Greetings all,

I have heard that exhaust back pressure is required (to a degree) by naturally aspirated engines, but have no idea why, and have been puzzling over this for a while. Can anyone shed some light on this?

Ignoring noise output and any catalytic converter, what would be wrong with using no exhaust at all, or a really fat one to reduce flow resistance? Or is a short pipe tuned to resonate at the same frequency of the exhaust valves the optimum?

Also, how can the optimum-for-desired-rev-range back pressure or exhaust diameter be found?

My current logic says that any pressure needed to force gasses out of the exhaust, is pressure against the piston on the exhaust stroke, and will therefore reduce available power. What am I missing?

Thanks,

Julian