Whats the best shift RPM for best acceleration?
01-04-2007, 02:45 PM
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I looked at the E60 torque curve for the 545. colejl provided this link http://www.e60.net/information/options/engines/N62B44 for the torque curve. It seems that if you shift at 4200 rpm and drop 800 rpm with an upshift, you will have the most torque available for acceleration and thus quicker acceleration times. Most people and most autoshift programs bring the car up to redline before it shifts. Is torque king for acceleration or is it Horse Power?
01-04-2007, 03:22 PM
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Originally Posted by pennetta' post='373735' date='Jan 4 2007, 11:45 PM
Is torque king for acceleration or is it Horse Power?
However...
a) Gearshifts take time. So you may be better off redlining and saving one shift, depending on what you are trying to measure (i.e. 0-60 mph, 0-100 km/h, 0-200 km/h, quarter mile, ...) especially if the critical value occurs just after an "optimal" but early gear change
b) You are effectively demultiplying torque (and therefore force) if you upshift, even though torque at the crank may be higher when the revs drop.
Just to clarify what I mean by b), let's make a simple example:
Imagine a car with two gears, total transmission ratio 1:3 and 1:1 respectively
Max torque is 100 Nm at 2000 rpm, max power torque is 80 Nm at 6000 rpm, and red line is at 7000 rpm with 60 Nm.
In first gear, effective torque at the wheels is 80 x 3 = 240 Nm at 6000 rpm. If you shift at this point, you go back to 2000 rpm, so your crank torque goes up, but your wheel torque decreases to 100 x 1 = 100 Nm. Whereas, if you keep going until red line in 1st gear, your torque at the wheels is 60 x 3 = 180 Nm.
End result: you accelerate faster by staying in gear and redlining it, than by shifting at the optimal point. And that's ignoring the gear shift time.
01-04-2007, 05:30 PM
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Yes that was great dlev67! Its also more fun to run up to redline. So I guess if you were designing an engine for max acceleration you would design the highest torque output to be closer to the redline.
01-04-2007, 10:37 PM
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Cool explaination. I knew redline was best but could never explain it... Thanks!
01-04-2007, 11:53 PM
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great explanation dlev67! ....made perfect sense 
01-05-2007, 12:58 AM
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Originally Posted by dlevi67' post='373746' date='Jan 5 2007, 12:22 AM
In theory it's torque. F = ma, as Newton was fond of saying. Given that the mass of the car is constant (give or take a little fuel), acceleration is greater when force (i.e. torque) is greater.
It's always great to try to illustrate answers to deliver meaning (and your post conveyed that extremely well as always) but using erroneous analogies doesn't aid erudition!
To simplify my particular point:
Torque and Force are not interchangeable physical entities. At the most basic, force is measured in Newtons (N) [lb wt] and Torque is Measured in NewtonMetres (Nm) [lb ft].
Particularly, F=ma (or, bizarrely when I studied it, P=mf!!) does not in any way refer to a turning moment (a la torque) but rather to uniform motion in a straight line.
ABC
01-05-2007, 10:04 AM
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Originally Posted by dlevi67' post='373746' date='Jan 4 2007, 04:22 PM
End result: you accelerate faster by staying in gear and redlining it, than by shifting at the optimal point. And that's ignoring the gear shift time.
01-05-2007, 10:36 AM
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Originally Posted by Rudy' post='373768' date='Jan 5 2007, 04:32 AM
Sounds better too! 
Rudy!I'm with you.
But didn't Znod and his friends do quite a lot of testing and track times for the 1/4 acc?