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      12-07-2007, 12:33 AM   #183
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Originally Posted by lucid View Post
Bruce, not sure what to make of this statement. What makes you say gearing matters during the initial 60 ft and doesn't matter after that? The car is accelerating both before and after 60ft, so how can torque at the wheel and a high redline not matter after 60 ft?
First let's dispose of the redline issue: It's immaterial except as a potential method for making more horsepower, so it's power that directly matters, no matter the rpm.

As for gearing, perhaps the best way to illustrate my point is to visualize identical vehicles except for gearing. One is stock, and the other has been treated to a more aggressive final drive ratio.

In a drag race, the geared car, given reasonable traction, will launch harder than the stock vehicle, and begin to pull away - and the reason for this is that it's making more power because it's operating higher in the rev range. Sure, you can do the mechanical advantage/torque at the drive wheels thing if you so desire, but you'll find that horsepower is a great shorthand in this environment. Enigma may be impolite, but he's exactly correct.

OK, now the cars are well underway, and the geared car grabs second. At this point the stocker is still in first and higher in the rev range so it's now accelerating harder than the geared car - because now it's making more power at that point.

Then the stocker has to shift, and the geared car is now higher in the rev range and making more power, so it will again be pulling harder at that point. Then the geared car shifts, and...

You see the point. Once the cars are well underway, it becomes a seesaw battle. First one car is pulling harder, then the other.

The geared car has the theoretical drag race advantage because it's got the early advantage, mostly off the line given copious traction, but also because it gets an early advantage in each gear, while the stocker's advantage points are delayed. This is mostly an ET thing, but terminal speeds are often mildly affected because the geared car may have an overall average power advantage during the course of the race. This is by no means a given however, and is dependent on the vagaries of shift points and where the finish line falls in the race.

For a good, though incomplete, analysis of a gearing change on an E46 M3, just click on this.

That note illustrates the see-saw battle quite well, I think. It does in fact ignore traction issues, however, and since the M3 in our garage always has trouble in first gear even with stock gearing, that may be an issue. Perhaps more important is the fact that it also ignores rotational inertia, and the additional rotational inertia penalty given as a freebie to the geared car will limit its advantage - primarily because its penalty is enacted early in each gear, while the stocker is penalized only after the geared car has shifted (each time). The geared car still wins, but by a reduced margin.

I know that nobody talks about this (rotational inertia), but it's far from a tiny issue. As an example, if you have a car with a three to one first gear and a two to one second gear, you would naturally expect first gear to accelerate 50% harder in first gear than in second. Not so, however. The least loss I've encountered during instrumented testing on many cars over the years is about 12%. The most is near 20%. That particular record was achieved in a German car, and we well know that German cars use flywheels that may as well have been liberated from old Panzer tanks, or U-boats.

OK, I'll shut up now. Too many points

bruce.augenstein@comcast. is offline  
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