Originally Posted by Caravanshaka

3-4 hp was the difference between the 2 cat-backs, not the overall gain.

DDM did a baseline, then another pull with the HFC's and did 19 hp...later on down the road he did a NEW baseline, then added the exhaust and got 13 hp. Yes, the temperature was low for the HFC's compared to 110 degrees he ran in with the exhaust, so adjusted for temp and take away 10% of the original 19hp gain and you are still looking at 30hp to the wheels, or over 10% gain.