I did a lot of research on Header and exhaust design.
Here's the short, of what I found. Don't fall asleep, there's a test on this latter!
Your right in saying, an internal combustion, is like a pump.
The burning of fuel, heats and expands air, forcing a piston down. When the exhaust valve opens, hot gasses escape. The exhaust rushes down a pipe and out. So why an exhaust pipe at all? If it's a pump, wouldn't an open exhaust, have the least restriction and proved the most power? How can a pipe, with no moving parts, help make more, or less power?
As you guessed, there's a lot more going on, than meets the eye, or in this case, ears.
Lets slow things down, to one step at a time.
As the exhaust valve opens and pressure is released, a Shock wave
is formed. This wave moves down the pipe, at the speed of sound and out. We hear it, as a "POP". It's what happens behind
this wave, that helps make power. There is a small vacuum
What if we could make this vacuum bigger? This would pull
, more exhaust out and the engine wouldn't have to "Pump" or push
it out. The end effect would be more power.
As the shock wave, travels out the pipe and into the open air, or muffler, some of the wave, bounces back. This travels all the way back to the valve and bounces off the back of the valve. If the valve opens, at that same moment, the two shock waves join and a larger wave is created. It has a larger vacuum, behind it.
All of this, takes a small amount of time. We can adjust when this happens, by changing the length of the pipe.
Now the pipe length comes into play.
A short pipe, the wave joining happens quicker or at a high RPM
A long pipe this happens slower, or a low RPM.
As far as back pressure
, you don't wan't any. The least amount of restriction, the better.
A muffler adsorbs, or it can break up the shock wave. Small shock wave = small sound. As long as it can flow large volumes of hot exhaust.
A Big Gun muffler is an adsorption type. Remember the packing? That's what its for, adsorbing shock waves.
A Gibson is designed to break the shock wave up or cancel it. That's what the baffles do.
There's a bunch of math that go's along with all of this, but I won't bore you with it.
Now, about that test ....
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