How an exhaust header works | GTAMotorcycle.com

How an exhaust header works

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bitzz

Well-known member
I have a thing, that if we are discussing exhausts and you say "back pressure", I stop listening... THERE IS NO BACK PRESSURE.
This visually demonstrates what is really going on
https://imgur.com/t4FHo5c
The air pressure is moving in one direction, away from the trumpet, the pressure WAVE moves back and forth.
More akin to sound waves than wind.
It's all about the WAVES.
 
What about a tuned 2 stroke expansion chamber where the wave bounces off one end and travels back towards the port and pushes some of the incoming charge back in?
 
bitzz said:
I have a thing, that if we are discussing exhausts and you say "back pressure", I stop listening... THERE IS NO BACK PRESSURE.
This visually demonstrates what is really going on
https://imgur.com/t4FHo5c
The air pressure is moving in one direction, away from the trumpet, the pressure WAVE moves back and forth.
More akin to sound waves than wind.
It's all about the WAVES.
Click to expand...
I was taught 'back pressure' in the context of discussing engines, is the pressure the hydraulic pressure needed to move exhaust gas through the exhaust system.

Think of breathing thru a straw, and then thru an open mouth. To do this test, stick a pressure sensor in one nostril and plug the other, then exhale thru the straw in 2 seconds. The recorded pressure is the back pressure. Repeat with an open mouth, what's tge difference?

Repeat the experiment while running up stairs to see how restrictions impact power delivery. This is the same impact in a enging running high rpm.

Overcoming 'back pressure' takes energy, so the more restriction, the more energy required(lost) the pressurizing the exhaust gas.

In the video above, I think the sound waves are just moving forward.
 
Last edited:
timtune said:
What about a tuned 2 stroke expansion chamber where the wave bounces off one end and travels back towards the port and pushes some of the incoming charge back in?
Click to expand...
Called "port stuffing"
The pressure wave part works exactly the same with a 4 stroke... but gets overly complicated with valves... but ends up being called "carb signal" at the throttle plate
 
Last edited:
Mad Mike said:
I was taught 'back pressure' in the context of discussing engines, is the pressure the hydraulic pressure needed to move exhaust gas through the exhaust system.
Click to expand...
That's my point, it's NOT hydraulic pressure, it's a pressure wave. The exhaust gases aren't supplying the pressure by moving, the exhaust gases only go one way: OUT, but there is a energy wave that flows through the exhaust gases, that does the work. It's more acoustics, than fluid dynamics
This works with an straight, open pipe, with NO restrictions on flow, the pressure wave will travel down the pipe to the open end and will rebound (the wave will invert if it hits an open end, it will rebound not inverted when it hits a closed end).
If you restrict flow of the exhaust you'll over heat the exhaust valve, but that isn't relevant to this discussion
 
bitzz said:
Called "port stuffing"
The pressure wave part works exactly the same with a 4 stroke... but gets overly complicated with valves... but ends up being called "carb signal" at the throttle plate
Click to expand...
Does the wave not exert "pressure" to stuff the port? Just asking for a friend.
 
The ART of exhaust design is to get this pressure wave to enter the combustion chamber as soon as the exhaust valve opens at peak positive pressure to drive exhaust gases out, then go to peak negative pressure when the intake valve opens. If you time it right you can increase performance by... a LOT.
We know the pressure wave travels at the speed of sound, so we can figure out how long the pipe should be, to maximize the effect at a given RPM.
Through the magic of fluid dynamics, we know that this pressure wave will rebound or reflect when it hits a change in area, like the beginning of the muffler... so then you get TWO rebounds of the pressure wave, one at the muffler, and another at the end of the muffler, so you get the effect at two RPMs. Put a megaphone muffler on it, and the muffler will amplify the wave, put a reverse megaphone muffler on it to amplify AND increase the speed of the wave.
If you have a 180 degree twin you can port this pressure wave from cylinder A into cylinder B, using a cross-over pipe.
If you have a twin, you can use a 2 into 1 pipe, and use the pressure wave off cylinder A into cylinder B... but you also get a rebound when the pressure wave hits the collector, then the muffler, then the end of the muffler, so you get 3 rebounds in each pipe, which results in 6 rebounds per cylinder
Then there's 4 into 2 into 1 pipes, then there's multiple cylinders, like V8s that each cylinder has a 180 degree opposite, that do 4 into 2 into 1 into 1 into 2 mufflers... that melts your brain trying to figure out where the 8 different waves are going.
 
My friend is now very confused.
 
bitzz said:
The ART of exhaust design is to get this pressure wave to enter the combustion chamber as soon as the exhaust valve opens at peak positive pressure to drive exhaust gases out, then go to peak negative pressure when the intake valve opens. If you time it right you can increase performance by... a LOT.
We know the pressure wave travels at the speed of sound, so we can figure out how long the pipe should be, to maximize the effect at a given RPM.
Through the magic of fluid dynamics, we know that this pressure wave will rebound or reflect when it hits a change in area, like the beginning of the muffler... so then you get TWO rebounds of the pressure wave, one at the muffler, and another at the end of the muffler, so you get the effect at two RPMs. Put a megaphone muffler on it, and the muffler will amplify the wave, put a reverse megaphone muffler on it to amplify AND increase the speed of the wave.
If you have a 180 degree twin you can port this pressure wave from cylinder A into cylinder B, using a cross-over pipe.
If you have a twin, you can use a 2 into 1 pipe, and use the pressure wave off cylinder A into cylinder B... but you also get a rebound when the pressure wave hits the collector, then the muffler, then the end of the muffler, so you get 3 rebounds in each pipe, which results in 6 rebounds per cylinder
Then there's 4 into 2 into 1 pipes, then there's multiple cylinders, like V8s that each cylinder has a 180 degree opposite, that do 4 into 2 into 1 into 1 into 2 mufflers... that melts your brain trying to figure out where the 8 different waves are going.
Click to expand...
I dont think this has much concern with modern gas engines as the intake charge pressure will clear exhaust at 3000rpm or greater. Old engines, low revving diesels yes, the benefit at low rpm, but modern powersports and performance cars... nope... they are not called to attention at low rpm.
 
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