teapot (intake charge) If you kick the football (exhaust stroke) the weight of the ball (backpressure) now pulls the teapot with it. If you swap the football for a balloon (no backpressure) and you kick it - the teapot isn't going anywhere, no matter how hard you kick the balloon. The real answer is an exhaust is a complex and useful tuning device which designers can utilise to shape power characteristics dependent on needs, and backpressure goes hand in hand with any exhaust system.

[@gigel99324]

My butt dyno says my car lost torque with a stainless steel free flow exhaust.

[@anthonykaiser974]

Alex Gunner my I-64 Southern Indiana hills Dyno said the same thing when I put a dual exhaust on a 318 Magnum in a 99 Ram, after gaining considerable torque from Gibson truck headers. Didn't lose all of it, thankfully.

[@lil3unnyfoofoo490]

Butt dyno😂😂😂

[@niFeforever]

Maybe because your ecu was mapped specifically to the stock exhaust?

[@artgoat]

If you don't have an exhaust pipe properly tuned to your engine, with the correct length headers, you're actually going to increase back pressure, because the natural tuning of the pipe is opposing the volumetrics of your engine. A larger pipe is not actually always going to produce better flow. If you think that pipes are only going to be tuned for a small range of RPMs, you're absolutely correct, which is why a perfectly tuned exhaust and a "perfect" cam is going to have one hell of a power spike at one RPM, making the power curve a lot less usable. A certain amount of detuning is essential to make a car drivable (and not horribly anemic up until 4,500 RPM, at which point it peels the rubber off your tires).

[@gigel99324]

artgoat I think this only applies to N/A engines. Any disadvantages with a turbo engine would be outweighed by the benefits.

[@YZJB]

Great video Jason! Happy new year from Australia!

[@EngineeringExplained]

Happy New Year!

[@johnparker4723]

a lot of people dont realize that while an engine may have no need of exhaust backpressure, the way an engine is tuned is usually accounting for some back pressure, and depending on how smart the ECU is, it is possible for an engine to run worse or have other adverse side effects from simply removing exhaust backpressure. looking at you newer dodge gasoline trucks!

[@RenoLaringo]

Whenever I feel backpressure in my exhaust, I know things are going to be bad...

[@ShaunJV12]

My lecturer at university said that you do want a small amount of back pressure for N/A engines because the residual exhaust gasses left in the cylinder effectively increase the compression ratio as you are compressing a larger volume of gas into to the same volume at TDC. With forced induction engines you might not want this as you can make up the difference by forcing more air and fuel into the cylinder

[@EngineeringExplained]

Residual exhaust gases can be beneficial from an emissions standpoint (this is what the purpose of EGR is), but if you're displacing air in the cylinder with spent combustion gases, there's less room for fresh air & fuel.

[@thetrollwhoknows9928]

AstonMan you traded spent gases for fresh air and fuel? I'm not buying it

[@ShaunJV12]

Fair point, for power that would make sense but if you wanted to an engine to be efficient then having that small amount of residual exhaust means you can have a higher compression ratio and use less fuel

[@thetrollwhoknows9928]

AstonMan NOx forms primarily when a mixture of nitrogen and oxygen is subjected to high temperature, the lower combustion chamber temperatures caused by EGR reducesthe amount of NOx the combustion generates (though at some loss of engine efficiency). ... For example methanol is more tolerant to EGR than gasoline. Adjusting timing and fuel mixture is the answer to efficiency and power and Optimum combustion temperature. EGR exhaust gas recirculation only reduces nox it is beneficial for nothing else.

[@korn38ktm]

With n/a 4 strokes it doesn't hold as true. Shorty pipes can ruin the engine, and in a midget car my grandpa just welded a full straight exhaust to the back for looks and destroyed every race. Every race they would tear down his engine thinking he was cheating and all that was changed was a longer pipe

[@pizzaparty-r1c]

This explains why low rpm at high load causes denotation. Some hot exhaust gases are still in in the cylinder on the intake stroke due to lack of scavenging.

[@thegallivanters825]

How would I calculate the correct size and tube length? Or as that just trail and error?

[@Laracrafttrabant]

My car has a 2-stroke sooo, my cars exhaust needs reflective sound waves :D but yeah planing to put in a 4 stroke and then that dose not matter anymore and i have to remove the components that are tuned to the 2 Stroke. ^-^" your videos help understanding what i have to consider with the conversion - keep it up your a big help :) kind regards Lara

[@donniebaker5984]

back pressure will severely harm the performance of a 2-stroke here again you are confusing back pressure with the reflective sound waves as it works the same way in a two stroke.... reflective sound waves are much more critical for the performance of a two-stroke to keep the intake from escaping the exhaust port. these are not my words but the words of Pop Yoshimura the greatest tuner of all 2-stroke engines there ever was and the inventor and designer of Honda's first two stroke engine

[@Laracrafttrabant]

@@donniebaker5984 edited it to be correct :)

[@danawilkes6174]

Tri-Y's are still being used today, as they form a higher vacuum condition at the exhaust valve for more torque as compared to 4 into 1 header design. I have been using Tri-Y's for apx. 50 years. They will out perform a 4 into 1 all the way to 6,000 rpm when setup correctly. With an automatic and most being overdrives now days, they make a big difference when used here.

[@iamakkkshay]

What if I remove the exhaust pipe and let the exhaust valve eject gas directly in atmosphere. *Just jocking

[@OldSchoolZ-wy2yx]

Where's your horse, then?

[@fordmx5]

@@OldSchoolZ-wy2yx 😭

[@Oblithian]

@@OldSchoolZ-wy2yx that would be jockeying. Jocking would be... wearing restrictive underwear? or behaving typical of the highschool 'jock'?

[@richardbostick8675]

So if I didnt have enough back pressure could this cause a popping noise from the combustion chamber

[@HoRS]

yup when I had a straight pipe, but of larger diameter than stock, with big tail end, it did popping noise with little flames, when u reved it... on 1.9 8V 205 GTi.... it literally run better at high revs, bcs of straight pipe

[@BigUriel]

"Back pressure" is just a term people who read too many car magazines and think themselves engineers use to overly simplify the very complex fluid dynamics involved in the tuning of an internal combustion engine, which real engineers use computers to simulate extensively before they build anything because the problem is so mathematically complicated that it's literally impossible to solve analytically.

[@infiniteseeker9643]

Happy New Year. Best wishes.

[@fishrrelaxing9361]

You kind of skipped the 101 version and went straight to 102 class. The 101 version would have been a better explanation. Back pressure exerts force back into the cylinder requiring use of some horsepower through another cylinder firing exerting force on the crankshaft to physically push gas out. The more back pressure the more horsepower is used up in rotating parts and not making it to the driveshaft and propelling the vehicle. Your explanation of pressure waves is more towards tuning of intake and exhaust timing and design differences of header lengths and exhaust sizing. EI was more about exhaust design then about back pressure being bad. I know they are interrelated but feel you really did leave out important info.

[@Dread_Not]

Wow, actually good criticism in a comment?

[@liloso60]

Fish R Relaxing well when you explain back pressure like that it sounds like version 100. If there is back pressure on a cylinder then it will take power to push it out. Bam the videos done. Take care see you next week.

[@fieldlab4]

It's my impression there are important acoustic effects after the collector, and the length of the exhaust pipe from the collector to the "resonator" is important, and removing the resonator but maintaining the total exhaust length can negatively impact performance.

[@RexinOridle]

Jason, what if we put a supercharger at the exhaust? For discharge of course.

[@DrewLSsix]

Rexin Oridle. As in a vacuum pump? Potentially beneficial but compared to a compressor on the inlet you are pretty limited in what you can expect in terms of improved output.

[@RexinOridle]

Judging by the numbers of likes, I don't think anybody likes our question, LOL.

[@pjay3028]

Rexin Oridle I think you would find that the benefits gained from better exhaust scavenging would be more than offset by the weight and power consumed by the exhaust fan/supercharger so you would actually achieve a net performance loss.

[@duncanb1981]

I was about to write a lengthy comment commenting on the problems with your idea when I had the image of a mechanically or belt driven turbo device with compressor wheels that simultaneously create pressure and vacuum in both inlet and exhaust sides of the engine. I guess if it used fans as supposed to a roots type supercharger it could work.

[@leealmond8655]

Actually, there have been some passive exhaust extractors used in Formula racing whereby the exhaust tip is placed in a high velocity area (think above the rear spoiler or outside the firewall) with an integrated radial venturi to create a lower than atmospheric pressure at the exhaust tip. Operates similar to the venturi in a carburetor -- it sucks better! Most effective on a short exhaust with naturally lower flow resistances/losses. Also, on very short exhausts the exhaust pipe can be flared to broaden the scavenging RPM range (similar to velocity stacks on carbs). Analogous to impedance matching in the RF world.

[@diffieq]

With need for a lower external pressure for exhaust gasses, do manufacturers consider aerodynamics of where the exhaust is placed to maximize vacuum or are the fractions of an atmosphere negligible?

[@MyLonewolf25]

Negligible unless you’re running serious drag car power That said top fuel cars create about 5-600 lb of downforce from their exhaust just because they move so much air Same reason pro mod drag cars will run bull horn style exhaust. Every little bit counts. But honestly in passenger cars you’re more worried about the physical exhaust route than you are the gasses coming from it. Same goes for “low” hp cars

[@2689vjavier]

It makes a difference on motorcycles as their aerodynamics are crap. For example changing the direction of the exhaust tip can mean that you increase back pressure and harm horsepower by trying to make the gas escape into a high pressure zone. Or moving it a little bit can mean having the exhaust gases exit into a low pressure zone and helping with the scavenging effect. They can't be measured on a dyno. Unless your dyno is also in a wind tunnel