How much back pressure should a high performance engine have. I have a 330HP 350 crate from GM and I think thelack of exhaust bak pressure is making it idle rough. I have virtually no back pressure becuse the cat is hollow and the stock muffler is too for the most part. THe cat is completely hollow. Is there such a thing as too little back pressure. The reason I ask this is becuase when you cover the tail pipe with a rag the engine seems to smooth out. IS there anyways to check for back pressure, or are you checking for vacuum level to find the absence of back pressure. I have a vacuum tester, but I don't know how to check for back pressure. Thanks for the help.