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      12-08-2007, 03:44 PM   #193

Drives: E92 M3 and Elise
Join Date: Oct 2006
Location: Land of the Microchip

iTrader: (0)

Originally Posted by bruce.augenstein@comcast. View Post
There's a chance we may have another misunderstanding here, but that said, gearing and redline essentially have no bearing on the matter, which in fact is only power and weight. In addition to the information I've posted on the topic in another string in this file, there are in fact only three caveats here that I know of. The first is that, having a redline a good deal higher than the power peak means that you can wind the engine out higher, and thus arrive in the next gear further up on the power slope. In that single case, a "higher" redline (in this case meaning higher than the power peak, rather than in absolute terms) will give you an acceleration advantage. The second is that, given an engine with an 8400 rpm power peak and another making the same 414 HP and 295 foot pounds at 4200 (each at their redlines), each with the same approximate internal rotating inertia and the 8400 rpm engine geared twice as aggressively as the 4200 rpm version (so that each engine can get into its power band with roughly the same alacrity), I'd bet on the 4200 rpm combo to win in a drag race, simply because it has less overall rotating inertia to contend with. The third is that I'm assuming any mass production car is geared in such a way as to take advantage of its power band in everyday use. No fair installing gearing tall enough so that the engine can't get into its power band until you've hit 80 mph in first gear, as an example. Hence my comment in another string that, after the first 60 feet or so, gearing essentially doesn't matter.

As we've said before, horsepower is the great simplifier in this context. You can certainly go through the torque-at-the-drive-wheels calculations, but when you do, you'll see that you didn't have to.

A few comments. High TQ engines tend to be heavier which partially negates the rotational inertia advantage.

The other point you touch on is what i always refer to a a good powerband. You want to choose your shift points to maximize the area under the HP curve from the HP plot. In a car like the M3 where peak HP is so close to redline its easy, you just shift at redline.

However, if you had the same engine but with a 10k redline you would want to shift higher, past the HP peak. In an ideal world you would shift very close to the point where the HP had dropped the same hp value the car is going to make in the next gear after your shift.

With the current motor thats not possible. However, this is why some engines that produce less peak hp sometimes get better acceleration numbers.

From the graph its likely the engine will spend most its time most its time between 6500 and 8400. In that range it will make between 360 and 420hp. You could integrate the area under the curve and get a good aproximation. Just eyeballing it we look like we will on average be using about 93% of the cars power. Sometimes 100% and sometimes 85%

The Lexus has a flatter HP curve in its dyno. As a result its going to utlize a greater average % of its peak power. This gives is a slight edge that just comparing peak numbers would not make aparent.