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      09-18-2010, 06:15 PM   #1
Mr. 5
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2007 e90 335i  [4.00]
When to Shift Discussion--The right RPM

Many have talked about the perfect RPM to shift and I thought I would put something out there to explain about how gear ratios have a lot to do with dictating when to shift.

Some people (including myself) have thought that when the horse power starts to drop then that is when one should start shifting, but it has more to do with that.
Horse Power is actually derived from torque so we'll focus on just the torque.

This is what you need to know:

1. The Torque curve throughout the RPM band with your car. Someone else's curve may not necessarily be the same as yours so their shifting points may be different.
2. The gear ratios of each gear.

I'm going to use a Torque curve from my GIAC Stage 1 flash.
I used the Dynograph to determine the torque at each RPM that I randomly chose. I tried to pick increments of 250 RPM.



RPM TRQ
2500 327.7
2735 333.15
3009 360.32
3264 357.89
3498 359.51
3767 363.05
4008 367.43
4270 368.73
4504 369.67
4737 369.7
5006 367.14
5290 354.53
5502 347.33
5743 334.45
5998 320.76
6246 307.97
6501 291.56
6763 271.74
6997 229.74

Since I have an MT, I'm going to use the MT gear ratios.

1st--4.06
2nd--2.4
3rd--1.58
4th--1.19
5th--1
6th--.87

Now, take the torque numbers and multiply the numbers by the ratio of each gear.

1st gear numbers will look like this:

1330.462
1352.589
1462.8992
1453.0334
1459.6106
1473.983
1491.7658
1497.0438
1500.8602
1500.982
1490.5884
1439.3918
1410.1598
1357.867
1302.2856
1250.3582
1183.7336
1103.2644
932.7444

2nd gear numbers will look like this:

786.48
799.56
864.768
858.936
862.824
871.32
881.832
884.952
887.208
887.28
881.136
850.872
833.592
802.68
769.824
739.128
699.744
652.176
551.376

Now, graph each curve. I used excel.
Get the peak number of the gear you're shifting into (in this case it's 2nd gear) and draw or make a horizontal line ratio numbers.
The peak number is 887.28.

When 1st gear numbers, 2nd gear numbers, and 2nd peak number is graphed, it looks like this:



Notice that even at redline (7k RPM) you will never reach the amount of torque that 2nd gear puts out so there is no need to shift early.

When doing the same thing with 2-3 shift, you’ll see that at approximately 6800 RPM is where the peak of the next gear torque hit’s the curve. This is the optimum shifting point.



Do this with each gear and you’ll find the following:

3-4 Shift 6600 RPM



4-5 Shift 6200 RPM



5-6 Shift 6000 RPM



So, to summarize, the best ultimate shifting point for this car making this amount of power is the following:

1-2...7k
2-3...6800
3-4...6600
4-5...6200
5-6...6k
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