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      01-12-2013, 02:17 PM   #51
driverman
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Drives: 2008 328i 6MT - SOLD
Join Date: Nov 2008
Location: El Dorado Hills, CA

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I should be doing other things, but my love for cars is trumping duty this morning.

I worked up some numbers to illustrate my point based on ROUGH APPROXIMATIONS of the N54's torque curve and a 6MT. Torque after tranny is T * gear ratio.

1st gear = 4.11
RPM T mph T after tranny
1500 250 8.4 1027
2000 300 11.2 1233
2500 300 14.0 1233
3000 300 16.8 1233
3500 300 19.6 1233
4000 300 22.4 1233
4500 300 25.2 1233
5000 300 28.0 1233
5500 280 30.8 1150
6000 260 33.6 1068
6500 240 36.4 986
7000 210 39.2 863

2nd gear = 2.32
RPM T mph T after tranny
1500 250 15.0 580
2000 300 20.0 696
2500 300 25.0 696
3000 300 30.0 696
3500 300 35.0 696
4000 300 40.0 696
4500 300 45.0 696
5000 300 50.0 696
5500 280 55.0 649
6000 260 60.0 603
6500 240 65.0 556
7000 210 70.0 487

In this example, if you wind out 1st gear to 7K, you'll have 863 lb-ft of torque on the output shaft of the tranny and you'll be going 39 mph. When you shift to 2nd, you'll drop down to about 4000 rpm and have 696 lb-ft of torque on the output shaft of the tranny. Therefore, even though T and HP are dropping off in 1st gear, you are better off revving to redline to take advantage of the shorter gearing 1st gear provides.

The same thing might not be true for other shift points. You have to run the numbers. A good cross check would be to see how far the car magazines rev the engines in each gear when they do their road tests. They are always trying to get the fastest runs.