Which Headers

[nkaminar]

When I was in college, a lifetime ago, I studied scavenging and learned about wave propagation and how that could suck exhaust out. Now I have learned how the design of the exhaust valve and port at the head (or block) can be made to suck exhaust out. I still think that the wave propagation works and is important, but that the whole exhaust system is more complicated and all tied together.

For an exhaust system to be helpful it has to be designed for the engine and the rpm that the engine is used at. Even a low compression stock Model A could benefit from the right exhaust system, but the "header" pipes would have to be very long and probably be collected somewhere in the middle of the car. The collection is important because the pulses from the different cylinders reinforce each other.

When in high school I experimented with a Briggs and Stanton engine from a lawn mower by changing the length of the exhaust pipe. Those old engine had a pipe thread where the exhaust went into the block so it easy to change the length by using different length water pipes. This is an interesting experiment to do and easy with the right engine. A 10 foot pipe makes the engine run badly as the exhaust pulse returns to the exhaust valve at the completely wrong time.

The exhaust positive (pressure) pulse travels down the exhaust header at the speed of sound in that hot gas. It then reverses at the end and the negative pulse travels up the header, again at the speed of sound, to suck out the exhaust. So the length can be calculated based on this. But the final cut is made by testing. If there is a significant overlap of the intake and exhaust valves the suction at the exhaust valve can also help to suck in some fresh air and fuel.