Excerpt from Chapter 8 - Taking It to the Track

Torque, Power and Gearing

As you watch the races on Speed Channel, or in person at a road-racing track, you’ll notice the wonderful change in the engines’ song as the drivers slow down for corners and then accelerate out. What you’re hearing, of course, is the driver downshifting the car before the corner, and then upshifting as the car gathers speed out of the corner. What’s this all about?

What it’s about is always keeping the car’s engine at its strongest power point when you need pick-up. In an automobile, that relationship is measured not by horsepower, but rather by “torque.”

In technical terms, torque is the twisting power exerted by the engine crankshaft as it rotates. In simple terms, torque is the power to get the car to go faster. It’s that push you feel in the small of your back as you get on the throttle and start to accelerate.

Car designers often point out that while owners argue about which car has the most horsepower, the measure that matters more is the torque of the engine, since it is the torque that gets the car to start off from a stop and go faster when needed, such as when passing.

If you’ve ever looked at the plots of engine torque shown in the car tests in the automobile magazines, you’ve noticed that as the engine speed (“rpm” in gearspeak short-hand, which stands for revolutions per minute) rises, the torque increases, but only up to a point. At some point, as the engine speed continues to increase, the torque levels off, and then begins to decline.

For example, in stock condition, the MINI Cooper S produces about 135 pound-feet of torque at 2500 rpm. Torque rises rapidly with engine speed, reaching about 150 pound-feet at 3500 rpm, then more slowly until it peaks at 155 pound-feet at 4500 rpm. At that point, as rpm continues to increase, torque declines gradually to 120 pound feet at 7000 rpm.

What this means in practical terms, is that when driving your MINI, you want to have the engine running between 3500 and 4500 rpm at those times when you need greatest responsiveness and pick-up, such as when passing another car on the highway or pulling away after executing a pass on the track.

If you don’t upshift as you accelerate down the straight, or failed to downshift when entering a tight corner, you’ll find yourself on one side or the other of peak torque just when you need the additional pick-up. That’s why shifting gears is important. For best acceleration, you want to keep the engine revs in the range where the engine is generating the greatest torque.

By the way, downshifting is never used in high-performance driving to slow the car and it shouldn’t be used that way on the street, either; that’s what the brakes are for. (The one exception is in highway driving on long descents down steep hills. There it can be a good idea to downshift to a lower gear and use the engine compression to slow you down. That way you keep the brakes from overheating in case you need them before you get to the bottom of the hill. However, that is a different matter than spirited backroad or track driving.)

Similarly, as the car accelerates, the good driver doesn’t want to push the engine past its physical limits, so as they accelerate they shift up to a higher gear. That way the engine is producing as much power as necessary, but at the lowest possible engine speed.