How to Calculate Car Transmission Loss

Written by richard rowe
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How to Calculate Car Transmission Loss
When you've got 10,000 horsepower at your disposal, you don't need a power-sapping transmission. (Rusty Jarrett/Getty Images Sport/Getty Images)

The bad news is that there's no way to accurately predict exactly how much power a particular transmission is going to suck from your engine. There are simply too many variables, including transmission efficiency, gearing, heat and even the type of fluid used for a particular application. However, there are a few rules of thumb that apply to most scenarios -- in particular, regarding manual vs. automatic transmissions and whether or not the automatic transmission uses a lock-up torque converter. However, a little bit of homework can get you fairly close to quantifying how much power you're losing through your vehicle's transmission.

Skill level:
Moderately Easy

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Instructions

  1. 1

    Calculate wheel horsepower, given the known crankshaft horsepower and transmission type. Automatic transmissions and the associated drivetrain usually consume about 20 per cent of the engine's power, and a manual transmission takes about 16 per cent of the power. Multiply your known crankshaft horsepower by 0.20 for an automatic or by 0.16 for a manual, then subtract that from the known crankshaft horsepower to derive wheel horsepower.

  2. 2

    Calculate crankshaft horsepower, given the known wheel horsepower and transmission type. Multiply wheel horsepower by 1.25 for an automatic or 1.193 for a manual transmission. Double-checking our work from step one, we'll calculate horsepower loss from a 300 crankshaft horsepower engine through a manual and an auto: Wheel horsepower should be 240 for the auto (300 x 0.20 = 60; and 300 - 60 = 240) and 252 for the manual. Working backward, the auto measurement of 300 horsepower (240 x 1.25 = 300) and the manual measurement of 300.63 (252 x 1.193 = 300.63) are close to dead-on.

  3. 3

    Recalculate for an 18-percent loss if your car uses a lock-up torque converter. Even the best non-lock-up torque converter never achieves full lock; the lock-up converter clutches positively lock the engine and transmission together, resulting in a bump in efficiency. Multiply crankshaft horsepower by 0.18 or wheel horsepower by 1.219 to derive the measurements from each other. Using the 300 horsepower engine mentioned above, we find that a lock-up torque converter reduces wheel horsepower to about 246 (300 x 0.18 = 246); multiply that wheel horsepower by 1.219 and you get 299.87 crankshaft horsepower.

Tips and warnings

  • Bear in mind that these are fairly general guidelines intended to cover most road-going cars and trucks; a very large transmission, regardless of the type, is going to take more power to turn than very small one. If a particular transmission requires 50 horsepower to turn, it will require that same 50 horsepower whether its bolted to a 100 horsepower engine or a 500 horsepower engine. So, consider the application before espousing any estimation as gospel.

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