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Old 11-20-2012, 10:29 AM
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Bönz Bönz is offline
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Location: Michigan
Join Date: Jun 2012
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Mein Auto: 2015 M5
Originally Posted by haastx10 View Post

Stock: 328 WHP on dyno, 18% loss to wheels calculates to 400 hp stock at the flywheel
Stage 3: 457 WHP on dyno. Calculates to 539 hp at flywheel, with Dinan exhaust
I think what you meant to say is IF stock crank HP is 400, then 328 at the wheels calculates to an 18% drivetrain loss. But since we don't really know crank HP, we cannot accurately calculate drivetrain loss. Although 18% is a realistic figure, there is no hard fast rule for this sort of thing especially with complex modern drivetrains (not to mention the frictional and inertial effects of the dynomometer itself).

Your Dinan calculation is backwards, as you took 18% of the 457 and added it to 457. Assuming the DT loss is linear (which it is not), your Dinan HP at the crank would calculate to 557. Dinan's figure for stage 3 with exhaust is 511, which at an 18% loss means 419WHP - which seems more reasonable.

There are many different kinds of Chassis Dynos and without getting into details, the best information to get from these machines is before and after (at the driven wheels). What your dyno work shows is a 39% increase in WHP which seems highly unlikely. Assuming the 328 stock number is accurate, a 28% increase from 328 to 419 seems more likely. This also matches Dinan's crank HP increase exactly, from 400 to 511, a 28% increase.

Notice also that Dinan's torque increase is about 22% (450 to 550) and there's no peak torque difference between stage 2 and stage 3. Engines produce torque - HP is calculated as a function of torque and RPM. Clearly the exhaust change effects the torque curve, NOT peak output, which makes sense on a turbochaged engine. This also explains the increase in peak HP (versus torque) as it all has to do with the torque curve.

All that said, I would love to have a 28% increase in HP. Hell, I'd settle for 10%. But I just cannot justify the $$. The stock car is more than adequate. Other vehicles are just obstacles on the road. The power just flows so easily from this car, it never ceases to amaze me. And I'm coming out of an M3.

Can't wait to see your dyno graphs. Bottom line, if you recall basic calculus, it's the area under the curve that matters - not the peaks. This is why the flat torque curves of BMW's turbo engines feel so strong. And why, I suspect, it advertises as such (ie. 480/2000-4500).

Last edited by Bönz; 11-20-2012 at 03:25 PM. Reason: added word "NOT"
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