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Re: Shift points - help please (long and tangled) (archive)

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Posted by Steve T on December 16, 1999 at 11:26:42:

In Reply to: Shift points - help please posted by Stan on December 16, 1999 at 08:54:16:

Alright, I'm just going to go into some theory here since I don't have an M3 or any dyno charts on an M3 to go from, just a crummy 325i. Anyway, here's how I understand it, I think it was explained in EC before they got to be a completely worthless magazine. You want to shift out of every gear at the point that will give you the maximum total horsepower in the next gear. For example, lets say your M3 peaks at 6900rpm (220hp?) and redlines a 7500 200hp, if you were to shift at 6900rpm you would maybe go down to 5200rpm 170hp and your total power in the next gear would be the integral of the power curve from 5200rpm 170hp to 6200rpm 220hp assuming you shift in the same place. If, however you waited to shift until maybe 7500rpm 200hp you would end up at about 5650 rpm which interpolating linearly would be 192.5 hp. So your total hp in the next gear would be the integral from 192.5hp to 220hp and back down to 200hp, which would give you a higher total power in the next gear.

To summarize (poorly perhaps, you need to look at a dyno sheet to really understand this) you want to space your shift points to maximize the area under the hp curve. If you were driving a Z28 you might peak at 4000 and fall all the way to the redline at 6200 rpm. You might then shift at 4700 and fall to the upside of the curve at 3400 and keep you shifting centered on the torque peak. So it isn't always at redline, although I'd imagine its pretty close on peaky M3 motor.

To summarize again, keep the torque peak centered in all of your shifts. Of course, that may not be true if the hp falls off rapidly after peak, or peaks rapidly and falls gradually but it'll probably be pretty close.

And of course since the spacing between all gears is different you have to shift at different points to keep the total power maximized.

On the other hand you may also benefit from keeping the car in a lower gear longer because of the torque multiplication at the rear wheels.... Anybody got any thoughts?

Steve


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