TORQUE: USEFUL CONCEPT or AUTOMOTIVE RED HERRING ?
By Bob Palmer
VIN:
March 4, 2000
Practicalities (cont.):
To see what these numbers mean in terms of engine rpm changes, let’s be somewhat arbitrary and see what happens if we make all our shifts at 6,400 rpm. A little simple math gives the following:
| Shift |
Close Ratio
|
Avg. rpm
|
Wide Ratio |
Diff. rpm |
| First-Second |
6,400 - 4,662
|
5,531 (2nd)
|
6,400 - 4,443
|
219
|
| Second-Third |
6,400 - 4,885
|
5,642 (3rd)
|
6,400 - 4,576
|
209
|
| Third-Fourth |
6,400 - 4,961
|
5,680 (4th)
|
6,400 - 4,638
|
323
|
In the first column we have the rpms at each shift point for a close ratio and in the third column are the corresponding numbers for the wide ratio tranny. The second column gives the average rpm in second, third, and fourth gears assuming we also run out to 6,400 in fourth (first gear is unspecified since we haven’t said what rpm we start at). We can see by this example that the motor must operate over a 1,500-2,000 rpm range in each gear. The fourth column are simply the differences between the wide ratio and the close ratio rpms at the shift points, which are only about 10-20%. In fact, the gear changes on a wide ratio toploader are actually even closer that on the newer Mustang five-speeds; especially between first and second gears and between fourth and fifth gears.
Let’s get back to the main issue here which is about getting the most out of the motor. If you have dynamometer data for your engine (or at least a similar engine) that shows horsepower versus rpm, you can then compare the dyno curve with the shift point numbers and see how well you are doing relative to maximizing the power between the shift points. We see in this particular example that while we are running in second gear our average rpm is 5,531, in third gear it is 5,642, and in fourth gear it is 5,680 (assuming we run all the way to 6,400 again). Given this information, obviously the motor should be making its best horsepower around the average rpm we’re running which, in this case, is a little over 5,600 rpm. The optimum average rpm will actually fall a little bit lower than the peak horsepower rpm because the horsepower curve is not symmetric and typically falls off faster past the peak horsepower point than before it.
Before delving into further details, let’s take stock of what we’ve learned from this hypothetical (but not unrealistic) exercise. Imagine building a real engine that is matched to the above example in terms of its horsepower curve. This motor would probably make its peak horsepower at around 5800 rpm. Then, to get the most out of this motor, we need to run it about 600 rpm past the horsepower peak in each gear. This is really a pretty general conclusion, although the exact numbers will depend on the exact characteristics of the motor, and whether you have a close ratio, wide ratio, five-speed, etc.
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