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http://www.canadiandriver.com/articles/jk/020327.htm
I get asked a thousand times a year - how can I get more power from my car/truck/SUV/motorhome - you name it! What many people really want is better acceleration, or improved fuel economy, or to be able to pull a bigger load. Before putting a pile of money into the engine, we should take a look at what power really is, and how it moves our vehicles.
Look at almost any automobile review and you will find horsepower and torque listed in the specs. For example, my "Quickmobile" has 200 horsepower at 9000 rpm and 57 ft lbs. torque at 2000 rpm. Is this vehicle really quick, or do numbers sometimes play tricks on us? Well, it has 200 horsepower. What does that mean?
Horsepower is related to torque, and torque is turning force. Place a lever on the engine's crankshaft and measure the twisting force produced, and you have torque. Note that time is not a factor - torque can be constant over time. Horsepower, on the other hand, is the rate at which the torque is produced.
Horsepower is a measurement of the engine's ability to do work. One horsepower can lift 33,000 pounds up one foot in one minute. Horsepower is measured over time. The more horsepower a vehicle has, the more mass it can move in the same amount of time, or it can move a fairly constant mass (the weight of the car) in less time. In simple terms, to get a vehicle accelerating quicker, we need to produce torque faster.
There are many different ways of measuring the power of an engine. Brake horsepower is a common term and refers to the power measured at the crankshaft using a "prony" brake. A prony brake was a simple lever that was connected to the crankshaft with a brake. As the brake was applied, the lever would deflect and power was indicated. Nowadays, computers and strain gauges are used to measure the power.
Rear wheel horsepower is just what it implies - horsepower applied by the rear wheels. A chassis dynamometer is used to measure the power transferred by the drive wheels onto the dynamometer's rollers. Brake horsepower is much lower than the horsepower specs provided by the manufacturers, because of all the power used by the transmission, final drive, and accessories.
Gross horsepower is the power measured at the crankshaft without any engine accessories. This includes removing the air cleaner and ductwork, exhaust system, water pump drive, and any other devices driven by the engine. Prior to 1973, horsepower specifications were listed by the manufacturers as gross horsepower. After 1972, manufacturers provided net horsepower specifications.
Net horsepower is also measured at the crankshaft before it goes into the transmission, but this time the engine is operated as it is installed in the vehicle, complete with accessories and ductwork.
Occasionally, usually when reading overseas vehicle information, I find reference to taxable horsepower. Taxable horsepower is a simple mathematics calculation used to determine the taxes to be paid on the vehicle. It has very little to do with actual horsepower because it doesn't factor in key elements like compression ratios, cylinder head design, and camshaft specifications. To calculate taxable horsepower multiply the cylinder bore x cylinder bore x number of cylinders x 0.4, and don't forget to pay the tax!
Many engineering documents and European manufacturers list engine power in Kilowatts. We more commonly use kilowatts to measure electrical power, but it is easy to convert from kilowatts to horsepower. One kilowatt is equal to 1.341 horsepower. While you are converting, you can also change torque measured in Newton metres (Nm) to ft lbs. by multiplying the Nm by 0.737561.
There are other measurements of horsepower. It can be converted to British thermal units (BTU) or it can be listed in PS. PS is short for Pferdestarke, the term for metric horsepower. Don't ask - I haven't seen it used anywhere either, but if you want to dazzle your friends, one horsepower is equal to 1.0139 PS.
So back to my Quickmobile. It really isn't very quick. With only 57 ft lbs. of torque, there isn't a lot of twist to turn the wheels. Sure, it has 200 hp, but it comes at a high 9000 rpm so it isn't very useful for normal driving or accelerating from a stop.
To achieve good performance, operate the engine at its peak torque, and the lower the rpm this occurs, the stronger it will pull from a stop. I see too many "hot-rodded" engines where all they go after is maximum horsepower at high rpm. If you have a light vehicle, steep axle gearing, and are willing to constantly rev the engine high, then high horsepower numbers will make you go fast. Great for racing.
For most of use however, it is better to have an engine with higher torque ratings and the lower or broader the rpm range it achieves it the better. Torque is what gets your vehicle moving. For everyday driving, I would pick a vehicle with high torque ratings at low rpm over a high horsepower, high rpm one any time.
Want the best of both? Then consider vehicle with a supercharged or turbocharged engine. By packing more air into the engine, torque is increased dramatically, but the engine can still be built for high rpm operation. Who says you can't have your cake and eat it too. Unfortunately, cake costs money, and so do superchargers and turbo's. There is always a catch.
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