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      01-12-2013, 10:09 AM   #45
driverman
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Drives: 2008 328i 6MT - SOLD
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Quote:
Originally Posted by dtc100 View Post
From a 335i guy.

Keep in mind it is not all about the curves. Even if the curve has fallen, keeping in the lower gear longer would still deliver more power to the wheels than switching to a higher gear, that is why there is this optimum shifting point that does not exactly follow the dropping curve.

As engine revs climb, Torque reaches a peak or plateau and then drops off. Some people think you should shift here. As revs climb higher, HP peaks and then drops off. Others think you should shift there. And others think you should always shift at redline. What's correct?

The key point is that you must consider tranny gearing AND engine T at different RPM when computing the best shift point.

Torque to the drive wheels = engine Torque * tranny gearing * axle gearing - drive train losses.

When you shift from 1st to 2nd, engine revs drop back into the power band of the engine and you get more Torque, BUT you loose the gearing advantage 1st gear had over 2nd gear. So the right shift point is when the engine puts out enough additional torque in 2nd gear to make up for the gearing advantage you had in 1st gear. If that doesn't happen, you shift at redline.

Here's the gearing for a 335i with 6MT:
1st gear 4.11
2nd gear 2.32

Can you see how it might make sense to stay in 1st year after T and HP have fallen off because of the huge difference between 4.11 and 2.32 gearing?

The Ideal shift point from 1st to 2nd can be computed if you gather the following info:

- engine T at different rpm
- 1st gear ratio: 4.11
- 2nd gear ratio: 2.32
- multiply 1st gear ratio by engine T at different RPM
- multiply 2st gear ratio by engine T at different RPM
- compute what the RPM drop would be when you shift from 1st to 2nd

If you did this, you could compare how much T is being delivered by the tranny output shaft in 1st gear and how much T you'd be getting at lower RPM in 2nd gear and see when 2nd gear's output exceeds 1st gear's output. There's your shift point. If 2nd gear's output never exceeds 1st gear's output, you shift at redline. If I had more time I'd do the computation and present it.

For most gasoline performance cars, the best shift point is redline. Just check the road tests in R&T, C&D, Automobile, Motor Trend and other car magazines. (Example: http://media.caranddriver.com/files/...ril-2012-1.pdf) Most manufacturers of performance cars build their engines to rev high because it's fun and it lets their cars stay in the lower gears longer to take advantage of the lower gearing.

Last edited by driverman; 01-12-2013 at 10:44 AM.