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I don't agree that they need backpressure but I do agree you dont want to go to big. If you go to big the exhaust will expand causing it to cool down which then makes the gases tumble and slow down. So 2 1/2" is as big as I would go on a small block, 2" is to small. X pipes do work well but in my experience all of its gains are in the high rpm range (above 6000) thats why nascar uses them, so on a street driven vehicle you dont get the advantage over a H pipe. Mufflers are very important because they cause the exhaust to expand creating a scavaging effect (vacuum) which is good. I personaly like magnaflows over the flowmasters because they flow better but probably not enough to feel the difference, mostly sound difference. If you use a glass pack make sure it is a good quality glass pack not a cheap thrush cherry bomb that you buy out of a trash can at the local auto parts store, it should have a 4" outer core so there is more room for expansion. But even then they are not as good as a good turbo muffler.