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Discussion Starter #1
I've noticed that shifts from about 5K RPM in 2nd, 3rd and 4th all result in a 1K RPM drop - nice, closely spaced, even gear ratios. (With 6 to choose from, I seem to always be able to find one I like - along with 3-6 upshifts after getting on the freeway
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Anyway - how does one go about determining where the optimum shift point is for each gear to get maximum acceleration? I've been told it usually is NOT redline, but I'm not sure I understand why. My intuition says it should be the max RPM before the torque curve begins to drop. Is that correct? Does anyone have this figured out for the M5?

Thanks -

/Greg

 

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I have a book at home ( I am traveling right now) that details this out. It is BMW performance tuning book. I will get you more specific info when I return home this weekend. It essentially involves the balancing of the torque curve falling off at the limit and the need to maintain the revs when shifting to pick up the other end of the torque curve. I also have some rough charts made up comparing rear wheel motive force for some of the "top" performance cars.

You have to remember that torque provides the force along with gearing to accelerate the car. But you also have to remember that the higher the rpm the torque is generated at, the greater the gear reduction that can be used to multiply the force. That is why high horsepower cars depend on high rpm, it allows the gearing to turn their relatively low torque output into great force at the rear wheels.
 

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The idea is to keep the revs within the powerband which is between maximum torque (3800) and maximum horsepower (6600). You should shift in such a manner that the rpm drop reaches maximum torque.

Originally posted by greg:
I've noticed that shifts from about 5K RPM in 2nd, 3rd and 4th all result in a 1K RPM drop - nice, closely spaced, even gear ratios. (With 6 to choose from, I seem to always be able to find one I like - along with 3-6 upshifts after getting on the freeway
)

Anyway - how does one go about determining where the optimum shift point is for each gear to get maximum acceleration? I've been told it usually is NOT redline, but I'm not sure I understand why. My intuition says it should be the max RPM before the torque curve begins to drop. Is that correct? Does anyone have this figured out for the M5?

Thanks -

/Greg

 

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Take a look at the dyno graph that Jonh Hennessey has on his website to find out where the torque starts to drop. Make your shift just before the torque starts to decline, and continue through the gears, shifting at that same amount of RPM. Heck, the damn thing is so fun to drive just waste an afternoon altering you shift pionts (5,000-5,500-6,000 etc.)and find out what works best. Also keep in mind the surface you are driving on.
 

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Just to add to the confusion...

Remember that in 1st gear (and somewhat in 2nd) the engine revs so fast that the tach is 500-1000rpm behind when you're doing a full throttle wind-out.

Rick
 

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I was told you want to shift at a point where the engine has the maximum torque in the next higher gear. Usually this occurs at redline, but it depends on how the car is geared and the torque characteristics of the engine.
 

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Discussion Starter #7
Thanks! This makes sense. For the M5, this would mean shifting around 5000 I believe. (but, to the earlier post, YES the thing is SO fun to drive - just drive it. Yesterday I took the long way home. Now my mouth is sore. Couldn't stop grinning! This car is SO awesome - it just blows me away.)

Originally posted by Alan S:
I was told you want to shift at a point where the engine has the maximum torque in the next higher gear. Usually this occurs at redline, but it depends on how the car is geared and the torque characteristics of the engine.
 

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According to my calculations the most effective shift points are:

Shift RPM
1-2 6353
2-3 5757
3-4 5159
4-5 4674
5-6 4578

Each of these shift points should yield 3800 RPM (max torque in the M5) in the next higher gear.
The acutal shift points are probably slightly higher due to RPM loss when shifting. I have the spreadsheet if anyone is interested. Does anybody know if I can post the spreadsheet on this message board?
 

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Using the max torque point for the shift point is not necessarily correct. For one reason, if I am in first at a lower torque point than second, I will still accelerate faster in first because of the effective gearing gain. So if you shift too early you are giving up the gear advantage too early. This is why a car accelerates much quicker from 0-60 than 60-120 (ignoring wind resistance).

The correct way to determine the shift point requires calaculating the area under the curve between the two gears based on the torque curve and the rpm. This is detailed in the "BMW Enthusiast's Companion" published by the BMW CCA and available in most chain bookstores. It is an excellent source for these types of questions. When I get a chance I will construct these graphs for the E39 M5 and post them to the web. Fortunately, John Hennessey has provided us with Dyno results. I would highly recommend this book for anyone interested in performance modifications or who is just a gearhead.
 
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