Dang it Terry, you know backpressure is a really poor way to describe it
It's all about velocity and flow. Backpressure is such a poor way of describing the situation its pitiful.
Ideally, you want the exhaust to be able to flow all gases the engine can throw at it, but at the same time, if you use too large an exhaust pipe, the velocity of the exhaust gases goes in the toilet. This is a problem because of somehting called scavenging; you see, if exhaust velocity is high, it creates a scavenging (or suction) effect on the cylinder when the exhaust valve is still open, allowing more complete cylinder filling (ie less exhaust gas is left in the cylinder, so more oxygen rich air can enter, and make a bigger explosion
)
Anyway, the end result is that, just like anyhting else, the exhaust is part of a larger system, and all the parts must be sized appropriately to work together well. Too large is bad, and too small is bad. Do your homework, and make your exhaust the right size for your application; for optimum results anyway
.
Backpressure, for the sake of backpressure is dumb, and ill-informed. If properly sizing the system for your engine's flow, and to maintain adequate gas velocity results in some "backpressure" so be it, but don't go hunting for backpressure, all that does is choke things off.
