How To Get Maximum Acceleration Part III

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By: Michael R. Clements
(This article is part of a series. To read the previous article click here)

Example Engines

A couple of real world examples should demonstrate how these ideas apply equally well to two completely different engines. First, a 46ci Suzuki Katana motorcycle engine; next, the 454ci Chevrolet V8. In the charts shown below, the green/grey line is torque and the red/black line is power. In these examples, I assume that shifting up one gear lowers the engine RPM by 25% -- if you're at 3,000 RPM in a gear and you shift up, you will be at 2,250 RPM. While this is pretty close for most vehicles, the actual number used does not affect the conclusions (try using a different number and see for yourself).

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Here is the torque/power curve for the Suzuki:

The torque peak is 48.5 ft.-lbs. at 10,290 RPM; the power peak is 100 hp. at
11,550 RPM. Following the torque curve, you'd shift up just past 10,290 RPM to
keep the engine near its peak torque output. Following the power curve, you'd
shift up just past 11,550 RPM to keep the engine near its peak power output.

Nobody would advocate shifting before the torque peak at 10,290 RPM. The question is how far past this point to go before shifting up to the next gear. In the chart below, each row tells you what happens before and after shifting gears at any point in time. For example, the first row says that if you are at 10,290 RPM in one gear and you upshift, you'll be at 7,718 RPM. The two shaded columns give you the final torque at the wheel of the bike before and after the shift -- in this case, 48.5 before the shift, vs. 34 after the shift. The actual wheel torque will depend on the final drive ratio, but that is irrelevant because it will be proportional to these numbers.

BEFORE SHIFT			AFTER SHIFT
RPM	CRANK T	RATIO	RPM	CRANK T	RATIO	FINAL T
10290	48.5	1	7718	45	0.75	34
10500	48	1	7875	45	0.75	34
11000	46.5	1	8200	45	0.75	34
11500	45	1	8600	42	0.75	31.5
12000	40	1	9000	43	0.75	32
12500	38	1	9400	46	0.75	34.5

As the table above shows, the increased torque produced at
lower RPM never overcomes the gearing disadvantage of the higher gear. The
engine hits redline before reaching the optimal shift point. Thus, the optimal
shift point is redline (12,500 RPM).

Here is the torque/power curve for the Chevy:



The torque peak is 440 ft.-lbs. at 3,500 RPM; the power peak is 333 hp. at
5,000 RPM.

Again, nobody would advocate shifting before the torque peak at 3,500 RPM. The question is how far past this point to go before shifting up to the next gear. This chart works just like the last one.

BEFORE SHIFT			AFTER SHIFT
RPM	CRANK T	RATIO	RPM	CRANK T	RATIO	FINAL T
3500	440	1	2625	400	0.75	300
4000	407	1	3000	427	0.75	320
4500	376	1	3375	435	0.75	326
5000	351	1	3750	421	0.75	316
5500	306	1	4126	399	0.75	299

As the table above shows, the increased torque produced at lower RPM never overcomes the gearing disadvantage of the higher gear. As with
the motorcycle, the optimal shift point is redline (5,500 RPM). This probably
comes as a surprise to a lot of big V-8 owners!

When to Shift Gears

Finally, let's answer the original questions. First, what is more important for acceleration, power or torque? The answer should by now be obvious: POWER. The above discussion should make it clear that neither torque nor RPM is alone sufficient to determine acceleration. Since one is force and the other is leverage, it is their product that determines acceleration. And their product is power. The single most important factor determining acceleration is a car's power to weight ratio. For good acceleration you want a light car with lots of power.

Second, what determines the top speed of a car? The answer is much impler than it seems: once again, POWER. The whole question of acceleration vs. top speed is a red herring. Both depend on the amount of torque at the wheels of the car. And power at the engine crankshaft determines how much torque you can get to the wheels of the car. So the single most important factor determining top speed is the ratio of a car's power to its coefficient of wind resistance. For high top speeds you want an aerodynamic car with lots of power.

Third, when should you shift gears for maximum acceleration? The answer is that you shift on the POWER curve. As the above examples demonstrate, if you shift just after the engine’s peak torque RPM you aren’t maximizing the torque at the wheels of the car. Your shift points should surround the peak power RPM. More precisely, the optimum shift point for any engine and transmission is always somewhere between the peak power RPM and redline. If you're not inclined to get into the physics of it, just split the difference and shift halfway between the two -- you'll always be very close.

To be precise, the optimum shift RPM is the point at which the engine’s power output is the same before and after the shift. So your ride the power curve to the peak and over just far enough so that when you shift, you’re at the same power output but you’re climbing the curve again. Thus, how far past the peak power RPM you go, depends on how close together your gear
ratios are. Since most cars have the lower gears spaced farther apart than the higher gears, the optimum shift point is usually redline in 1^st
gear, dropping by a hundred RPM or so in each successive gear. But since the
peak power RPM and redline are usually close together, splitting the difference and using the same point for all gears is a good strategy.

Conclusion

Here are the key points which the above discussion and
examples have demonstrated:

1. Torque is not conserved through the drive train -- do not mistake
crankshaft torque with torque at the wheel of the car.

2. Engine RPM and Torque are both equally important for
acceleration.

3. Power tells you what the engine can do – torque and RPM alone
each mean nothing.

4. For maximum acceleration, always shift between the engine’s
peak power RPM and redline.

To contact Michael R. Clements send an email to: mike@mclements.net