I got a chance to do before and after dynos for my Von Hep exhaust. First off... it saved over 20 lbs so I'm happy with that aspect
Second, I ran the car HARD for 2 full tanks of gas so the ECU could re-learn any possible gains away.
Ironically the atmospheric conditions were almost identical between the two runs.
Baseline: 81.56 degrees F, 27.30 in-Hg, 11% humidity
Von Hep: 81.58 degees F, 27.35 in-Hg, 13% humidity
Using the "aftermarket part seller" technique I gained 5.56 HP and 3.26 ft lbs of torque. This is comparing the worst baseline to the best after run. I decided it was more even to compare the best baseline run versus the worst after run. Using the honest method I gained 1.53 HP and 1.80 ft lbs of torque (I'm attaching this dyno graph to be more realistic). The cool thing is that there was an increase in torque across the entire RPM range. Especially under 3000 rpm
This was apparent on every run
It sounds GREAT. Quieter then a stage II but deeper sounding then an Arqray single tip. I love tunnels as the car sounds like a pure racecar under WOT at high RPM