Exhaust theory.

[Gen1GT]

This is important exhaust theory, written by Dave Coleman, appearing this month's issue of SCC.

Here's the phenomenon in a nutshell: Exhaust does go out your exhaust pipe in a steady flow. At the tailpipe, after several cubic feet of pipes and mufflers, it's reasonably consistant, but in the exhaust port, it's a different story. Every two revolutions, the exhaust valve opens, some air is pushed out, then the valves closes and the air had to stop. During the closing of the exhaust valve there is an opportunity.

If you imagine the cylinder full of air that just got pumped into the exhaust as a solid, it will be a big tube, or column, of air. This air column has some intertia, so it isn't terribly thrilled about having to stop when the valve closes. As the valve starts closing, the air column tries to keep going. This inertia can be used to help suck the last of the exhaust out of the cylinder. And because the intake valve is starting to open as the exhaust valve closes, this sucking also helps start the flow of fresh combustibles into the cylinder.

Inertia is 1/2MV², so increasing the V(velocity) of the air column is the best way to increase inertia. Make the exhaust pipe smaller, and jamming the same amount of air through it will require one of two things. Either that air will have to get compressed to fit into the smaller pipe(that's backpressure), or it will have to go faster to all get through in time.

In reality, both things happen. With relatively low exhaust flow at low RPM, the velocity helps scavenge the cylinder. At higher RPM, though, it takes too much energy to make the exhaust go fast enough, and backpressure starts to build.

Generations of hot-rodders have seen the small pipe/big torque phenomenon and wrongly assumed it was the backpressure causing low-RPM benefit.

The whole air column inertia thing really only applies to the header primaries anyway. As you get farther down the exhaust, the air doesn't really have to stop when the valve closes, so its velocity is irrelevant. Bigger, at that point, is better at any RPM. There is a point where bigger pipe won't help any more, but you can't boost torque by going smaller

[Killer 8]

so its saying that if you want to gain in the upper range having bigger headers will help the engine exhale easier do to the fact that it will reduce the mixture of exhaust Old air and intake New air from the overlap that comes with most cams in cars during the initial down stroke?

[Gen1GT]

He saying, accurately and correctly, that header primaries control most of the flow characteristics of an exhaust system. They need to be optimized for a particular build, ie, as small as you can go by not increasing backpressure. By the time the exhaust pulses are past the collector, the flow is mostly uniform, and exhaust diameter benefits from being large. He also reconfirms that backpressure is undesirable, and it's exhaust gas velocity that is important.