How to calculate wheel horsepower to a crank all-wheel drive

Written by richard rowe
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How to calculate wheel horsepower to a crank all-wheel drive
All-wheel drive vehicles, like this Subaru STi, can lose quite a bit of power through their complex drivetrains. (voiture de rally image by RomainQuéré from Fotolia.com)

Throughout most of automotive history, the only way to test a vehicle's horsepower output was to pull out the engine and bolt it to an engine dynamometer. In the early 1980s, a man named Mark Dobeck built the very first chassis dyno, which was essentially just a very heavy steel drum set with its top edge protruding through a platform. Dobeck extrapolated engine output by measuring how long the vehicle took to accelerate the drum up to a given RPM. Dobeck quickly discovered that this "wheel horsepower" was often significantly lower than known "crankshaft" horsepower, owing to drag created by the transmission and axle gearing.

Skill level:
Easy

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Instructions

  1. 1

    Research other vehicles in your car's model line to find out if the base model was originally front-wheel or rear-wheel drive. If your car doesn't have a two-wheel drive equivalent -- as is the case with most Subarus or the Mitsubishi Evo X -- then look at the engine. Transverse (sideways-mounted) engines have a basic front-drive configuration; longitudinal (front-to-back) mounting is a rear-drive configuration.

  2. 2

    Measure your car's horsepower on a chassis or "wheel" dynamometer equipped to handle all-wheel drive cars. Such dynos use two sets of rollers to measure the front and rear wheels simultaneously. Try to replicate average driving conditions as closely as possible. Turbocharged cars are particularly sensitive to environmental conditions, such as temperature, air pressure and humidity.

  3. 3

    Account for power loss through the transmission. Use a table, such as one provided in the Tips Section, to subtract driveline loss percentages for your particular configuration. Horsepower losses, through the driveline, will typically increase the size of the component. For example, a manual transmission used behind a tiny four-cylinder will require far less power to accelerate than a massive Rockwell 18-speed truck transmission.

  4. 4

    Account for power loss through the axles. This approach will vary by your engine and drivetrain layout. Transaxles require only about 1 per cent of power to turn their driveshafts, but you'll lose about 10 per cent of your vehicle's power whenever it has to turn 90 degrees. For instance, a Mitsubishi Evolution uses a transverse engine (with a transaxle) and an integrated transfer case that sends power to the 90-degree rear axle. As such, the Evo will lose only about 11 per cent power through the axles.

  5. 5

    Multiply the rear axle's power loss by whatever percentage of power goes to that axle. For example: if a Subaru STi sends 60 per cent of its power to the rear axle, then you'll only lose 6 per cent of the engine's total output instead of the full 10. If the STi sends 40 per cent of its power to the rear, you lose 4 per cent. You only need to do this for vehicles that use a transverse engine with AWD; longitudinal-configured vehicles use a pair of 90-degree axles, so it comes out even.

  6. 6

    Total up your percentage losses then place a decimal point to the left of the 10's place of the percentage. Multiply that number by your vehicle's wheel horsepower output to determine the losses in horsepower. Add the losses to the wheel horsepower. Example 1: a Mitsubishi Evo utilises a transverse engine with a manual transaxle (1 per cent for the transaxle plus 7 per cent for the transmission gearing equals 8 per cent power loss) and a 90-degree turn rear axle for a total power loss of 18 per cent. Example 2: A Chrysler 300 AWD uses a longitudinal engine with an automatic transmission (17 per cent loss for the transmission) and a pair of 90-degree turn axles (10 per cent apiece) for a total power train loss of about 37 per cent.

Tips and warnings

  • Loss Table for Transmissions:
  • Four-cylinder manual transmission: 7 per cent
  • Six, eight or larger manual transmission: 10 per cent
  • Four-cylinder automatic: 12 per cent
  • Six, eight or larger automatic: 17 per cent

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