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Old 04-08-2009, 04:11 PM   #8
John V
No more BMWs
 
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Join Date: Apr 2005
Carmudgeonly Ride: Ram, MS3, CX-5, RX-8
Location: Glenwood, MD
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Quote:
Originally Posted by Rob View Post
So you are basically saying that somehow the corvette loses the advantage of 140 ft-lbs, or more than half the M3's total torque, through drive train loss? What am I missing? Is it all from the gear ratios? If it is, what's the benefit of giving up that torque advantage? I suppose gas mileage would improve.

From a practical standpoint, does the M3 outrun the corvette b/c of it's torque advantage? I haven't paid attention to acceleration numbers b/c I have started to just think of cars as "fast" "not so fast" and "slow." I have come to realize that, unlike some of you, shaving a half a second off a one minute autocross course doesn't mean anything to me and there are a lot more important factors that go into car choice than 0 to 60 or quarter mile times.
FWIW, this didn't stem from an autocross-related discussion. And the point wasn't that the M3 outruns the Z06 or vice-versa (I don't even know which one tests faster in 1/4 mile acceleration runs, but they are probably pretty close). The original discussion was one of area under the curve when it comes to torque... which I still haven't had time to calculate. But I digress.

I neglected drivetrain loss for the reason that Nick mentioned - it's probably the same for both cars and certainly for comparison's sake it's in the noise here.

I just find it interesting that people (Corvette people in particular) like to pick the peak torque value of an engine and declare that it's "torqueless" without taking into account gearing. The M3's torque curve is very flat, and it spans a very wide RPM band. The Corvette's engine is more peaky in terms of torque delivery and it doesn't rev as high. See below.



The LS6 is over 300ft-lbs from 2000 to 7000 RPM (redline is 6500, but it revs to 6800 before the rev limiter kicks in). Sure, the peak is 400, but the engine overall is limited by its two-valve architecture. Even if a stock LS6 could rev to 8400 RPM without grenading, you probably wouldn't want to - the engine can't breathe up there.

I wasn't trying to draw any particular conclusion other than the fact that the cars are very similar in terms of measured performance but they go about it in very different ways. WHEEL torque is what matters, and WHEEL torque is engine torque times the reduction factor of the transmission. For the M3 it's around 15:1 in first gear and for the Z06 it's around 10:1. Big difference!

And fuel economy is very closely tied to engine RPM (and weight, and to a much smaller degree throttle position) so yes, the BMW gets worse mileage.
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