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Torque and BHP

Published on Jan 19, 2016


There are two values that are generally published for an engine which tell you how strong the engine is, and they are torque and bhp - brake horsepower. Torque is a measure of the twisting power of the engine. Torque is basically directly related to acceleration. The better the torque, the more twisting power the engine has, the better your car or bike will accelerate. Power, or more specifically bhp, is directly related to the torque readings.


In England and the US, horsepower means Imperial horsepower. The technical definition of this is "the power a horse exerts in moving 550 pounds of cargo a distance of one foot in one second." This calculation can include just the horse and its own weight. Horsepower can be defined many ways. One horsepower equals 746 watts, and as such, proper SI units are normally used instead. The term horsepower is more a legacy term than anything else.

The term brake horsepower came about because of an apparatus called a water brake that can be used to measure horsepower. Today all manner of brakes are used from hydraulic to electrical. They all perform the same function though, and that is to load up the engine and measure the torque with strain gauges. BHP figures can be calculated from the measured torque values to determine the power of the engine at any given rotation. If bhp figures are published without any other data, you've got to assume they're measured at the crank.

The problem is that once you add on clutches, flywheels, gears, driveshafts and all the other components between the engine and the wheels, the actual power at the wheel is often noticably less. So sometimes you'll see bhp figures noted as "at the wheel". This means the torque has been measured with the wheel being turned through all the above connections to give a more accurate power reading.
no) and get true at-the-wheel readings.


In thebad old days, bhp readings would be taken with the engine running in "optimum" condition, ie. with no oil or water pumps attached, direct cold-air injection, super-cooled coolant, no exhaust back pressure or catalytic converters and so on and so forth. Fortunately today there are standards that have to be maintained. Most recently, in 2005 the SAE made some changes to the test procedures to eliminate some of the 'slop' in power measurements, and for car manufacturers to be able to make valid SAE-certified bhp claims, their tests must now be monitored by SAE representatives.

The results of this change were interesting if only because the bhp values for engines changed without the engines themselves being modified. For example the Honda Element engine remained exactly the same, but its bhp rating dropped from 170bhp to 165bhp, simply because of the new procedures.

It's worth pointing out that whilst the rest of the world used bhp or kW (kilowatts) to publish power figures for engines, in America they typically used to use hp(SAE) instead, meaning the rated power of the engine as installed in the vehicle, ie including all the engine components, pumps, drivetrain etc. Having said that, even today, all hp(SAE) or SAE-certified bhp figures are taken at the flywheel and thus still don't really tell you how much power is getting to the wheels. The only way to know that is to put your car on a dynomomet

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