Horsepower and Torque

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Horsepower

The definition of 1 horsepower is the ability to do 33,000 pounds-feet of work in one minute. So horsepower is a measurement of torque over time. The measurement of horsepower was invented by James Watt when he replaced horses turning a water pump with a steam engine and wanted to know how many horses the engine could replace in order to rate the engines power. He figured that a horse could pull with 180 lbs. of force. The horse traveled in a circle with a 12 ft. radius, and could make 144 complete revolutions in 1 hour. This means that the horse traveled 181 feet per minute. 180 lbs. of force times 181 feet equals 32,580 pounds-feet of work in 1 minute. Watt rounded that up to 33,000 pounds-feet of work in 1 minute.

Figuring Engine Horsepower

Watt's definition of horsepower is a horse pulling on lever, so the power was applied to the lever. An engine does not work like this, the power is made at the crankshaft. So to know the horsepower of an engine you will need to imagine a 1 ft. lever at the crankshaft and figure the circumference of that one ft. lever and how many rotations per minute it is traveling. A 1 ft lever will have a circumference of 6.2831853 ft., so the formula for hp would look like this:

HP = (6.2831853 × RPM × Torque) ÷ 33,000

And can be simplified to:

HP = (rpm × Torque) ÷ 5252

By using this formula you can see that HP and Torque curves will always cross at 5252 rpm. To help put horsepower into perspective, imagine two engine that both make 300 ft. lbs. of torque One engine makes it at 3000 rpm and the other makes it at 6000 rpm. Which one makes more power? The one revving at 6000, because the same amount of torque is being used twice as fast. Gearing the engine down to the the same 3000 rpm will double the torque at the final drive making it 600 ft. lbs. If both engines had the same final drive, they would be able to do the exact same amount of work (300 ft. lbs. worth), but the 6000 rpm engine will do it in 1/2 the time.

Torque

Torque is simple, imagine a 1 ft. lever with 1 lb. of force pushing on it. That is 1 pound ft. of torque. The formula to figure torque from a known HP is:

Torque = (5252 × HP) ÷ rpm

Brake Horsepower and Torque

Dynamometers measure torque by use of a brake. Brake torque is simply a measurement of how much resistance is needed to hold the engine at a steady rate of speed. This is commonly referred to as a step test, usually taken in 250 rpm increments. Brake Horsepower (BHP) is then figured using the above formula. The problem with brake torque is that it is not effected a measurable amount by the inertia of the engines rotating and reciprocating parts. For an acceleration engine, there is a better way.

Acceleration Horsepower

For an acceleration engine, you want to measure power as the engine is accelerating. You don't only care how much HP the engine has, you also want to know how fast it can rev up. This will take into account all of the inertia of the rotating and reciprocating parts. So instead of a brake holding the engine at a steady speed, the brake holds the engine at a steady acceleration rate, 300 rpm per second and 600 rpm per second are commonly used. Hp and Torque readings will be lower, due to less braking force required, but the mass of all the moving parts will be better measured. You could have two engines put out the exact same brake power readings, but if one has much lighter pistons crank and rods, it have more acceleration power and be quicker at the track. To put this into perspective, it is easy to lose 6 lbs. from a stock small-block Chevy crankshaft and actually make it stronger in the process. My Camaro has a 3.27:1 1st gear ratio and a 3.50:1 rear gear ratio making a total ratio of 11.445:1. Losing 6 lbs. rotating weight is like losing over 68 lbs. off the car in 1st gear. That's just the crank, using lighter pistons, rods, pins, and aluminum flywheel can get the rotating and reciprocating mass over 20 lbs. lighter. Now 229 lbs. lighter in 1st gear is a big difference. You see why people get crazy to loose a few lbs.

Building Power

Many engine builders will say that torque is what wins races and too much emphasis is put on horsepower. It is true that torque is what actually pushes the car, but torque and horsepower are directly related, you can't have one without the other. The chart below shows how you can increase the rpm range of an engine to get more horsepower, at the cost of torque, but still make more rear wheel torque.

In this example, both engine have the same rear wheel speed, but the higher rpm engine, which makes less flywheel torque, makes more rear wheel torque. Torque does win races, but you need horsepower to put it to use, so saying torque is more important than horsepower is really a contradiction. For a street car, an 8000 rpm engine and steep gears are not practical, but for all out drag racing, build as much horsepower as you can and just gear it to suit. If you build the horsepower, the torque will be there through gearing. The engine with the most average hp for the rpm range used will have more average power, period.