NA is different then turboed.
You need some kind of backpressure especially on mostly stock NA engines...

 

The back pressure theory is about flattening the power band, so it won't suck as bad.

 

I'm not sure what you mean by that, but I think it helps with "scavenging" of the exhaust gasses. My way of thinking on this is that it creates some kinda "suction" to pull out the gasses.

Exhaust is gas flow, heat and a pressure wave. All of the designs are a compromise - logs hold the heat better, equal length distribute the pulses at more even time spacings and have better gas flow.

From what I've read, for a yutz like me at 7 - 10 psi it makes no real difference. When you start really going ***** out, like 300+ HP, the difference might be like 5Hp or something. You could probably get more by porting the head or something.

 

Old MYTH....not true at all. Backpressure is something you don't need. Just look at the dragsters that run 3" open pipes that are only about 2 feet long. They aren't worried about backpressure, only providing a way to get the heat away from the engine.

 

what you guys are forgetting is that most turboed engines run about 30-50 psi of exhaust gas into the turbine inlet of the turbo, so if you have bigger piping and more flow and velocity out of the turbo (its a velocity vs flow problem) you have to have enough volume in the runners that it isnt restrictive but they cant be soo big that youre losing velocity all over the place, with a log manifold and no real collector, the velocity is all messed up because the exhaust gas has to make sharp 90* turns and isnt forced primarily into the turbo (as opposed to other runners etc) so its not as efficient, whereas an efficient collector on a race manifold smooths out the sharp turns and prevents a loss in velocity and makes more power due to the better scavaging