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Old 04-08-2009, 02:26 PM   #1
John V
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Speaking of torque (i.e. no wonder the E90/92 M3 is so fast...)

Inspired by dred's thread on torque, HP and work, a buddy and I got into a discussion about how people frequently misunderstand torque and gearing. For some reason, we found ourselves looking up dyno charts for various cars and came across one for the E90/E92 M3, which looked basically flat at 260ft-lbs from 3,000 RPM to 8,100 RPM. Given how fast the car is, I figured it had to be geared pretty short. And it is. But what's shocking is how much torque the car puts to the wheels, particularly in first gear.

Below are plots of wheel torque for the E90/92 M3 (peak torque 260-ish-something) and C5 Z06 (peak torque 400ish) versus MPH for the first four gears, assuming shifts at redline. The M3 puts almost 50% more torque to the wheels than the Z06 at low speeds... and the cars are pretty evenly matched after that.

The other plot is torque to the wheels versus MPH for the gears over the entire usable RPM range.

Of course this all discounts weight. But if you convert torque to thrust and divide that by the weight of the car, the M3 still wins in first gear (even with a 600lb weight disadvantage). That's the third plot.

The long and short of it is that most people would probably not describe the M3's engine as torquey (at least when compared to a big 2-valve pushrod V8). But engine torque is pretty irrelevant. Torque to the wheels is really what matters, and in this case the M3 is very impressive.

Maybe this isn't interesting to you guys, but I was bored waiting for a teleconference to start late last night...
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Old 04-08-2009, 03:12 PM   #2
Rob
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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.
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Old 04-08-2009, 03:19 PM   #3
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Good stuff.
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Old 04-08-2009, 03:24 PM   #4
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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.
It's all gearing. I'm assuming that JV ignored drivetrain losses (which should be about the same in two front engined, RWD cars anyway).

The M3 has shorter gearing and more revs to work with.
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Old 04-08-2009, 03:29 PM   #5
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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?
The difference is reduced by two things:

1) Different max rpm. M3 is 8100 vs C5's 7000. It allows you to reduce the gearing to to increase torque at the wheels.

2) Torque curve. Max torque is only relevant while you're at that RPM.

John, I assume the Z06's torque curve is less flat than the M3's?
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Old 04-08-2009, 03:30 PM   #6
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The other thing to consider is that it's one thing to look at how a car is set up under an optimal gear selection for a given speed (which counts for optimal performace) and the range of performance being offered in a given gear. Basically, to borrow from that article, what if both cars had a 3-speed tranny? The corvette would be better.

That is a similar situation to feeling lazy, not wanting to hold revs up due to sound level comfort, or a heavy family car with a so-so slushy. In all those situations, you benefit from broad torque down low.

If I keep the 330i near 3krpm, it is a blast and pretty darn fast.

OT: I've started to dislike revving past 6krpm becasue the revs take to damn long to drop and shifting is not as fun. 3-6krpm is my fun zone - 3.5-5.5krpm is even better.

As always, everything is a compromise - unless you don't have to.
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Old 04-08-2009, 03:31 PM   #7
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Quote:
Originally Posted by rumatt View Post
The difference is reduced by two things:

1) Different max rpm. M3 is 8400 vs C5's 7000.
Fixed.

But yes.

I happened to dig up and save this yesterday...

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Old 04-08-2009, 04:11 PM   #8
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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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Old 04-08-2009, 04:30 PM   #9
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Quote:
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WHEEL torque is what matters, and WHEEL torque is engine torque times the reduction factor of the transmission.
Aboslutely.

In machine design in general it's all about the final delivered torque. My biggest machine, for example, has an electric motor that puts out a max of just 21Nm. But in my case, I had functional requirements that led me to know I needed about 1600Nm "at the wheeels" (with some room to spare given only approximations of drivetrain losses). At that point, it's a matter of finding the most (cost-)efficient way to supply that torque. Given the available gearing I could package/supply (and its trade off wrt the speed requirements) the 21Nm motor was the best option I had. And that guy gives me the 1600Nm I need.

In my case, I have no transmission. I have one gear ratio. I rely on the characteristics of the motor to allow me to work through the speed range.

As an example, decades ago, electric motor performance was poor and required us to actually have 2 speeds. Nowadays with brushless motors you can get away without that. Again, as with cars, since I can't change gears at ALL, I need the torquiest motor I can find that can deliver the most torque as far up in the rev range as possible. And I do run out of torque anyway (that's just physics). Fortunately, the performance envelope of my machine is such that I don't need much torque as speed goes up.

EDIT: This also explains why the little extra hp/tq that the ZHP gets over regular 330i's go a long way. It's not just an extra 8lb-ft, it's 8lb-ft AND the shorter final gear, AND an extra 300rpm to use it all.
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Old 04-08-2009, 05:21 PM   #10
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I wasn't trying to challenge or criticize. I was asking b/c i didn't understand.

The autocross comment was a defense to the realization that I had no idea how these cars perform from a measurable metric. I know both are fast. I know I would pick the M3 as a daily driver out of the two b/c I have kids.

But according to the graphs you have put up, the M3 should be faster than the Corvette at least for a time off the line. That's surprising to me and I would be interesrted to know if it is true in the real world.

I also don't understand why the 'vette would leave so much potential untapped, if that is what its doing. Is this the choice of the engineers for "fast enough?" Perhaps the compromise point they chose? Or are there other considerations that require or at least make the torque drop desireable? I suppose fuel economy might be enough by itself.
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