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back pressure and expansion chamber


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ok i read the backpressure myth post and expasion chamber 101 post. My brother talks about how a two stroke needs back pressure and i told him he was wrong. He said that is what the expasion chamber does is create back pressure. The back pressure myth post says good exhaust is when you have the lowest backpressure possible. what i got out of expansion chamber 101 post is that the expasion chamber is to put fresh mixture back into the cylinder before the exhaust port closes...so is that consider back pressure? Am i wrong can someone explain it simply how these two posts or thoughts converge? Does a two stroke need back pressure?

Edited by specialkcrf
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In my SIMPLE mind...

Two strokes use an effective "echo" to push a little back in the port for maximum efficiency. Ths happens most effectively at it's "tuned" exhaust velocity.

Four strokes use more of a vacuum effect to help scavenge the mixture through the valves.

In an aircraft the pipe is straight and ports directly overboard. no back pressure at all. In fact the tips are longer on the forward facing side to prevent cool ambient air from getting to the hot valves and cracking them. No back pressure at all. But Aircraft engines are usually operated near max power only and blade pitch changes forward thrust produced.

Dirtbike engines are reved from idle to Redline 1000 times a lap... So power is produced throughout the rev band. Pipes produce slight back pressure at certain rpms to produce power gains where the band would normally be flat. Tuning a pipe is important and very particular. Several companies have spent millions on the development of exhaust systems to make them optimal for off road use.

-BIG DAN:thumbsup:

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A 2 stroke expansion chamber - when built right will pull fresh fuel mixture thru the cylinder and into the pipe , then the reverse wave will push the fresh air/ful charge mix back into the cylinder right after the intake ports are closed off by the piston. , it is like a supercharge affect .

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In my SIMPLE mind...

Two strokes use an effective "echo" to push a little back in the port for maximum efficiency. Ths happens most effectively at it's "tuned" exhaust velocity.

Four strokes use more of a vacuum effect to help scavenge the mixture through the valves.

In an aircraft the pipe is straight and ports directly overboard. no back pressure at all. In fact the tips are longer on the forward facing side to prevent cool ambient air from getting to the hot valves and cracking them. No back pressure at all. But Aircraft engines are usually operated near max power only and blade pitch changes forward thrust produced.

Dirtbike engines are reved from idle to Redline 1000 times a lap... So power is produced throughout the rev band. Pipes produce slight back pressure at certain rpms to produce power gains where the band would normally be flat. Tuning a pipe is important and very particular. Several companies have spent millions on the development of exhaust systems to make them optimal for off road use.

-BIG DAN:thumbsup:

so two strokes do need back pressure then?

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i don't know if you could really call it "back pressure". Its almost more an echo. This pressure wave is caused by the last tapered or cone section before the stinger. The wave bounces back and pushes any unburned charge back into the cylinder

at a certain rpm range. The first cone shaped section helps to scavenge the burned charge out of the cylinder. For more specifics about headpipe sizes, cone tapers, main section diameter and legnth and stinger diameter, BOOKS have been written on design and yes, millions have been spent.

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I wouldn't call it back pressure, as that indicates constant pressure back towards the cylinder, which is the worst thing an exhaust could do. On both a two stroke and a four stroke exhaust you experience pressure waves, since an engine is a cyclic pump. This unsteady nature of a reciprocating engine creates waves of high and low pressure in the exhaust. To optimize power at a specific RPM range you have to design the exhaust so that the pressure wave frequency matches the RPM of the engine. When the frequency is matched then the exhaust port will open while a low pressure wave is in the pipe near the cylinder, causing the exhaust in the cylinder to be pulled out (as well as being pushed out by the piston). Then if the exhaust is matched to the engine speed the high pressure wave will make its way back to the front of the pipe right before the exhaust port closes, forcing the fresh charge that overshot the cylinder back into the cylinder. I hope this makes sense.

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Yes and no. Like KJ said, its not necessarily backpressure. You do want a certain amount of restriction in the stinger to keep the pressure wave from blowing out, but restrict it too much and you get an exhaust system that wont flow enough, along with the possibility of burning a hole in the dome.

Take a look in your silencer endcap. You will probably find a ring welded in about 1/4" from the outlet. Many refer to this as a backpressure ring. Its there because most manufacturers use standard id tube for their silencers and its simply more economical to weld in a ring to get optimal "backpressure" than it is to use a custom id tube for every application.

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No engine needs "back pressure". This implies simple resistance to the outward flow of gases, and it's detrimental to performance in every case.

A two-stroke does require that a timed pressure wave should arrive at the exhaust port just as it is closing so that the wave can stuff the fuel and air mix that will have naturally escaped into the exhaust tract back into the cylinder. In a very real sense, a properly tuned expansion chamber is a de facto supercharger.

Read:

http://www.bridgestonemotorcycle.com/documents/do_you_really6.pdf

http://www.chuckbunnell.com/kart/jennings/jennings.html

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ok my brother kkeps saying that that would be called back pressure..the timed pressure wave. And now he is sayong a four stroke needs back pressure to i told him what you all sayed but he said ok you beleive that stupid internet and wants to bet me a $100 so how cdan i officially prove him wrong his friend is putting up a $100 too,

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i do nt think he will ever understand i think he is just stupid he keeps sayind that you need back pressure to make the piston move faster and all kinds of stupid shiz. i sayed no that is what the spark plug igniting the gas does its called combustion. then he said it need back pressure so air flowss through the cylinder better and so it has more torque i think im going to give up he oviously doesnt know anything about an engine and doesnt want to learn

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I liken trying to educate people like that with wrestling with a pig in order to teach him a lesson. You get really beat up and dirty, and the pig has a good time. But he doesn't learn anything.

It's not worth your trouble, trust me.

A four stroke actually is just the opposite of a two-stroke. In a four-stroke, the intake valve starts open before the exhaust valve has completely closed. This is referred to as the overlap period. There are a number of complex dynamics at work in any engine type, but we'll consider only one here. As the exhaust opens, the exhaust bursting into the header produces a pressure wave, which travels down the pipe to the end. As this leaves the pipe, sudden drop in pressure causes a negative pressure wave (vacuum if you will) to run back up the pipe toward the valve. The idea is for this wave of vacuum to get back to the exhaust valve just as it closes so that it will accelerate the incoming intake charge.

At one point, I had a set of formulae for use with open exhausts, but they wouldn't be applicable with a muffler in place.

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I liken trying to educate people like that with wrestling with a pig in order to teach him a lesson. You get really beat up and dirty, and the pig has a good time. But he doesn't learn anything.

It's not worth your trouble, trust me.

A four stroke actually is just the opposite of a two-stroke. In a four-stroke, the intake valve starts open before the exhaust valve has completely closed. This is referred to as the overlap period. There are a number of complex dynamics at work in any engine type, but we'll consider only one here. As the exhaust opens, the exhaust bursting into the header produces a pressure wave, which travels down the pipe to the end. As this leaves the pipe, sudden drop in pressure causes a negative pressure wave (vacuum if you will) to run back up the pipe toward the valve. The idea is for this wave of vacuum to get back to the exhaust valve just as it closes so that it will accelerate the incoming intake charge.

At one point, I had a set of formulae for use with open exhausts, but they wouldn't be applicable with a muffler in place.

Im interested in this quote , and presume that the length and diameter of a four stroke exhaust will greatly affect the returning vaccum waves speed, which brings me to the question of why a big bore header should improve performance on a four stroke ?? is it the case bigger bore of equal length produces more top end power only, ie gases escape quicker,and again curious about would a narrower pipe increase mid or low power ??

reason i ask is because I will be having a custom exhaust made up for my 1991 dr 441 which i converted from a 350s last year, and the old original exaust system weighs an absolute ton compared to modern light weight alloys.

I have a gsxr after marcket can to stick on as a muffler, it is still of original length some 600mm and plan to shorten it but not so sure whether to increase the inside diameter of the header ?

to summerize i am happy with the bikes performance , low - mid high , and expected a loss at top end due to the displacement increase , but I would like another few mph on top end without loss in mid is this possible ???

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From what i have read two strokes need both."The stinger section of a tuned pipe,which is the part where the pipe gets really small before the silence is also called a bleed pipe.It's function is to restrict gas flow out of the exhaust and create back pressure by slowly bleeding off the exhaust gas.This serves to assist the positive the positive pulse wave in pushing any spilled fuel/air charge back into the motor" This is straight from A. Gram Bell's book "Two stroke performance tuning" ....I want some of that $100

He can find the book at amazon.com .If he still doesn't believe you disown him immediately....

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From what i have read two strokes need both."The stinger section of a tuned pipe,which is the part where the pipe gets really small before the silence is also called a bleed pipe.It's function is to restrict gas flow out of the exhaust and create back pressure by slowly bleeding off the exhaust gas.This serves to assist the positive the positive pulse wave in pushing any spilled fuel/air charge back into the motor"

The stinger needs to be small enough to maximize the strength of the reflected pressure wave, yet large enough to allow chamber pressure to drop to near zero prior to the next exhaust opening. Slowly (relatively speaking) bleeding off pressure is a result of balancing these two requirements, not the primary design consideration.

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The stinger needs to be small enough to maximize the strength of the reflected pressure wave, yet large enough to allow chamber pressure to drop to near zero prior to the next exhaust opening. Slowly (relatively speaking) bleeding off pressure is a result of balancing these two requirements, not the primary design consideration.

This is true the only point in the reduction of size on the 2 stroke pipe is to get that "sound wave" or pressure wave back to scavenge the excaped air/fuel mixture. If the small part of the pipe was too big then you would not get an adequate pressure wave back to the cylinder to scavenge the unburnt air/fuel. If it is too small then you would have a good pressure wave but not enough exaust getting out of the system. the trick is to find the ballance between maximum pressure wave and airflow.

I once heard somebody tell me that it was like using sound waves as a supercharger of sorts

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