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If a vehicle has a known crank or wheel horsepower rating, is there a method that can reasonably estimate what the other horsepower value should be? If so, what is that method?

For example, if a vehicle has 300 wheel horsepower, is there a way to estimate its crank horsepower? And vice versa?

Note/Disclaimer: Accurately measuring horsepower on a car is difficult. A car on one dynamometer could produce one value, but on a different dynamometer, could produce a slightly different value. Different dynos can employ different means of measuring the power a car is producing. In addition, the weather and other atmospheric conditions can effect the amount of power a car is capable of delivering. For those interested in performance mods, the key metric to pay attention to is the amount of power gain across the RPM range of your vehicle.

Azhdeen
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2 Answers2

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The general "rule of thumb" for horsepower ratings between crank HP (CHP) and rear-wheel HP (RWHP) is ~18-20%. An automatic transmission will be closer to 20%, while a manual transmission around 18%. As you stated, and for the same reasons, these are just a general guideline. If you have the CHP number available, multiply that number by .8. (ie: 450hp x 0.8 = 360rwhp). To go the other way, divide the RWHP number by .8 (ie: 360rwhp / 0.8 = 450hp).

NOTE: Please remember this is just a generalization. No one can figure the actual numbers going either way with just math alone. Even between identical cars the numbers may differ. Only through testing on a dyno can you get an accurate number, and then the dyno must be calibrated correctly in order to give an accurate reading.

Pᴀᴜʟsᴛᴇʀ2
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There's not really any such thing as "crank horsepower" and "wheel horsepower" in any sort of definite sense. They both describe general categories of measurements, with many variables left up to the discretion of the tester.

All dyno measurements can be calibrated to ambient temperature and pressure. This introduces the possibility for shenanigans. For example, a car equipped with a turbocharger that maintains a constant 200 kpa of absolute pressure will produce the same horsepower at sea level and at 10k above sea level. Testing such a car at a high altitude and adjusting for ambient pressure will inflate the horsepower figure.

Crank horsepower is generally the car's power, measured at the crankshaft. However, do you hook up the accessory belts to the engine? Does the engine drive its own oil and water pump or are those driven by the testing lab? How about cooling fans for the coolant? Are power steering and AC hooked up to the engine during the test as they might during real world use? Is the source of air for the engine room temperature or the temperature found under the hood of the car?

Wheel horsepower is generally the car's power, measured at the drive wheels. One problem is that all dyno manufacturers produce different results measuring the same thing. Some only measure two wheels while some measure all 4. Some provide varying levels of resistance to simulate wind and road resistance, some just have the car spin a weighted roller and measure the rate at which it spins.

Even if you could settle on a totally consistent way of measuring crank and wheel hp, there would be no reliable way of translating between the two because different drivetrains (including different choices of transmission, diff, lubricant, stall converter, etc) and drivetrain styles (fwd, awd, rwd, hyrbid electric) all produce different levels of parasitic loss between the crank and the drive wheels.

Jim W
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