it depends on alot of things alot has to do with the heads/cam and compresion......if your trying to quit down a v-8 becase its to loud....step one don't go from headers to mufflers while using 3 1/2 inche exhaust.and don't use flowmasters or u will be in the same postion that i'm in...i can't even start my car in pinalles county....it really sucks can't even put a tune up in my car.