The Difference Between Horsepower & Torque - How It Works

What’s the Difference Between Horsepower and Torque? The Answer Might Surprise You

Stephen Kim Jun 1, 2011 0 Comment(s)
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Harold Bettes: For people who say that they would rather have torque than horsepower, you have to think about where in the rpm range that torque is produced. Torque multiplication with the transmission in Reverse is greater than in any of the forward gears. If that’s the case, a car will produce the most torque at the rear wheels while in Reverse, so shouldn’t you just leave it in Reverse and mash the gas? Of course not. Likewise, if you prefer torque over horsepower, maybe you should try leaving a car in First gear all the way down the dragstrip. Let’s say you have a motor that makes 400 lb-ft of torque that’s matched with a 2.50:1 First gear ratio and 4.00:1 ring-and-pinion set. That equates to 4,000 lb-ft of torque that’s being applied to the axles at the engine’s torque peak. However, since the transmission gear ratios get taller with every upshift that means the torque applied to the axles drops significantly in each successive gear. Obviously, if you leave the car in First gear just to maximize the torque that’s applied to the axles and wheels, you’re not going to go very fast. This example clearly illustrates that it’s not torque that pushes a vehicle down the road, but rather horsepower.

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RPM = Power

Judson Massingill: In the racing world, the motor that makes the most explosions going down the track is going to win the race. In other words, the motor than turns the most rpm is probably going to win. Again, that’s because torque per cubic inch is difficult to increase, so the next option is turning more rpm. Not surprisingly, unless the rule book restricts it, engines in every form of racing turn more rpm every year. With today’s cylinder head technology, torque curves stay flat even at high rpm so turning more rpm is the key to making power. In most race engines, horsepower will continue climbing even after peak torque, and as long as rpm increases faster than the rate at which torque drops off after its peak, horsepower will continue to increase. An extreme example is Formula 1. Since the rules limit displacement to 2.4 liters, the motors barely make more than 200 lb-ft. By revving them up to 18,000 rpm, however, they make an incredible 800 hp.

Jon Kaase: Horsepower is torque multiplied by rpm, so if you have a drag car with flat torque curve, the horsepower will keep climbing for a while because of the way the math works out. In other words, if an engine produces lots of torque at high rpm, it will make a ton of power. A Pro Stock motor makes about 1.7 lb-ft per cubic inch, while a very healthy street engine can make as much as 1.5 lb-ft per cubic inch. The big difference is that a race engine will continue to maintain high torque output even after its torque peak. All they’re doing in a Pro Stock engine is raising the rpm where torque is made, and they don’t make that much more peak torque than they did six to eight years ago. Most engines will usually turn a couple thousand rpm past their torque peaks. If horsepower drops off any quicker than that, it usually indicates a problem with the motor. Either the heads, camshaft, intake manifold, carb, or headers are too small, or it’s running out of valvetrain. Scott Shafiroff: In racing, rpm is critical. If you think about it, the greater the number of explosions per second an engine produces, the better it will accelerate a car down the track. In race cars, you look at the horsepower an engine makes and the rpm it turns, and gear it accordingly. You never want a motor to be below peak torque, which requires keeping the rpm up. On each gear change, you want the rpm to drop down to the torque peak and then accelerate beyond power peak. Engine design is about making compromises. You don’t want to trade a lot of torque for only 2 hp, but you do need to sacrifice some torque in order to make horsepower up top.


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