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Old 04-08-2009, 06:05 PM   #11
clyde
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Quote:
Originally Posted by John V View Post
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.
It may be splitting hairs and going in a different direction than you're talking about, but fuel economy is also tightly tied to throttle position. If you are traveling at a specific RPM with a constant throttle position, your fuel efficiency will be much better than if you rapidly alternated between wide open and closed throttle positions. If you jump on and off the throttle, obviously, the RPM won't be static, but if you do this over a set distance and turn the same number of revolutions and have the same average throttle position over that distance as the constant, you'll use a lot more fuel (how much depends on lots of variables).

How big or small the difference is relative the addition or subtraction of 500 pounds of load or 500 RPM...
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Old 04-08-2009, 06:29 PM   #12
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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.
More torque at the wheels, though, does not necessarily equal faster off the line. In addition to the weight issue (which is beyond my ability to quantify) there are other variables like traction--the Corvette has much wider rear wheels than the M3 does, and thus may be able to better take advantage of the wheel torque it does have than the M3 is.

JV, are your calculations torque at the contact patch, or torque at the hub? In other words, do you include the effect of tire diameter in your overall gearing calculations?

Also, WRT fuel economy, another huge variable (in fact, probably the biggest variable) is wind resistance. Since the Corvette has a smaller frontal area than the M3 and probably a better coefficient of drag besides, it simply takes less work to move the Corvette through the wind at a given speed.
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Old 04-08-2009, 06:39 PM   #13
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Torque at the hub. Once you take into account the diameter of the tire, you're talking thrust (i.e. force).

Fuel economy is tied to throttle position but not nearly so much as it is tied to RPM for gasoline engines. Anyone who has ever delved into the programming for a gas FI system knows this, because injector firing rate is ONLY determined RPM and duty cycle is tied to airflow. The trim functions here are throttle position, intake air temperature, coolant temperature, and some other minor contributors. The other factor here is pumping loss - which is lowest when the throttle is wide open. The most efficient way that we know of to operate a gas engine is to have no throttle and manage the speed with valve lift and duration (aka Valvetronic for BMW)... but it's not necessarily practical for all applications (read: it's expensive to implement and not easy to build).

In diesel engines, fuel economy is tied very closely to "throttle" position because there is no throttle - you're directly controlling the pulsewidth of the injectors.

The bottom line is the BMW is damned fast - especially for how heavy it is - but once first gear is done, the BMW is going to get smoked by the vette.
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Old 04-08-2009, 07:26 PM   #14
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The bottom line is the BMW is damned fast - especially for how heavy it is - but once first gear is done, the BMW is going to get smoked by the vette.
i guess that's the beauty of it, though. that a 3er is as fast for the first whatever period of time.

that's always the appeal of the M series. that a gran turismo is THAT fast.

one obviously pays the price monetarily, carbon emissions-wise, as well as gearing-wise, for that 1st gear performance. for most people of a certain means, it's certainly worth it.

my own personal journey with a leaden sled that has a rocket booster attached to it is that the curb weight really wears on you after awhile. the whole back end of the thrust proposition is that whole braking thing. no matter how good the brakes work, weight is something that is so visceral and it does literally weigh on you over time to constantly fight that static weight from a standstill as well as coping with that sheer mass on decel.

engine to engine, the BMW is almost peerless for a DOHCer in street guise.
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Old 04-08-2009, 08:18 PM   #15
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The M3 is not THAT bad... 3600lbs is the new 3000. A 997TT is as much. But yes, it is a gt, it is never going to feel like an s2000 or a cayman.
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Old 04-08-2009, 10:24 PM   #16
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The M3 is not THAT bad... 3600lbs is the new 3000. A 997TT is as much. But yes, it is a gt, it is never going to feel like an s2000 or a cayman.
True. And as I've mentioned too many times already, the E93 vs. E90 M3 test drives proved how much of a difference 3700 vs 4200lbs is dynamically.
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Old 04-08-2009, 10:36 PM   #17
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I love this thread. I really have nothing to add. It's a rare occurrence that I'm unable to out-nerd somebody. The fact that there are several posters in this thread alone that out-nerd me is either awesome or totally sad.

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Old 04-09-2009, 06:33 AM   #18
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The M3 has been a GT for 15 years-- like every other BMW with an M badge, other than the E30 M3.
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Old 04-09-2009, 07:19 AM   #19
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I'm still thinking about an M3 or 997S. They are both GT's, and right now, after 2.5 years of the more hard core cayman (not that it is faster, but it is light, loud etc.) I'm kinda looking for a gt.
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Old 04-09-2009, 11:31 AM   #20
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I'm still thinking about an M3 or 997S. They are both GT's, and right now, after 2.5 years of the more hard core cayman (not that it is faster, but it is light, loud etc.) I'm kinda looking for a gt.


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