My experiences using dynos as a measure of HP before and after mods has been baffling.
My M6's Mods are:
Carbon air filters removed
K & N stock filter replacement
Supersprint Headers Supersprint Cats
Superprint X-Pipe w/ SS Resonator
Supersprint Sport Mufflers
AA Cam ECU Tune
3.91 Dinan Differential
Schrick Kelleners cams
The cams showed no HP gain - maybe even a loss, the Pulleys a VERY small gain (on one dyno the pulleys showed a loss, the 3.91 diff showed a power loss (see my thread to understand this), the 3 combined components: SS cats, SS resonator, and SS sport mufflers showed a 7.5 whp gain. Headers with software showed a gain of 37 hp...or something (dont recall exactly) (see ALL my old threads for complete information on these tests and reviews) Get this.....I have even done dyno runs after all my mods and have shown HP numbers that were LOWER than when my car was mildly upgraded.
Bottom line, there are simply too many factors that influence the numbers coming out of dynos. I dont fully trust these numbers - I only use them as a guide to see if my car upgrades are moving in the right direction.....but my most recent runs have been "so off" I dont even bother with dyno results anymore.
All I know, is that my car is trapping 120-121 in the quarter mile. It gets consistant 12.1s spinning 1st, 2nd, & a bit into 3rd. The car will run an 11.99 or better if it had traction.
Regardless of what the dyno says, the car moves. So, I wouldnt get too bent out of shape over your dyno results. The best way to gauge your results is to baseline your M5 against another M5. Then install your upgrades and run the cars again. You may be surprised that even though the dyno showed a loss, the race may prove different.
My recent run against my friends Nitrous M6 (nitrous off) I beat him by 7-10 cars or so.....Gustav was in the cars when we raced...he also has the videos of the race with and without Nitrous (hopefully he will post them soon)
Bottom line - what I am trying to say here is that the dynos didnt show me anything close compared to what I saw in the race.
Maybe u can chimp in on my dyno thread I posted especially a/f ratio. I pulled 399hp (CF removed, section1&2+stock muffler, K&N filter). Then after Evo pulley+exhaust, the car pulled 402hp. Then it pulled <400hp after BMC filter installed. I used Dyno Dynamic all on 5th gear (temp 82-92F) I'm