I have new car that has a set if short 90 degree turn outs, the fomer onwer put them on to get away from blowing dust and dirt up under the car from the straight out headers, he siad his motor slowed down by adding them years ago I put a set of 14x 3.5 inch Bullit racing mufflers on my old 440 Dart that had a set of Hooker 1 7/8 inch fenderwell headers on it, no extensions, those muffs slowed that combination down I had convinced myself that they would act just like collector extensions, they didn't Test, test and tset some more for real world results