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Horsepower, Torque and Gears

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Horsepower, Torque and Gears

Whether I’m dyno tuning or talking amongst racers, one thing that constantly frustrates me is the common misunderstanding about the difference and relationship between torque and horsepower. Now that we’re in the off-season, there's no better time to delve into the subject. My goal here is to explain the difference between torque and horsepower and the role gearing plays in the whole deal. For those of you that are too busy to read the whole article, I’ll make it simple: horsepower is everything – you can make up for lost torque with gearing.

Okay, you might be skeptical of that rather bold statement, but let me remind you a Formula One car has over 750 bhp but less than 300 lb-ft of torque. Even the big V8s in NASCAR’s Sprint Cup Series have 865 bhp but only 530 lb-ft of torque. If peak engine torque was that important, don’t you think these guys would have it? The reason these engines create so much peak horsepower relative to their peak torque, is simply because they rev.

Let’s break down how torque and horsepower are related. Torque is a measure of how much turning force the crankshaft is putting on the flywheel. Horsepower is a representation that tells us how much energy the engine is producing. If the engine continues to be able to apply a high torque force while the engine is spinning quickly, the horsepower is high. In fact, if an engine was able to produce the same torque at all engine speeds, the horsepower would be a straight line that increased with rpm. Horsepower shows us that it takes a lot more energy to apply the same turning force to the crankshaft at high speed, than it does at low speed.

It’s simple:

High force = high torque. High force at a high engine speed = high horsepower.

Based on that statement, you can see why you would want to tune the engine for high rpm. The faster you spin the engine, the higher the horsepower will be! The problem, however, is engines run out of steam at a certain rpm and the torque falls off to such an extent the horsepower goes down despite the fact that rpm is continuing to rise. To get the torque to stay up we need to make modifications to the engine that optimize its performance and breathing at high rpm, which means sacrificing performance down low – reducing peak torque.

The way we calculate the relationship between torque and horsepower is simple. If you know the torque the engine is producing and you know the rpm, you can calculate the horsepower. Say, for example, that torque is 300 lb-ft and the rpm is 6,000. The horsepower is equal to the torque multiplied by the rpm factor. The rpm factor is rpm / 5,252 (the rpm factor is 1.0 at 5,252 rpm). Torque and horsepower are the same at this engine speed. Below 5,252 rpm, the torque is greater than the horsepower. Above this speed, the horsepower is greater than the torque.

HP = Torque * (rpm/5,252)

HP = 300 * (6,000/5,252)

HP = 300 * (1.142)

HP = 343

This sounds great, but you know from practical experience that torque is important and modifying an engine for peak horsepower alone is a sure way to lose time on the race track. If you run a 16,000 rpm tuned F1 engine at 4,000 rpm, you’ll be driving a snail. You need to run the engine where it makes peak horsepower and ensure the horsepower throughout the powerband is as close to the peak as possible.

An engine with a sequential gearbox can get away with a much smaller powerband than a four-speed since it does not have to operate over such a broad rpm band. That doesn’t mean modifying your engine for higher horsepower and higher rpm will make it slower out of the corner though. Not even if that additional power comes at a loss of midrange torque! Let me explain.

Let’s look at two hypothetical engine setups. Both engines have identical blocks, but Engine 1 has a smaller cam as well as longer intake and exhaust runners. Engine 1 makes more torque but less horsepower. Engine 1 is only revved to 8,500 rpm as anything higher results in less and less horsepower. Engine 2 is tuned for higher rpm and revs to 9,000 rpm. Its peak torque is 20 lb-ft less than Engine 1 but, as a result of being able to keep the torque up at higher rpm, Engine 2 has an additional 25 horsepower.

Because we have enabled the engine to rev an additional 500 rpm, we can shorten the gearing by 6% (9,000/8,500 = 1.059) and have the same top speed in each gear. Where we might have were exited a corner at 6,000 rpm, we will now be at 6,350 rpm. Because of the increased rev limit and higher powerband, the shift points and gears around the track are unchanged. But, because we were able to add 6% more torque to the axle, we have significantly reduced the loss in low-end power compared to the graph above. This is made clear when we graph the horsepower between the two engines versus the ground speed:

See how the losses down low are almost non-existent? Notice how pronounced the power gains are over 120 km/h? The additional rpm allowed us to use mechanical advantage through re-gearing to get back the lost torque. When driving the vehicle, you would never know the difference, other than the additional power up top and the fact the rpm was a little bit higher, despite the fact the engine was making significantly less midrange power.

The point I’m getting at is that you can always re-gear to make up for lost torque, but you cannot re-gear to gain horsepower. If you can create more horsepower out of your engine, even at the loss of torque, it is usually worth doing, assuming you’re using the same powerband. If the engine cannot be revved any higher and your proposed changes lose torque at all but the top 1,000 rpm, it’s most likely not worth it unless you have a 20-speed gearbox. Usually though, more horsepower equals faster lap times!

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