2 stroke has almost nothing to do with automative tho 😅 not alot of 2 stroke cars

A lot of people think that based on the tons of dumb comments on our exhaust videos!

lord yes

I got friends thst can not grasp that an engine is just an air pump and they want to breathe.

@@AtomicReverend There are still people out there who believe the "kadency" theory of exhaust flow. This was disproven many, many years ago.

​@@motoiq they are to ignorant to realize the effect is "exhaust pulse scavenging" the exhaust pulses traveling through the pipe pulls th next pulse behind it.

Was it Toomi in Lawndale I think? He used to make pipes for factory teams. Old school cut and try. EC Burt too.

@@motoiq Name drop. For pipes, it was Paul Turner (sp) and for shocks it was Mike McAndrews. A search using expansion pipes, and shocks with each respective name will reveal. We all lived in a tiny beach town called La Selva Beach, CA Be good Stan, I love your stuff! My freshly rebuilt Honda A20A3 will roar again soon with an MFactory diff. and I got the last bearing and synchro kit in the USA for that trans!! Good old machine guy from Portland, OR did the work. He is retired now and doing a limited amount of work, because he just can't quit! I am a very lucky man! Who knows? Maybe one day I will send you an EJ25! It still runs great in my 2003 Forester with 218,000 miles!

So you are the one that got the last parts! People been talking about you.

Is that the Mike McAndrews from mountain bike stuff? He was the first guys to do digressive damping on bikes. I designed the Elsworth ICT suspension. ellsworthbikes.com/

@@motoiq YES! I remember him toying with shocks when I was a "kid". He and Paul were heavy into motocross. You mean Ellsworth stole your design!

This is an exhaust, intake resonance tuning is something else.

​@@motoiq Please do a video on intake resonance tuning 🙏

"sound" is in fact the human interpretation of pressure :)

@@davidsandberg3109 don’t you mean vibration not pressure

@@louisedayao1562 No just think of how a speaker works, like a subwoofer. The sound is made from the speaker cone moving in and out, together with the speaker enclosure (box) it changes the pressure inside the enclosure. If you have ports on the box the pressure is forced out the holes and it gives the low bass more punch. Its all to do with air pressure.

Has a chill tone in his voice. Can’t imagine him being angry. Lol

Backpressure in the sense of mufflers isn't good for it.

Dynamic > static.

The angle, length, and exit geometry are all very important, do some research, I didn't make this stuff up.

Some two strokes have used superchargers to blow out of the crankcase, experimental low overlap ones for low emissions and diesel two strokes. Expansion chambers don't suck stuff out they push stuff in to prevent overscavanging.

​@@motoiq The Detroit Diesel already proved that a 2 stroke can have an oil sump, but they banned it under the claim that it was burning too much oil, which is too bad, because the only real problem with the 2 stroke is the needle bearings, the linkage needs proper lubrication, scavenging is no longer a problem thanks to injection and resonance tuning. Expansion chambers do indeed suck, they pull so hard on expansion that vacuum is generated all the way through the air filter, increasing intake after the pressure in the crankcase is spent, all before the back compression wave. But it cannot accomplish the scavenging alone. So if the expansion chamber was in a partial vacuum then could it, would have been the question.

Diffusers like the first cone of the expansion chamber are considered steady-flow devices that increase the pressure of the exhaust gas by reducing their kinetic energy or in other words reducing the fluid moving velocity.

That's a lot of how the pipe works

The average flow is not pressurized, it is in vacuum with every cycle. The reed valve, etc does not block the exhaust, it prevents back flow into the intake.

It controls the back pulse and a properly designed exit is done to maximize the strength of the reflected wave and minimize the mean backpressure. That is the function of a stinger or nozzle style exits.

⁠​⁠@@motoiq The amplitude of the return “pressure” (there’s that word again) wave is dictated by the angle of the conical baffle. The return pressure wave occurs long before the end of the conical baffle.

The length of the end cone, diffuser and belly and the overall volume of the pipe are more critical than the angle for tuning the pipe, the angle is just a function of the other dimensions.

​@@motoiq I'm sorry but you're just plain wrong here, see Gordon Blairs SAE work iirc on two stroke expansion chambers for example. A certain gas density and pressure in the pipe is important, if lower was simply better we'd all just use multiple stingers from the mid section or a perforated mid section to bleed everything off as freely as possible. But that won't work well, because contrary to four strokes a certain amount of pressure in the pipe is necessary for proper function, you actually want even more with high performance engines tuned for a narrow peak power band than with general use engines as a rule of thumb. Besides increasing the strength of the pulse there's also an important "plugging" effect and a lot of sht going on even major manufacturers weren't easily able to wrap their engineers heads around. Everybody who has designed and tested pipes knows that after a certain outlet diameter there's losses. Substantial losses at that. And it's definitely not due to the Stinger somehow interfering with the pulse generation by the baffle, exhausts with mid section stinger for packaging reasons and "suspended baffle" designs that bleed through a small annular gap suffer from the exact same effect and constructions with different baffle arrangements than cone-cone do so as well. All in all the fluid dynamics and sht in a two stroke are orders of magnitude more complicated and intricate than people usually think. But it's absolutely certain that lower pressure doesn't just equal more power. Therefore - in two strokes backpressure is indeed important.

​@@heinzhaupthaar5590motoiq was on about volume and saying the angles don't matter?? Ive never heard any renowned 2 stroke/expansion chamber expert mention volume. You calculate what size of pipe you need based on powerband average temp yata yata, volume is just the outcome i don't think theres any inference to come from it at all. volume is shapeless, And the principles depend exactly on shape and geometry.

Which is a pressure wave that pushes the mixture BACK into the cylinder. ALSO CALLED BACKPRESSURE. This guy ain't a 2 stroker, he should stick to 4's until he actually knows his stuff. He's got no excuse other than lack of actual experience or interest.

the stingers function is to prevent the pulse from escaping while letting exhaust gas out.

They are really strong!

Actually via Bernoulli's principal the pressure increases in a cone shaped diffuser. Also we are taking about mean backpressure, not periodic pressure pulses are we explained simply in the video. Mean pressure raises the floor of the gas exchange portion of the two stroke cycle which reduces the amount of work that can be recovered as power.

@@motoiq i see. I had it backwards. I was thinking without bernoulli's principle which would mean that mixture would be sucked from the tranfers due to the low pressure at the diffuser and then the wave reflected back squeesed going into the baffle would push leaking mixture and leftover mixture back in but it must be the opposite. Diffuser causes high pressure which prevents leakage and then the baffle causes a lower pressure to pull mixture from the crankcase. If thats not it then im having a word with god and his mad fluid dynamics he created with expansion chambers.

You got it half right! The cone diffuser thing is counterintuitive but you are trading velocity for pressure which is what Bernoulli's principle is all about.

It depends on a lot of things but generally you want the uninterrupted stinger to be 6XD

For shorts, you only have 60 seconds.

@@motoiq true that

Have had personal experience with 50cc two-stroke engines and a pipe makes a big difference

Yeah you can probably double the power.

@@motoiq way past double the power

They change the port timing for a wider powerband, like variable cam timing in a 4-stroke.

​@@motoiq Some of them also change expansion chamber volume and tuned length.

really what motors?

@@motoiq I know Honda's power valve and Kawasaki KIPS do it. They have a chamber by the exhaust port that is either opened or closed to exhaust flow, effectively varying the tuned length of the header pipe. Honda's earliest PV did this only with no change to port timing or geometry. I believe KIPS was the first to combine the chamber system with a variable-height port. Obviously they weren't doing anything as radical as tromboning the pipe, although I've read some PWCs inject water into the pipe to change temperature and therefore the pulse timing.

it does but we are talking about mean backpressure affecting the floor and energy recovery potential of the PV curve.

I am not even talking about pipe design theory, you are, I am talking about mean back pressure. You understand mean means average right? I am talking about VE, thermodynamics and gas exchange of the engie being hampered my excessive mean backpressure, not debating the importance of pulse timing, etc. I am talking about the entire engine, you are taking about just the pipe.

@@motoiq Wow what a disconnect, your entire short titled “2-strokes don’t need backpressure” was about the pipe and pulses within it and now to avoid admitting your title was inaccurate as I said, when confronted with evidence from a source you cited, you deflect to VE? And FYI the mean pressure in the pipe is positive thus creating back pressure. I’d be happy to show you pressure traces I’ll be at Rok Fest in Vegas next week look for me in the scrutineering tent I’ll be the on wearing a shirt that say’s “Official”

I am not deflecting, I am just stating facts. What I said is accurate. Instead of trying to argue with me, your time would be better spent studying how the rest of the engine works. Look up the pressure-volume diagram for a two-stroke engine and you can easily see that if you raise the pressure base, less work or power can be recovered from each cycle. That is just one of the reasons but it's the simplest and easiest to visualize and there should be many examples for you to look at online. You understand how the chambers work 100% but are not looking at the entire engine. I didn't make this stuff up, every second-year engineering student learns this and it was well-understood over 100 years ago. If you look at the pressure profile of a chamber, you can see that pressure goes negative in significant parts of the cycle, especially in the diffuser area of the pipe. You probably know this. If you squeeze down the pipe exit to the point where you are intentionally increasing mean pressure, the timing of the pipe becomes faster due to the increased density and temperature of the gas inside and sometimes this gives favorable results. It's more of a tuning Band-Aid as a pipe with the desired tuning with lower mean pressure is going to be better for power recovery and production. Like I said, it's not just me making something up but well well-understood phenomenon that I learned in school and applied later in life.

That is not how it works although heat has an effect on pipe tuning.

@@motoiq *lol* I DO know how it works, but i seriously doubt you know. I´ve been tuning twostrokes for about 35years now, and if i would go by your example and NOT having backpressure the pipe would be very very hard to tune at correct rpm as the heat would be lost. The 'same' pipe length would tune at very low rpm. And by that, trying to tune at say 7500rpm(250 dirbike) it would be so short that you would need to have such abrubt angels on the different sections that you would actually make it EVEN HARDER to tune it due to the pulse is 'getting lost' 0.2-0.3bar pressure is a normal value to aim on, this allows the pulses to fluctuate above and below atmospheric pressure, and by this makes the engine breathe properly due to pulses. And getting this correct, the pressurelevel actually helps at higher rpm(above tuned point) to push back some of the fresh charge for a longer duration where he pulses have started to fade(above tuned rpm) Hence many misunderstands this. 'Higher backpressure gives more power' Yes it might, but less torque down low. Correct backpressure is the key!

You don't understand thermodynamics and have only a practical knowledge of how a chamber works. Purposely increasing the mean backpressure to do something is a crutch to proper pipe/port/intake design.

@@motoiq do not judge me! I know a lot but might have problems with language. But i do see that you are trying to use conquer technique in this conversation.

Don't judge me either, I know a lot.

if it needs back pressure why can it run without an exhaust

it will run but it will loose as much as 50% of the power. You need to watch and comprehend the video.

It needs an expansion chamber. The muffler will hurt it though.

@@fish_R_stinky69 muffler wont hurt it

@@11sjv It'll make less power with a muffler lol.

The pipe is homolgated under the rules with it so that would be illegal for the class it runs in so I would never test it that way.

You need the diffusion to help get the stuff out of the cylinder.

@@motoiq gotcha.

You can have the stinger go into the pipe chamber. That way you can have a larger stinger for less backpressure but maximum reflection.

​@@motoiq I read a very interesting old article where they inverted the stinger (put it entirely inside the chamber). They noticed almost no change in performance, but it made the engine as quiet as a typical MX style silencer. I believe this didn't catch on because cleaning the inside of the pipe became extremely difficult, and it looked doofy, and silencers were required most places anyway.

I can believe that. I know stingers have been put inside pipes to shorten the overall length for packaging reasons.

No

@@motoiq so the expansion chamber doesn’t use PRESSURE to push the mixture BACK into the cylinder?

That is not backpressure in the sense that we are talking about, we are talking about mean pressure.

It doesnt help combustion other than the reasons of preventing overscavanging during overlap

No direct intrinsic increase to cylinder pressure, but a proper exhaust-scavenging technique will effectively reduce how much 'pushing' the piston has to do, as well as 'pull' fresh fuel/air mixture into the cylinder (4stroke speak here - I'm not 100% certain how it translates to 2stroke) ... so, more/better combustion will probably create more cylinder pressure sustained if not peak

In this way yes!

​@@davidsandberg3109 The pulses from the expansion chamber can be somewhere in the neighborhood of 5-8psi (positive or negative) depending on pipe design. When the negative pressure wave arrives, all the ports are open, and that negative pressure can make it all the way to the intake/carb on a reed-valve engine. You can imagine how useful 8psi of suction at the intake port would be. In really hot engines with big port timing, this drives intake volumes more than crankcase pumping, and is the reason reed valves finally beat rotary valves for absolute performance. This huge negative pulse deliberately over-scavenges the cylinder at low RPM, and then the 5-8psi positive pulse from the baffle cone comes in and crams the air back into the exhaust port. In ideal circumstances this does act like a sort of resonance supercharger. It's worth noting that the guy who invented the expansion chamber, Walter Kaaden, was one of the main engineers behind the V1 pulse-jet program in WW2. Using the very same principles, pulse jets make pretty significant compression with resonance alone and no moving parts.

Rotax would be the best source of that.

It's not the mean pressure in the chamber which is the actual backpressure.

@@motoiq youre just creating a narrow definition of back pressure.

Not narrow but the correct description of what is going in regards to extracting work from the pressure volume cycle. If the pressure at the bottom of the cycle is higher due to backpressure, less work or power is extracted. vocal.media/journal/what-is-the-difference-between-pv-diagram-of-two-and-four-stroke-engines

Stock TM spec exhaust.

You can see the whole exhaust very clearly. It's a muffler, its straight through perforated core, no backpressure.

rough formula to start is stinger length is 6x diameter or more. Another trick for a larger collector is to have it extend into the chamber, this way you can get away with a bigger one.

The stinger is made to reduce the shock wave energy leaving the chamber so more can be reflected. It does not serve as a restrictor. If you restrict an expansion chamber with too small of an outlet, the EGT's go up and you can seize the motor. The stinger can be smaller than the exhaust port because the exhaust diffuses and cools and thus shrinks in volume in the chamber. It's good to have a low mean pressure in the chamber with strong properly timed pulses for the best powerband and overall power. You can use a smaller in diameter stinger to increase the mean pressure in the chamber and speed the wave up for tuning, but that can reduce overall engine efficiency. I can't get into this detail in the 60 seconds you have for a reel.

The exhaust gas looses a lot of its temperature, more than half, and its volume decrease.

It's not the engine liking pressure, it's changing the tune of the pipe.

@@motoiq back pressure is the baseline that the pipes return waves ride on and so is very important. There could be engines that don't want any back pressure but each engine would have to be tested with different pressures to see what it likes. All normal 2 stroke pipes (expansion chambers) have a stinger that allows some back pressure. Must be a reason for that.

All engines don't want backpressure, you raise the bottom of the PV Curve due to backpressure there is less energy or power to recover, don't mistake having some backpressure speeding the reflected wave and the engine liking it as the engine liking backpressure. More power can be found by improving the pipe design to have the proper pulse timing with less backpressure. The stinger isnt there to increase backpressure but to make it more difficult for the reflected wave to leave the chamber while allowing exhaust gas to easily pass through. That is why the minimum dimension of the stinger is 6x the exit hole size.

@@motoiq make all the changes you want but none of them will raise the baseline for both return waves like the back pressure can (and does). Raising that baseline increases the plugging pulse but decreases the vacuum pulse

I am not repeating what I read, it is from the laws of physics, thermodynamics and racing two strokes. There are no rules such that the bleed of an expansion chamber should be 24.3mm for all sizes of two stroke engines. There are general but not absolute rules that a good point to start your stinger length should be 6x its diameter. The stinger's or exit venturi's purpose is to be the acoustic end of the chamber, not act as a backpressure increasing flow restrictor. I mean many pipes are side bleed and the exit isnt even at the end of the chamber and the chamber is still cone shaped. Of course changing the mean pressure affects pipe tuning as it changes the speed of sound in the chamber, but increasing backpressure decreases the engines volumetric and thermal efficiency. So tuning the pipe via backpressure is a bandaid to making a correctly dimensioned pipe. Jennings and Alexander Graham Bell have simple formulas that seem to work pretty good and there are good inexpensive software calculators for designs now. What you are doing is observing the changes in performance by making changes in the pipe exit, you know what that does and partially understand what is going on in it but you don't understand how the entire system works. Now my daughter races in classes that have homologated spec pipes and bikes are all four strokes so haven't had the need to develop anything lately except playing with scooters for fun but I sure just don't read something and spew it out.

@@motoiq You are misquoting my statement, I never said 24.3mm was a standard for all engine sizes for example an 80cc runs well around the 18mm diameter whereas a 550 twin two into one exhaust runs well around 40mm. The current Homologation for the TM line specification is 26.2mm however that looks more like a Vortex shifter pipe with a Homologation spec of 24.3 +/- 0.5. A good starting point for diameter at the end of the conical baffle is 3.1 x the Sqrt of Hp at the crankshaft. Your statement regarding back pressure would suggest the diameter of the end of the conical baffle doesn’t matter.

That is an OEM TM pipe not a Vortex. The diameter does matter but not as much as you might think, a lot depends on the stinger and if you can package the right length in whatever you are doing. But going back to the original subject, increasing the mean backpressure on the engine hurts power output, there is no doubt about it.

@@motoiq That is not your original statement, Adjust your pipe so there is zero back pressure and run it, you will lose power.

The pressure goes well below zero in a chamber under different parts of the cycle, it's why I keep saying mean pressure. What you are saying is to purposefully not make an expansion chamber work. Of course is going to make about 50% less power.

yes very much so

What I said is totally true, no piston engine works better with retained backpressure. it limits the amount of work that can be recovered from the cycle. Adding a restrictor to that pipe was a bandaid for improper pipe design, back pressure increases the speed of the reflected wave in the chamber. Pipe design uses wave propagation and timing, it's all about that, you want all of that with the least amount of backpressure possible. The exit of the expansion chamber is called the stinger, a rule of thumb is that its length is 6x or more of its diameter, It not only serves as the exhaust exit but also is part of the pressure wave tuning. If the stinger exit area is less than the exhaust port, it is that the exhaust gas diffuses, loses a lot of energy, and greatly cools down. It no longer needs as big of an exit cross-section. If that pipe was designed correctly and tested correctly, it would work better with no restrictors.

​@@motoiq MIKE DROPPED

@@motoiq I agree with all the principles and theory’s your laying out, but I can’t go against the evidence I have seen with my own eyes. Another example i had was in 2001 when Yamaha recently the GPR 1200 watercraft, We started r&d on one of them, we started with the exhaust as we always do on a new vehicle, and started eliminating back pressure by modifying the muffler, and what do you know we yet again dropped RPM and top speed. Now this was completely stock, so Yamaha designed the exhaust system, yet when you went to get rid of the back pressure, we lost performance. Maybe Yamaha didn’t design the exhaust correctly? Possibly? I was only part of the race team so we weren’t involved in the actual development of the power plant and only received new models a few month before the public, but I trust that OEM’s don’t purposely do things to lose power (aside from Meeting sound/pollution regulations) because on the showroom floor they are competing with all the other OEMs and even saying you have 5 more horsepower helps sales. I really enjoy all your video and in-depth break down on lots of things. But on this subject we’ll just have to agree to disagree.

You have to understand the principles of how two stroke pipes and physics work, getting rid of a muffler and back pressure, if the chamber was designed for the back pressure of the muffler you will loose power. That being said a chamber designed only to make power and not to comply with noise regulations with minimal back pressure is going to make even more power. Look up the pressure volume cycles and the thermodynamics of two strokes and see. I am not making this stuff up. Of course your observations are correct but the reasons why are not.

www.mecholic.com/2016/02/working-of-two-stroke-engine-with-pv-diagram.html, if you retain pressure inside the engine due to back pressure, you raise the bottom of the PV curve and loose area inside the PV loop, the area inside is the work or power. Two stroke expansion chambers are like musical instruments, if you do stuff to change something, they can get off tune very easily. By reducing the backpressure by removing the muffler, you are reducing the density and temperature of the gas inside the chamber, this means that the acoustic and pressure waves will travel slower getting the pipe off tune. A simple way of getting it back without adding backpressure may be by playing with the stinger length or adding a stinger before the new muffler. A lot of times this won't package in the space you have though.