Beyond Clever - How Two Stroke Exhaust Pipes Really Work

[@d4a]

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[@ArcticTraveller-o7s]

During the ’73-'76 years I raced motor-cross with Honda's CR Elsinore in both 125cc -250cc class events, they were beautifully constructed bikes that were the closest thing to factory works bikes money could buy straight off the show room floor. It had one major flaw in the design of the expansion chamber exhaust pipe, Honda engineers designed the pipe to run under the engine cradle, so each race event that had lots of jumps I'd crush the expansion chamber as well as my nads 😢 So once race day was over and was able to walk upright and speak without a high pitch, the cleaning began hosing the mud off the bike then the damage assessment began, I became proficient cutting out the flattened damaged portions and wire welding mild steel patches that matched the contours of the original pipe, after that sprayed it with heat resistant flat black paint. By the next race day the bikes were looking brand new. There was no internet back in the day, information relied on the phone or letter writing and what one could get through magazines, fortunately and with a bit of luck I was able to get a peak and dimensions of Honda's factory works engine head and barrel, though being dissimilar in port design and shape I was able to cc volume match it, few porting stones I was able to increase the abrupt and narrow power band with a torquier and wider power band that kept me competitive during the winter of '75 when Yamaha released with their new white tanked mono-shocked YZ series Motorcross bikes which was a game changer over the rough stuff, no longer was engine brute power the sole reason for winning, it was combination of frame, long travel front forks and rear mono shocked cantilever suspension. The good old days they were.

[@erkinalp]

Why don't they use uniflow two strokes, which lack this problem, then?

[@yogibarista2818]

Perhaps needs a more accurate title than just "two stroke" - obviously this is a crankcase intake/transfer 2-stroke, but there are diesel 2-strokes, and mazda even has a working poppet-valve petrol 2-stroke.

[@erkinalp]

@@yogibarista2818 Also applies to other ported engines such as wankel and liquid piston.

[@EarthSurferUSA]

Man, It is you again! I saw the vid headline and came here to critique your knowledge. But you know what you are talking about (rare on YT), and took all the fun out of my morning. :) Doug in Michigan

[@williamromine5715]

I am an 82 years old American who rode 2 stroke trail bikes in the Rocky Mountains of Montana and Wyoming. I never really understood how the shape of the exaust pipe worked. It has been many years since I stopped riding bikes. Today's video, as all of your videos, was very informative and entertaining. Thank you.

[@thomasauslander3757]

Time to get a scooter 🛵

[@jeffrandolf5673]

I'm 62 & still have my '84 XR250R 4- stroke. *_The REASON I haven't ridden it the past 7 years_* is because at age 55, I realized... if I get even the slightest injury, I will not be able to work 100% at my very physically, demanding/gruelling job & NOT GET A paycheck. No work=NO PAY. Since I'm at an age where: 1) I have a finite number of years working at the highest physical level possible & 2) I am not satisfied with my retirement savings AMOUNT... I do not want to injure myself, be unable to work & be unable to acquire the most retirement savings possible in a shortening time frame. I do start the bike annually & ride it in a circle for 5-10 minutes to keep it "alive". 30 years ago i broke my collar bone by merely falling over to the side at 2 mph on this bike. The recovery was very, very long due to no surgery.

[@thomasauslander3757]

@@jeffrandolf5673 smart Men 👍

[@merkel2750]

I’m a 348 year old animatronic dwarven made golem, I am two stroke.

[@msromike123]

@@jeffrandolf5673 Man you only live once. The average person has an injury producing accident every 70 years of street riding. Approaching life from the viewpoint of micro economics is very limiting and soul crushing.. Get some good gear and go make the most of your short time on this earth. My 2 cents.

[@gweasel7270]

This guy should teach teachers how to teach. His voice, his cadence, his scripting. Awesome. love this channel.

[@chrispy1965]

As a teacher myself makes me wonder the following question, what makes you think that teachers dont already practice this?

[@gmailisaretard]

@@chrispy1965 Some teachers are in a job, other teachers are educators. This guy is an educator, which can't be taught to be fair. I had at least one perfect example of someone who taught and had no business doing so growing up back in high school, she had a job and she didn't care about it. Another teacher was also doing a job but they spent time trying to do it right, they cared even if it wasn't a natural thing to them. And I also had several amazing teachers who truly were educators.

[@gmailisaretard]

I remember there was one point a long time ago where he said he felt his accent got in the way of his ability to communicate effectively sometimes. Back then and still now though, he is articulating complex concepts that I as someone who is reasonably intelligent, but ain't smart, am able to learn something every time I tune in.

[@johnnunn8688]

@@chrispy1965you mean, ‘that teachers’?

[@SteelyEyedMissileDan]

​​@@chrispy1965I've spent most of my 28 years of existence as a student. The majority of teachers I've had, both in public schools and universities, clearly did not care about the material they were trying to teach, nor did they care whether it made sense or was interesting to their students. They were just going through the motions enough to meet the bare requirements of their curriculum in order to get a paycheck. Teachers like that really make you appreciate when you get a fantastic teacher who loves their job and is damn good at it.

[@mahadhiahamedimtiaz4099]

This guy doesn't know how much his video has open my mind to automotive engineering....my biggest thanks to you mate

[@nate9479]

it's insane how much i've learnt through this channel

[@danielklopp7007]

Excellent description. I have a university degree in Physics and my senior thesis was titled, "Tuned Exhaust in Loop Scavenged Two-Stroke Engines". I got the best grade in my class because I did the theoretical work AND the practical application work (by cutting, rolling and welding exhausts of various shapes for a 2-stroke model aircraft engine). I even built a small dynamometer (using a small disc brake and a fishing spring scale) to measure the power from various shape pipes. The only thing I would add is that the varying speed of sound (depending on ambient pressure and temperature) does have an effect on the optimal shape of the pipe (when measuring peak power). I went to university in the northeastern part of the United States and did my senior thesis in the fall semester when the temperature in the early morning was around 0 C, and afternoon temperatures were over 20 C. For peak power, I built a "morning pipe" and an "afternoon pipe" 🤣

[@johngalt97]

The vapor pressure of humidity affects tuning, also.

[@danielklopp7007]

@@johngalt97 the vapor pressure of water (i.e. humidity) is a function of temperature and pressure (in other words, you restated what I had already pointed out).

[@Catatonic2789]

Very interesting! I love the bit about the morning and afternoon pipes. So it sounds like perhaps a very advanced future version of the 2-stroke combustion engine (a somewhat oxymoronic term) might feature a "variable geometry exhaust" that can physically alter its shape to respond to ambient conditions and provide peak power at a much wider range of RPMs. Sounds like something airplane people would get up to, it reminds me of how the inlets on SR71 do the same thing to automatically respond to speed and pressure changes throughout the flight to maintain efficient engine operation.

[@danielklopp7007]

@@Catatonic2789 - your suggestion (of variable geometry exhaust) has already been done. I believe it was Honda (and perhaps others) that made a motocross bike with an electronically controlled valve that altered the geometry of the expansion chamber (in addition to the electronically controlled variable exhaust port height). I'm an old guy and didn't have access to cheap stepper-motors when I got my university degree, but if they had been available, I would have tried building a variable geometry pipe back in the early 70's.

[@makantahi3731]

have you tested the oem exhaust with your . I deal with aircraft models with 2-stroke glow engines, 4ccm and I was thinking about a different exhaust with an exhaust valve that would limit the flow of exhaust gases as a function of efficiency: I noticed that with the oem exhaust the engine consumes a lot of fuel in all modes, especially at idle up to 50 % of power, so I put a 2-3 cm long pipe with a smaller diameter on the exhaust opening, and with that I significantly reduced fuel consumption at lower powers, but I also limited the max power by 20%, so it would be good to have a valve that would open at full throttle

[@ParkerAnderson-dc4ef]

20:24 bro really said "tried" as if he didn't just explain it in a way that made it seem like an incredibly simple concept. Every time I watch one of his videos I am amazed at how well he explains things and how he makes them all feel simple, he is truly a genius.

[@d4a]

Genius is a stretch 😁 but thank you, I appreciate the kind words

[@Donal-dw2we]

​@@d4a You are genuinely one of the smartest youtubers I watch, and I watch a lot of smart youtubers lol

[@michaelbrinks8089]

​@@d4a I have to disagree with you when you say it's the sound 🔊 waves that are creating the power increase. Because the tiny amount of moving air created by sound waves. Isn't creating any noticeable HP increase. It's actually the air pressure & hot gasses from the piston/cylinder acting like an air pump. To create air pressure with some of this air pressure being bounced/pushed back into the exhaust port. Think of a piece of paper near a loud 🔊 speaker. The paper only moves around a tiny bit from the sound waves. Due to the vibration/pulsing of the speaker creating a tiny amount of moving air 🆚 paper in front of an exhast pipe....Engine compression of 150 psi creates lots of air pressure & moving air out the exhaust.

[@TlalocTemporal]

​@@michaelbrinks8089-- That air pressure and moving air *is* sound. That's all sound is; pressure waves. These ones are even in the same range as human speech; between a hundred and a few thousand waves per second. The hp increase also isn't coming from the power behing the sound waves, it's coming from the extra fuel and air the sound waves help put in the cylinder. The sound doesn't fight engine compression either; it just makes the air-fuel mixture in the cylinder a bit denser before compression begins.

[@michaelbrinks8089]

@@TlalocTemporalSorry but wrong, an engine is like an air pump...An compression guage checks hoe much psi an engine is making. Even without fuel added, if you crank an engine over. You'll feel lots of air coming out the exhaust pipe. Which aren't sound 🔊 waves. It's compressed air made by the engine. When the piston is on the compression stroke, it's compressing the air . Then the compressed air gets sprayed out the exhaust port.

[@damianoferro]

Your clearest video yet. It's incredible how you're getting even better, whilst you already were the best.

[@aaxa101]

Sorry, the video is wrong and is mixing concepts.

[@adrunkenloner]

​@@aaxa101 care to expand on the wrong/mixed concepts?

[@THESLlCK]

@@adrunkenloner no, he's only here to be negative

[@YAMR1M]

@@THESLlCK AAXA101 is correct it is mixed and wrong. The visual example is not correct. The sound of the engine has NOTHING to do with he pipe design. His engine stuttering will happen on ANY 2 stroke at any RPM and is more about the throttle position and a simple fact of the port timing and the engine not being able to pull a full charge into the cylinder. The closed throttle chokes the engine down so the exhaust cannot do what it is designed to do At partial throttle the cylinder is getting fuel but the port timing is not going to allow the exhaust work fully and the exhaust design only works fully at wide open throttle as the specific RPM range it has been designed for. If the exhaust has been designed to work at 8000rpm or 12,000 rpm then it will not simple increase torque or power at lower RPM by opening the throttle. Yes power valves do help but this is port timing and NOT due to exhaust design.

[@adrianzmajla4844]

​@@YAMR1MSo why not a pipe same diameter? Why use an expansion pipe?

[@negativeindustrial]

I’ve been with you since the beginning of this channel and it is my opinion that this was your most effective video of the series. I’m not into motorcycles, or two strokes in particular, but I now feel qualified to pass this information on to some of my younger friends who are. Thank you for this.

[@d4a]

Thank you for your continued support, always happy to see you in the comments my friend.

[@ehtikhet]

Get a motorcycle! They’re bloody brilliant!

[@tomdenny8507]

I have understood expansion chamber exhaust pipes since learning about them by reading all the motorcycle magazines in the early 1970's. I have always thought of them as a "Tuned device", like a musical instrument. Never have I been able to explain it as well as you have. Foot note: there was a private racing team in the seventies running a Bridgestone 350cc twin two stroke with the cylinders turned around backwards, making the exhaust ports face towards the rear of the motorcycle. This caused their exhaust pipes to be too long and it wouldn't pass tech inspection. They cut off the stingers (the small pipe at the end of the exhaust), shoved them inside the expansion chambers and welded them in place. The result was no discernable loss of power but a greatly reduced noise output. As one of their competitors stated, "That thing doesn't make much noise but its fast as hell!" An entertaining anecdote from the 1970's for you.

[@danielklopp7007]

I did this exact same thing (i.e. cutting off the "stinger pipe" and re-welding it inside the convergent cone) to my TZ250 Yamaha... now that I'm an old man, my hearing thanks me for this insight I had as a young man!

[@PrinceAlhorian]

Would have loved to hear that bike sing. I fly model airplanes with two stroke nitro engines, for weight reasons the little glow engines don't have tune tubes on most models (those that do have this looooong expansion tube under the model), and a "quiet" two stroke twin sounds like something I've got to have on a model racer.

[@johngalt97]

Amplitude.

[@danielklopp7007]

@@PrinceAlhorian - As a physics major, I did my senior thesis on "Tuned Exhaust in Loop Scavenged Two-stroke Engines" (see my response elsewhere in these comments)...

[@michaelbrinks8089]

It's not the sound 🔊 waves that create the extra HP...It's actually the air pressure & hot gasses created by the engine that makes the hp increase. The expansion pipe is causing some of the air pressure & hot gasses to be bounced/reflected/pushed, back into the exhaust port & combustion chamber. The actual sound waves 🔊 plays no role or only a tiny unnoticeable role. When you change the stinger ID for example to a smaller or larger ID. You're increasing or decreasing the airflow & back pressure inside the expansion. Which is affecting the engine performance. Not the actual sound waves. Screaming into an expansion pipe doesn't do anything but blowing into it & creating air pressure does.

[@NeoAcario]

Since riding dirt bikes in the early 90s... I always wondered what the pipe shapes were for. Unfortunately the internet didn't really exist back then. 12yr old me thanks you for the info!

[@We-Do-NOT-Consent-303]

The explanation is in the name itself they are called resonator pipes orr resonator exhaust. It took me 5 minutes of thinking to figure it out how it works 40 years ago when I was a teenager. And I understood the same thing as in this video. I have had a 50cc bike with a resonator, that had about 11 horsepower at 13,500 RPM. Top speed 120 Km/h I love that bike.

[@jimmybob3756]

There were many text books written about the subject that were available in the 70's.

[@wingracer1614]

Back then we had things called book stores and libraries. Amazing what you could find buried in the back of an old book store. Even my high school library had a book on 2 stroke engines.

[@sntslilhlpr6601]

Jesus why is everyone giving this guy shit? When you're 12 years-old would you rather be riding or going to the library to look up every small curiosity you have? The internet makes answering questions like this a lot more convenient. Don't be a hater.

[@We-Do-NOT-Consent-303]

​@@sntslilhlpr6601who is giving him s*** I don't see any comment like that here😮 what are you talking about😅

[@tweed0929]

Simply the best channel on KZbin for engine mechanics.

[@Mcfreddo]

He is excellent. There is another guy who does fantastic engine videos. He's American. Not so busy at it recently I think. I will get his name later- can't remember it.

[@Mcfreddo]

Engineering explained. I so appreciate both!

[@KarrierBag]

What a brilliant explanation of how a 2 stroke works, I started my mechanics on them then 4 stroke bike and also car / truck engines and this has to be one of the best and easiest to follow tutorials on the subject I have seen.

[@squidcaps4308]

Extremely well explained. And to be fair, i never considered the expansion of the wave to create negative pressure wave but it makes perfect sense, there is a "void" that is left behind as the positive wave expands. They do use acoustic resonance with 4 stroke too, there the positive pressure wave is reflected back so that it hits the closed exhaust valve just before it opens, and is reflected back, and this creates a low pressure zone behind it that helps to suck exhaust gases out.

[@FedericoLucchi]

Yup! They also design the air intake to have its own resonance (the closing of the intake valve also creates a pressure wave)

[@Mis73rRand0m]

This is why you'll get specific "dead" chambers on either end of the engine nowadays; usually one or two plastic chambers on the intake side and one section of tube tee'd and capped on the exhaust. They often aren't for performance specifically, instead meant to reduce overall sound signature, but incedentally also increase efficiency because they disrupt aberrant reflected waves.

[@SoulTouchMusic93]

Yeah, same on intake. That's why those throttle trumpets require tuning. Plus on F1 cars and motorcycles you have a tiny hole to suck in air that expands later in the air box because slow air has more pressure.

[@squidcaps4308]

@@Mis73rRand0m That is fascinating. I'm sound engineer myself but also have worked in wind instrument repair shop.. There are so many things that overlap between sound and vehicles. Like, the equations that work to describe what suspension is doing has a ton of similarities with equations that are used to for ex to design speakers, and filters. Resonances, critical damping, hysteresis, saturation...

[@charlespascua9740]

It's fun how this channel is including motorcycles

[@ryalwyngarner3258]

I work on Sachs 2 strokes from late 1950's - the exhaust is so important... thank you for all this engineering detail

[@brodie.969]

you know you refreshed youtube at the perfect time when it says 22 seconds ago on a D4A video 🤝

[@joshuagibson2520]

Or you could just use channel subscriptions like a normal person and you'll be notified anytime there is a new video.

[@thevast9031]

@@joshuagibson2520🤡

[@zachkurth]

I have never heard someone explain these expansion pipes so succinctly and clearly. Thank you for making this video. I haven’t ridden a 2 stroke in a while, but maybe I need to get back on one.

[@d4a]

I highly recommend it 😁

[@graemewolfe1515]

@@d4a I second that 😁

[@autodidact537]

@@d4a Direct fuel injection like the Rotax ETEC 2-stroke engine that Skidoo has been using in their snowmobiles for quite some time now has solved the problem of the air-fuel mixture escaping from the open exhaust port.

[@Lina_Antoniou]

@@autodidact537 Rotax also makes karts if I am not mistaken?

[@sntslilhlpr6601]

@@Lina_Antoniou Yep, they're also pretty big in the aviation industry for ultralights and light-sport planes.

[@jcr6548]

i love the smell of two stroke in the morning! That sound takes me back to my youth. Long live the king!

[@JC-gw3yo]

Two strokes forever !!

[@cjpayne677]

I teach acoustics to High School students in basic sound engineering. I really appreciate how even though your channel focuses on motor enthusiast information, it's clearly obvious you have a love of all forms of engineering and physics. This video is a fantastic example of how a sound engineer can be brought into a motor development team and increase it's outcome, when gear heads would never have thought about asking a speaker designer to develop an exhaust.

[@raiyanislam3549]

you would be surprised seeing everytime sound engineers developed exhaust systems, always happen to make the most advanced ones

[@lvcifer-cloverfield]

I love fluid dynamics. What a wonderful video for me to enjoy with my sunday brunch

[@johnnyblue4799]

That was the hardest course in university. The math of it is absolutely horrible for my low IQ brain.

[@imnotusingmyrealname4566]

​@@johnnyblue4799Would be equally horrible for my ADHD brain except it's not as persistent :(

[@mickellis8747]

Great explanation, I built expansion chambers in the 70's for two strokes when they were unavailable for most bikes. The calculations were widely available at the time but fairly complex. The first chamber I made was for a mid 70's Suzuki TS185 formed from car panels by hand and oxy welded. It went like a rocket.

[@Handyman1199]

Any idea why the Exhaust on a Trabant only expands at the beginning but doesnt compress at the end? its a weird shape to explain, sorry. ill try to crudely draw it, hope it works on mobile too (here are the two cylinders) I I I I__________________________________________ I I __________________________ I I I________/ I I____________ I I \__________________________ I I__________________________ ______________I I I (to the mufflers)

[@roryhennessey1983]

Wow that's cool!. That was my first bike a 1974 TS185 I saw it in my neighbor's garage when I was mowing his lawn it needed a lot of help I totally rebuilt the whole bike in 1996. I always thought it needed a custom exhaust

[@msromike123]

@@Handyman1199 My guess is the muffler portion at the end is acting like the tapering end at the rear (without seeing it). It would be more efficient to have the classical shape with a silencer on the end, but I think you could get a weight reduction and overall shorter length with a design that uses the muffler/silencer to help reflect sound waves back (and still get a large portion of the efficiency gains required.) My two cents.

[@BarryJTaylor]

​@@Handyman1199 you need an individual expansion chamber for each cylinder on a multi-cylinder engine.

[@Handyman1199]

@@BarryJTaylor Thats what they do on Trabants when pushing the 600cc to 70ish HP, use two giant individual resonance chambers instead of the one inside the "pre-muffler". The original one is apperantly designed to increase low end toque, whilst also supplying heat to the cabin heater via a second shell (both the header and the pre-muffler/resonance chamber have a second shell, the engine fan blows air into them. the parts heat the air before pushing it into a big silencer, from where it enters the cabin)

[@80s_k1d4]

I am in my last 4 months of a 3½ year apprenticeship as a car mechanic. During this time, the videos from this series have been the best explanation for engine principles. Fast superior to school classes. Thank you very much for this series! Keep on!

[@chonglers1513]

This is so crazy! I randomly thought of 2 stroke bikes and have been watching a few videos, and then I log into my account to see this uploaded 13 min ago! The timing couldnt be anymore perfect friend. And yes, I was thinking why their pipes were that way

[@chonglers1513]

I finally got it after the second watch. Ive always wondered what the point was of its shape, yet have never looked for an explination. Im glad it was you that taught me the reasoning behind this clever piece of engineering🙏

[@joshpickles9022]

Dude. 1. That is genius 2. You are a fantastic teacher

[@KenLirette]

This video answered a question about 2-stroke bikes I've held in my brain for more than 50 years. Fascinating! Thank you.

[@rileymerson8781]

Just finished the whole video. The examples you use are phenomenal. You’re truly a wonderful teacher and one of my favorite creators on this platform! Always a good day when D4A uploads!!

[@TheSamler-31]

I already knew why the exhaust pipe of a two-stroke has this shape to maximize power, and I thought I didn't need to watch this video. I watched it anyway and realized I was wrong. I understood much better how it works and learned new things. I am still wondering why the shape changes so much according to the engine's CC, especially from a 125 to a 250/300. I loved the video and the explanation with examples was really good.

[@ericcsuf]

I used to desert race two strokes almost 50 years ago. I sort of understood how they worked and knew and experienced "riding on the pipe", but I didn't really understand it until this video! Absolutely the clearest explanation of how two strokes work and especially how expansion chambers actually work.

[@leftyeh6495]

Most people talking about riding "on the pipe" are actually referring to the power valve opening. These were generally controlled by a simple governor to open with rpm. The newest tech is not only electronic fuel injection but also electronic power valves to essentially vary the exhaust size over the rpm range. So now you can smooth the power out along with boosting it. The dirt bike world has stock 300s hitting the mid 60hp range where the old 500s left off. In the snowmobile world you have 850s and 900s hitting the 180-190hp range. And factory turbocharged 2t's hitting 200+hp. 😮

[@ashleyobrien4937]

I used to ride a 1992 Suzuki RGV-250 N with the factory hop up kit installed. The power band was like Heroin and cocaine totally addictive and a whole lot of fun. One guy I knew had his putting out close to 90 H.P. but track use only of course. The bike weighed less than 124 kg and was highly modded. Upgraded power valves, reeds, cylinder head, CDI, power control unit, suspension, expansion pipes, jetting, airbox, polished and slightly widened transfer ports, virtually every part that made the bike perform and handle was tweaked, I was lucky enough to find a guy who sold me the parts for cheap. The RGV-250 N was a damn fine bike, handling was psychopathic, the bike was so agile corners taken at crazy speed and the suspension was brilliant, the brakes Brembo with Kevlar pads felt like they could stop a train with only two fingers on the lever, great for stoppies and it would do power wheelies coming out of corners with just a little slip on the clutch and up she'd come weeee!..spent many weekends going up and down the Takaka hill terrorizing other road users....

[@chrisdaley9691]

Absolutely incredible video, I rode 2 stroke for many years back in the 90's and 2000's and always understood that the pipe and shape of it helped with power, but never understood why. This video explained it all so well, and not only that, the whole lesson on sound waves was incredibly fascinating too. I'll be watching a lot more of your videos going forward. Thank you!

[@Finite-Tuning]

I'm glad you literally spelled out "impedance" all over the place, because I swear what you were telling me verbally is that my exhaust couldn't get it up! Regardless, this is one of the best explanations I have ever seen or heard. Much respect! Cheers 🍻

[@thekinginyellow1744]

Traditionally in English first "e" is long, but I ain't gonna give the guy any crap for pronunciation since it's pretty clear that English ain't his mother tongue.

[@bowieinc]

Awesome! Consider adding “How Power Bands work” to title! I’m in 45 years of getting it wrong! This was top 10 best KZbin videos I’ve ever seen.

[@_s0prok]

I just started worked as a motorbike mechanic, one of my colleague keep telling me that I'm stupid because I only have little knowledge about motorcycle and doesn't have any formal education about automotive. Thanks to you and KZbin for automotive knowledge.

[@imnotusingmyrealname4566]

That helmet cam footage was so fun to watch. I can't imagine how fun riding a motorcycle must be.

[@oldschoolmotorsickle]

Yep, they are incredible fun.

[@classicreaction5340]

Being into snowmobiling in my youth I always knew that the exhaust pipes affected the power band but I didn't really understand the exact science behind it. This video is an absolutely perfect explanation of the entire process. You've got my subscription!

[@SavageShooter93]

I am so happy to see this channel thriving, mostly because its great information presented extremely well and it helps fund a sweet MR2 project. But I also get to say "I was here when he only had a few hundred subscribers!" and everyone enjoys the smug satisfaction of knowing about something great before everyone else.

[@igorhcc]

Cheers from Brazil! I'm 31 years old and always loved motorcycles. I've always been curious about 2 stroke engines (since I have several bikes, including a 1989 Yamaha DT180Z which I restored) and in my early days of internet those exhaust shapes was one of my first thins that I searched about on wikipedia, I remember using a printed English to Portuguese dictionary to translate the words... Now there's so much good content around, and you're part of that! As a mechanic engineer I think how much more information teenagers from this era can use to understand the world and machine around. You guys are really heros, keep it up with the good content!

[@benswilley7851]

One of the best videos I’ve watched in recent memory. Taught me something I’ve always wondered about, gave me terminology and vocabulary I never had, then made me laugh with the goggles falling through the helmet like I’ve done a thousand times. All with top notch visualizations to help it all be much more consumable. Thank you again for making this content and being bold enough to share it with the world.

[@jacobormsby9241]

this is precisely why different exhaust pipes can make a huge difference in the behavior of a 2 stroke engine. For example, both my wife and I ride Banshees ( twin cylinder 2 stroke quads). I run a more drag style pipe and my wife has a mid range pipe. My Banshee revs about 1500-2000 rpms higher than hers and has much more top end power while hers has less top end, the power band comes in sooner and has way more low end torque than mine. My power band is like a light switch. All of these differences despite having almost identical engine builds. 2 strokes are truly amazing pieces of engineering.

[@rightleftmiddle1020]

Fantastic video, and also the first genuinely well made explanation of two stroke exhausts I've yet seen on KZbin. I have been trying to learn more about the mechanics of two stroke engines for the better part of a year, and while there are many detailed videos most of them are borderline unwatchable due to scripting and production. I would love to see more content covering other two stroke details, especially stuff like porting, ignition mapping, and power valves. While the information on performance two stroke engines does exist online, it's not very accessible and I want to see someone change that.

[@artisanstyling]

Two strokes are truly a marvel of engineering. The fact that they are spewing during idle, as if begging to be brought to speed, is simply poetic. I've been a car guy all my life, but man what I wouldn't do for a project tzr125 right now...

[@WilliamSudek]

Riding 4 Answers once again knocks it out of the park. All that remains is to get you riding a desmo and the world will be perfect.))) R4A has always been one of my favorite channels... Even when it was D4A.)))

[@SamThullFrottle]

I have been a mechanic for over a decade. When multiple teachers explained two stroke pipes and their effect on the movement of exhaust gases, I always struggled with visualisation. Turns out they weren't quite explaining it correctly, as you've demonstrated it is sound waves and not the gas itself moving about that produces efficiency in a powerband. Well done mate, I often refer your videos to friends wanting to understand more. 👍

[@kiefershanks4172]

Imagine being this brilliant. That is freaking cool. Expansion chambers also look super cool.

[@LeonardoEngineering]

I teach some foundations of musical acoustics at the university level and I really appreciate this video. By far it is one of the best from your channel and I will suggest it to students who have an interest in engines!

[@igoradsilva]

I knew that 2-stroke engines used sound waves to keep more mixture inside the cylinder, but your explanation made it crystal clear how it happens! Now, let's make a video about YPVS???

[@moskavado]

Variable exhaust geometri Aka 2 stroke V - TEC 😂 In yamahas case a exhaust valve goes up or down sligt before piston tdc the Valve is Close so it can have very high port timing without loss bottom end. Exhaust valved 2 strokes powerband starts off idle and goes liniaer to max rpm with no elastics effects /turbo/on the pipe /boost mode

[@PSA78]

It's just a variable valve to change exhaust duration (and blow down timing). An easy way of getting a wider power band on things like motorcycles, for other purposes they might work on pipe temperature to help widen the power band.

[@Stopcolonizinglebanon]

50 years after hearing about this. I now understand the principle. Thank you.

[@imnotusingmyrealname4566]

If you can explain a thing well you understand that thing very well. This guy understands engines very well.

[@palco22]

I'm not an automotive mechanic but I believe I am somewhat knowledgeable, just the same, ... a back yard mechanic if you will but with your methods of explanation I go to sleep at night with a heck of lot more understanding of 'mechanics' ! Great video, as always. ... again, thanks.

[@annjacobs7764]

It's obvious how much work went into this!

[@JK061996]

Unlike this botted comment

[@SkunkPresant]

I have two two strokes (RD350LC and RD400). I spent quite a lot of time building chambers for the RD400 and trying to understand what was going on. I had the basics but you have brought it to a new level. Thank you.

[@mariansell]

Great video!! As a 2-stroke guy from Germany, i'd like to add something: we call these exhausts "Resonance exhausts", because like a musical instrument, they work best at specific frequencies, in our case RPMs. If you're using a long exhaust, the pressure wave takes more time to travel back and forth, which means that your "resonance" happens at lower RPMs, and shorter exhausts go into resonance at higher RPMs. Some 2 stroke guys wrap the first 1/3 of their exhausts with exhaust wrap to keep the gases inside the exhaust hotter, which let's the pressure wave travel at faster speeds, so the resonance happens at higher RPMs and as a result: more horsepower "the same amount of force as before, but it happens more often". It would've been great to mention outlet valves, that extend or shorten the blowdown time, depending on the RPM we're currently using.

[@robert-janvanhulst3996]

It's a shame tat only only 1 thumps up can be given This was spot on and my thump up is worth a thousands Keep up the output I wish I had you as a teacher. To late I am catching up

[@scottharris6411]

I’ve learned more about two stroke engine function and how they do not waste as much fuel as one would expect, and why the funny shaped shaped exhaust is intentional, in this one video compared to all the others I’ve seen. Well spent 21+ minutes! Your videos are very informative and educational- keep up the excellent work!!

[@tonyvince7]

new D4A video dropped... I click

[@Pons-GG]

When I was a kid, my friends and I who had dirt bikes always thought the two-stroke "powerband" was a physical component located somewhere in the engine that would kick in when you increased the throttle lol. Later learned it was wrong, but never truly understood it until seeing this video. Great explanation!

[@bazzingabomb]

The easiest way to think of a 2 stroke expansion chamber is that it's a non mechanical turbo, you can literally double your horse power when you add one of these exhausts.

[@Mis73rRand0m]

I think of it as a self-scavanged exhaust; instead of using other cylinders' gasses to create a pressure differential across the exhaust port, it uses clever resonance to help itself out.

[@bazzingabomb]

@@Mis73rRand0m Yup it very much is a scavenger system, and there is a very cool story behind it that sounds like a Hollywood movie. The inventor was a russian rocket scientist who defected during a grand Prix race.

[@danielklopp7007]

@@bazzingabomb Actually the man was East German, Walter Kaaden - an engineer working for MZ motorcycle company. And Walter didn't defect, his rider (Ernst Degner) defected and brought the technology to Suzuki.

[@bazzingabomb]

@@danielklopp7007 you are right, I said russian but meant soviet as east Germany was part of the iron curtain back then. I also new about mz and and the suzuki connection but not many people outside of Europe know about mz. But thanks for clearing up my mistake.

[@gorkzop]

Wanna buy an old dkw Rt200. Buy doubt I can double it's HP with just an pipe 🤣

[@grantm1891]

Great explanation. I remember being blown away watching a tether car with a 2 stroke engine that had an expansion chamber several times the size of the engine. It took a while to get rpm’s fast enough, but once it did the increase in speed really opened my eyes. Thanks for expanding my previously crude understanding.

[@karabenomar]

This isn't just a stroke of genius, it's a two-stroke.

[@jimsesam6797]

You are so good at explaining that even as a little English speaker, I understand everything - linguistically and in terms of content. I also feel your passion. I wish I had several teachers like you (I may not have been easy to teach at school, I think differently). I love your videos, thank you!

[@Onewheelordeal]

Funny how hobbies work. I was really into 2 strokes as a kid and studying building exhaust eventually lead me into speaker design and building

[@snarfarpher2416]

I have searched for an informed, properly explained answer, supported by factual information, for over 3 decades! Finally!! Finally at 44 years young I get that answer. I love your channel my friend. Educated, accurate, humble, clear and calmly explained with great illustrations. You provide an invaluable resource, my friend. Know that you are appreciated. Know this work is important and impactful. Thank you.

[@BandaMapper]

When you try to make more horsepower to beat your neighbor’s bike and know you are in the basement watching videos about longitudinal waves

[@der_da7982]

I'm doing my apprenticeship as a mechanic at Mercedes in germany right now and it's always so funny when I'm in school and know about what feels like 90% of the stuff already because I watch every video of you and do so much work on my cars ^^ (I bought a aw11 with a friend wich we are fixing up a few years back now because we saw your ITB video and that started it all ^^)

[@okagedesu]

Let's have a minute of silence for every backyard mechanic that have ever just put a tube as exhaust, and completely ruined their bike, without really knowing why.

[@shawnomack3927]

Your example using the water in the dish was an excellent way of visualizing this effect. Very easy to understand.

[@zogzoogler]

Ernst Degner & Walter Kaaden we tip our hats.

[@AUMINER1]

I'm impressed that you could ride down a dirt trail, at increasing speed, and clearly explain the vac and pressure cycles.. :) excellent job!

[@fredrich1706]

I’m a retired(72 years old)meat cutter working in a motorcycle shop as an assembler. I had asked my coworkers about the two stroke exhaust pipe. The answer they gave wasn’t as clear or concise enough for me to understand the benefits of the expansion chamber. Thanks for your wonderful video.

[@johnnywishbone831]

Thought I knew how 2T worked. Nope..... Thanks man. You took a very complicated subject and made it digestible to the average person. 10.5/10 Edit: Oh and 4T rules lol !!

[@lyricschannel842]

14:41 manual breathing mode: on

[@andrijasaviccsavic1124]

Insane how simple and understadable this video is. So logical, and since I ride 50cc 2t, you cleared a lot of things for me

[@Sir_Cactus]

13:26 I might be mistaken, as I am not an accustics expert, yet I believe all that was said about the accustic impedance is backwards. A medium with it's molecules closer together transmits waves better, not worse, than a medium, that has its molecules further apart. Furthermore, the pressure should increase in the expansion chamber, not decrease, due to Venturi's principle (or Bernoulli's principle, always mix em up). Gas pressure always increases in a diffusor (unless the medium is tranvelling at supersonic velocities). In other words, you made two mistakes that canceled eachother out, however, the explanation is still wrong

[@randomg0at]

I'm no specialist, either, but I think it's Bernoulli: Along a streamline the pressure + kinetic energy + gravitational potential energy is constant. So, as the gas enters the expanding section and slows, the pressure increases.

[@PSA78]

Yes the pressure increases with expansion, while velocity goes down. Good pressure recovery is key with engine tuning (from airbox to tailpipe on both 2 & 4 strokes), too much of a CSA change and you've wasted valuable energy by turning it into turbulence. 😄

[@enordquist1]

12:47 Another excellent video by D4A! I'm intrigued by the negative pressure wave initial reflection. It also makes me wonder about some multi-cylinder engines that use a single expansion chamber. Finally, in accordance with your 6-stroke video, your video, your pronunciation! However, it may be worth noting the (US) English pronunciation of impedance. We say im-peed-ance, rather than imp-uh-dence. Compare to pronunciation of impotence, which we pronounce imp-uh-tence). Great work! Looking forward to more.

[@laius6047]

just to clarify. if you didn't get the idea after this video, there's no hope for you

[@zaydabbas1609]

What an amazing description! I work with radio frequency electronics and the concept of “acoustic impedance” is one I’ve never heard before. It really lets you look at the entire exhaust as a transmission line.

[@saybrowt]

This is fking sick. Finally knowing why a 2 stroke has a powerband.

[@simplexicated]

This powerband is why two-strokes, especially bigger ones in the hands of less experienced 4 stroke riders can be quite challenging. The power increases from minimum additional throttle can be quite unexpected for the unestablished. Great video!

[@Simoxs7]

Can confirm on my first bike (which definitely had 125ccm) it felt like at 6000 RPM a turbo kicked in because you suddenly got a lot of torque. Also I‘m very envious you can go offroading, here in Germany its unfortunately illegal to ride anywhere offroad apart from the small closed off MX tracks

[@wingracer1614]

There are no trails to ride in Germany? That would suck. Trail riding is a great recreational activity.

[@alxazar]

This is probably the best explanation of waves you'll find in a video for motorheads. Respect.

[@Beni10PT]

4:20 we do not have valves, but what if we did?

[@jeredek9901]

We would have a 4-stroke lol

[@RealMrRobinson]

I was thinking about this when I clicked on the video it's two stroke with valves to increase efficiency

[@franciscodieguez5187]

For sure the best, clearest expanation to this date. Heard a lot of different people explaining and never quite got it 100%. Thank you very much!!

[@oc2phish07]

I did already know how two-stroke exhaust systems worked but I have never seen such a clear explanation before. I will be encouraging my students to watch your video and can then, hopefully, answer any questions they ask afterwards. Thanks for such a well made and presented video.

[@LyvTube]

It's extraordinary how your way to explain things that i've always wanted to understand is so crystal clear and baby-level-friendly 👶 I always love to watch your videos because it's so gratifying to finally know how things works after wondering it for so long. Thank you so much 🙏

[@garrysimmons111]

Way back in 1976, I, as a college-bound high school student, bought a book on 2-stroke exhaust design. That inspired me to pursue mechanical engineering because I wanted to design "go fast"" parts for dirt bikes. I ended up having a career as a software engineer for engineering companies, but that bit of motorhead engineer is still there (why I watch this channel)...

[@NateDog_WDE]

What incredible timing for you to release this video. I just recently went down the rabbit hole of learning about exhaust scavenging and back pressure and why long tube headers add so much power on v8 engines. Its crazy to think that something so detrimental to making power on a 4 stroke engine can essentially hsve the opposite effect on a 2 stroke motor. Great video and great explanation! You honestly made the principles of scavenging make way more sence to me as well!

[@rob5944]

This has to be one of your slickest vieos yet, and as ever, extremely informative. Congratulations for that. Not being particularly interested in bikes I didn't think I'd be so interested, but it answers one of those questions what one never gets around to researching. Why do 2 strokes cough and splutter. Actually the trials do look fun. Great work!

[@Vtwin_Superbikes]

I know all this but still watch anyway. Shows that your channel and explanations are worth listening too.

[@mxguy31]

Awesome explanation, your skill as an online teacher is improving greatly. Best explanation I have heard yet, I remember back when FMF started making the 'fatty' pipes that looked like someone beat the crap out of them from the inside, all kinds of bulges and odd bends. The FMF reps had a hard time explaining why it was more than just a weird look.

[@jasonneugebauer5310]

Very comprehensive visual explanation of how a two-stroke engine functions efficiently. This is the best explanation I have encountered. Thank you.

[@artysanmobile]

In my work as music producer, I became so fascinated with the science of acoustics I ended up designing loudspeakers and studios. The compression and rarefaction of the media - in my case, air - are the key elements in the creation of acoustic spaces and can perform truly surprising work. Every acoustic instrument is designed to leverage the compression and rarefaction of the air in which it is played. At lower frequencies, it is quite easy to imagine what is happening because it can be seen directly, and by moving the ear’s perspective to that clearly visual movement, it is possible to find a microphone position to pick up many different and desirable sounds. Mentally connecting these visual and sonic phenomena is a very powerful tool for the sound engineer.

[@walter6574]

Hey there, you have a remarkable talent for taking complex mechanical topics and breaking them down in a simple, easy-to-understand way. As a high school Electronics teacher, I've always prided myself on delivering clear lessons, but I have to admit, you've surpassed my skills when it comes to making challenging concepts accessible. Kudos to you! Over the years, I've attracted many high-achieving students, and I remember quite a few coming to me for help with Grade 12 Physics, particularly on how to simplify a complex resistor circuit into a single equivalent resistor. I would demonstrate this using diagrams, gradually replacing parallel and series resistors with their equivalent until the entire circuit was reduced to one representative resistor. I’m sure you’re familiar with this process, but I’m explaining it for any readers who might not be. I noticed that the Physics teacher from Lebanon would also used this approach, but his students struggled to grasp the concept. What was different? The key difference was in pronunciation. The teacher pronounced equivalent "e-qui-vah-lent" as "e qui-vay-lent," similar to how "valent" is pronounced in "valence electron." English, as we know, can be a confusing language for non-native speakers. Although English isn't my first language either, I attended a school where all the professors were native North American English speakers, so I learned to mimic the North American pronunciation. Why does this matter? Like that Physics teacher, you and I are non-native English speakers and can sometimes mispronounce words if we haven’t heard the correct pronunciation. This can make it difficult for listeners to fully understand what’s being taught. For instance, the word "impedance," which means to impede or restrict, is pronounced "im-pee-dence," not "im-pah-dance." Mispronunciations can unintentionally confuse students. That said, it's worth noting that pronunciations can vary across English-speaking countries. For example, "aluminum" is pronounced differently in the UK. So, this may not always apply universally. I want to emphasize that I have immense respect for your skills in sharing knowledge, and I mean no disrespect at all. It's often the small details that separate a superstar from a good professional-those little things can make all the difference. Peace bro

[@KevinJones-r3g]

I didn't know I needed to understand this. I was so impressed by the presentation that I kept watching. To explain a concept so concisely requires expert skill. Well done Professor.

[@tomop5603]

This is easiest example to why your one of my top youtubers for a number of years, clear and informative examples to show how engineering works, keep up the great work matey 🇦🇺

[@edwardneal4819]

I can learn more from watching 20 minutes of your content than I can from watching an hour and a half video from Veritasium (sorry Veritasium). But I'll still keep watching both. 🙂

[@donh8833]

During WW2 they directed the exhaust backwards on the spitfires, mustangs and corsairs. The engines exhaust was so powerful it actually added thrust to the aircraft. You just needed to lower the pressure close to atmospheric as possible to avoid jet wash thrust losses.

[@charliebailey2359]

I thought I totally understood two stroke pipes after studying them quite a bit. But now I understand them to a far deeper degree ! I race snowmobiles, jetting, pipe temp, pipe shape, elevation and air density mean everything.

[@MichaelBraid-xf3dw]

Yamaha IT465 I had never really had a power band It was power from just open throttle to to max RPM where the power delivery did fall off rapidly gone out of resonance Altering the exhaust chamber makes a huge difference to performance. Excellent video of explanation.

[@johnculbreth4543]

Over fifty years of reading explanations of two stroke pipes, this is the most understandable!

[@coozeem]

I knew the expansion chamber played a part to some extent, but the explanation and visuals really solidified my assumptions and primitive understanding of how the pipe really works. This is the video i didnt know i needed on a random October Sunday morning. Thanks!