That's what I've read is that if your exhaust is too large or too free flowing, then it will have a backwards vacuum effect and doesn't help to get the gases out as much.Daggar said:
I just think of it as the engine being an air pump, which, essentially, it is. So, it's all about pressure. For instance, a very mild 5.0 is going to have more pressure to help get exhaust gases out with 2.5" exhaust pipe vs 3" exhaust pipe. Now, if you have a heavily modified 408 stroker or something, it's going to be making a ton more exhaust gas and will have enough pressure to be able to use 3" exhaust efficiently.