If you are screaming through 3rd gear with your foot on the floor as the engine is passing through its peak torque RPM, you may think you're getting the best acceleration you can. And you are, for 3rd gear. If you drop down to 2nd gear the engine will be spinning at a higher RPM and will be producing less torque. But if you think this means your acceleration would diminish in 2nd gear you just fell for a mistake that has fooled thousands before you -- mistaking crankshaft torque for wheel torque.
Second gear is a shorter ratio so the torque at the wheel of the car is a larger multiple of the torque at the engine crankshaft. In 2nd, the engine may produce more torque at the wheel of the car, even if it has less torque at the crankshaft. Remember it's the product of torque and gearing that pushes the car, so if you have a little less torque with a lot more gearing, you're going faster.
Here’s another way to look at this: remember that 3rd gear is a taller (numerically smaller) ratio than 2nd, so it provides less acceleration. Suppose the difference between the two ratios is 30% (any percentage will do, but this is typical). That means that when you are accelerating in 2nd gear, the moment you shift to 3rd you will have 30% less torque at the wheel of the car – you will lose 30% of your acceleration. Thus, you should not shift to 3rd gear until you have passed the engine’s torque peak, and the torque has dropped at least 30% from its peak value.
Many racers try to max out their acceleration by “feel”. And since the acceleration of the car follows the torque curve of the engine, you can feel the acceleration decreasing as the engine passes the peak torque RPM. And the moment you feel the car’s acceleration start to diminish, you instinctively grab for the shifter. But then you shift too soon. You’ve maximized the torque at the engine crankshaft, but you haven’t maximized the torque at the wheel of the car. As you rev the engine past the torque peak, you can feel your acceleration gradually diminishing. But if you were to shift, the taller gear ratio would reduce your acceleration even more. So you must resist the urge to shift and let the engine keep revving a bit higher than is intuitively obvious.
The bottom line of all this is the following equation:
|
| P | engine power in horsepower |
| π | pprox.
3.14159265358979323846264338327950288 |
| n | engine speed in RPM |
| t | engine torque in ft. lbs. |
Removing constant factors (2π / 33000), this equation
becomes: |
| P = nt |
Which means power is the product of torque and RPM. Since RPM represents the potential gearing that can be used, power tells you how much torque you can get at the wheel of the car at any given speed[1].
Incidentally, we mentioned above that gearing trades rotational speed (in this case, engine RPM) for torque and vice versa. Torque is not conserved through the drivetrain (neither is rotational speed). But their product, power, is conserved. Thus, this equation shows that gearing tradeoffs between torque and rotational speed are always linear. In reality, drivetrain power efficiency is less than 100%, but it is very high, usually 90% or better. Usually, lower gears are less power efficient than higher gears. This is why dyno results show a slightly different torque curve for each gear.
We can draw other conclusions from this mathematical relationship.
First, since power is the product of torque and RPM, power cannot max out until torque is already decreasing. Thus, the peak power RPM is always higher than the peak torque RPM.
Second, whether the peak torque RPM and the peak power RPM are close together or far apart, depends on how rapidly the torque rolls off after it peaks. If the torque rolls off quickly after it peaks, the power peak will be close to the torque peak.
Third, every engine has a range of RPM just above the torque peak, where the torque is rolling off, but engine RPM is climbing faster than torque is dropping. Since power is equal to the product of torque and RPM, the engine’s power output is still increasing. Then, at some even higher RPM, torque rolls off faster than RPMs climb. The peak power RPM is the transition point where the rate of decreasing torque matches the rate of increasing RPM. Past this point, torque is dropping rapidly – “like a rock” as some would say.
Many people like engines that get all their torque down low because "you don't have to rev it up to get to the power". While these engines are easy to drive, they are slower on a racetrack or autocross. Because they get their torque at low RPM, they cannot take advantage of short gearing to multiply that torque. Thus it is no mystery why some of the fastest cars in the world are the hardest to drive. They are designed to achieve their maximum torque at the highest possible RPM. The higher RPM at which they achieve their torque, the more gearing they can use to multiply the torque even higher. If achieving peak torque at 2,000 RPM allows you to use a 4:1 gear ratio, then getting that same torque at 6,000 RPM allows you to use a 12:1 ratio, which means three times the torque at the wheel of the car. That's three times the acceleration, which is the difference between doing 0 to 60 in nine seconds, or in three seconds[2].
Of course, developing strong torque at high RPM doesn’t necessarily mean the engine will have weak torque at low RPM. Some engines manage to develop peak torque at high RPM, but still get 90% of their peak torque throughout the entire RPM range. Such engines used to be rare marvels of engineering. With today's sophisticated engine electronics and variable valve timing, such engines are becoming more common.
This article is part of a multipart series. You can read the next installment by clicking here
By: Michael R. Clements
I am in the middleof a build and sure would like tips for torque, lots of it! Mark
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