View Single Post
      04-13-2012, 12:57 PM   #35
M3PO's Avatar

Drives: '08 IB E92
Join Date: Sep 2011
Location: OC

iTrader: (7)

Originally Posted by signes View Post
The blue and green lines on this plot represent other E9x M3s runs on the same dyno (diff day.) No mods other than rear section exhaust.

Wow, the other two make sense which suggests that it's not the dyno or the operator. This definitely seems to point to it being an issue with your car.

Originally Posted by signes View Post
Interesting, others have thoughts on gear?
Sometimes this is the case. Read this:

Will different final drive ratios affect the dyno readings?

This one's tricky. First, there are potential discrepancies because different gears have different inertia values, generate more friction, and change the amount of tire slip. Higher numerical gears tend to be more inefficient, so as gear ratios increase numerically, power levels tend to slightly drop, particularly on an inertia dyno. When torque is multiplied by steeper gears, tire slippage also tends to increase.

However, there's another, often overlooked, factor in the brew: rpm and torque are inversely related to calculating horsepower, so changing the rear axle ratio or testing in other than a 1:1 transmission gear seemingly shouldn't change the horsepower numbers. But this doesn't take into consideration the fact that changing gear ratios changes the engine's rate of acceleration. For example: We know that on an engine dyno, if you change a sweep test's acceleration rate from, say, 300 rpm/second to 600 rpm/second, the flywheel power number (bhp) drops due to the faster rate of acceleration. As an engine accelerates at a higher rate, the power required to accelerate the engine increases, and a greater portion is consumed before it gets to the flywheel. Going to numerically higher gear ratios-whether in the trans (testing in a lower gear) or in the rearend-is like increasing the rate of acceleration in a sweep test. Whether this actually changes a given chassis dyno's reported results depends on how the specific dyno manufacturer does its math. For the most consistent results, always test in the same trans gear (generally 1:1) and rebaseline the vehicle after a rear-axle ratio change.

Read more:

EDIT: There's a big BUT though. I think if this were the issue your torque would be higher than the other two.
2008 IB E92 M3| BBS | KW | Arkym | Platte Forme A.G. | Active Autowerke | K&N | Fabspeed | Dinan | Evolve-R