the reason a 3" isn't optimal for smaller/no-turbo engines is exhaust speed. The exhaust moves faster through a smaller chamber, which dosn't necessarily have anything to do with backpressure. I guess I had 2 separate points: backpressure = not necessary, exhaust speed = good. I just kinda lumped them together, sorry for the confusion. THe whole old car statement was just in reference to the folks i know at autocrosses and stuff who swear by their supertrapps.