My experience, 4th gear acceleration from 40mph to 60 mph dropped by 3 seconds. Doug Thorley header, stock cat, 2" cat back Through a 2 1/2" Flowmaster. No righteous seat of pants action, but engines revs quicker and pulls better than it ever did before . No definite MPG gains. Also better pull in 5th gear on freeway. I'll install high flow muffler bearings next smile