Originally Posted by Back Road Runner
But this is what you want. You need that flow from the intake side to clear out the spent charge from the combustion chamber. This is the whole idea behind scavenging.
Backpressure is always bad. You're running a big air pump. You don't want restrictions. Restriction = inefficiency.
NO... backpressure is not bad... the correct amount of BP is a very good thing. You want some pressure to increase the velocity of the exhaust gas. Having a large very UN-restrictive exhaust will not keep your exhaust gas (and pulse waves) moving quickly which creates less of a low pressure area behind the pulse wave. this low pressure area is what helps "pull" the spend exhaust gasses out and the new a/f mixture into the cylinder. Having some of the new a/f mixture pass through the cylinder is a good thing because it ensures that no, or much less spent exhaust is left in the cylinder. It might waste a tiny bit of gas, but that is very neglegeable and if you are tying to make more HP, then fuel efficiency is something you have to sacrifice to some extent. Back to the pulse wave idea... tuned length headers (not just EL, but also tuned) will be a specific length from exhaust port to collector to create a reflected pressure wave to bounce and be back at the exhaust port ready to scavenge the next time the valve opens at the RPM the header was tuned for. Just wanted to share that so no one buys a 3.5" turbo exhaust for the n/a and then wonders why they dont make any power until just about redline.