# What is the general rule, if one exists, for determining the difference between crank horsepower and wheel horsepower?

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.

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.

• Pretty much exactly what I was looking for. Is there any difference when it comes to FWD or AWD vehicles? – Ellesedil Jul 11 '14 at 16:03
• @Ellesedil ... I've not heard a difference, but would assume there would be due to the extra power which is absorbed through the transfer and extra differential. If you are talking specifically about the difference between AWD and 4WD vehicles, I would suggest the difference would be minimal. Like I said, all of this is just rule of thumb, so don't bet on any of these numbers being "accurate". If by "FWD" you mean front wheel drive, again, I don't think there is any difference. It should still fall into the same ballpark. – Pᴀᴜʟsᴛᴇʀ2 Jul 11 '14 at 16:07
• Yep, definitely meant front wheel drive. – Ellesedil Jul 11 '14 at 16:32
• @Ellesedil, there is a difference. AWD loses more to drivetrain losses (which is what Paul is describing). You can think of it as if every mechanical bit between the engine and the wheels is adding to the frictional effect. So, for example, my AWD WRX is losing a greater percentage to drivetrain losses than a RWD BRZ. However, my WRX still ends up with a higher wheel horsepower number (plus the AWD is wonderful in the snow :-). – Bob Cross Jul 11 '14 at 20:43
• I can confirm that the 20% rule fits right in with my real-world experiences with my latest project car. – Captain Kenpachi Jul 14 '14 at 13:22

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.

• So... it sounds like you decided to ignore half of my question, which summarizes pretty much your entire answer, and have conflated the meaning of "general" with "consistent". So while I agree with what you've written here, you're answering a different a question. – Ellesedil Mar 7 '17 at 21:31
• I understood your whole question and if you read my final paragraph I directly answer it. The fundamental problem is that crank and wheel horsepower aren't fundamentally related to actual power or to one another in predictable ways. – Jim W Mar 7 '17 at 22:41
• Your comment is starting to make less sense to me. I didn't ask anything about "actual power". I just wanted a very general (generic, ball bark, estimation - note: not specific, consistent, precise) difference between measuring an engine's power and measuring a vehicle's power at the wheels. This is achievable simply through experience: measure an engine's power, measure a car's power, repeat with all different kinds of cars, figure out an acceptable average. That's it. I don't need to know if drivetrain loss accounts for 23.4769% decrease in power. I don't care about that level of precision. – Ellesedil Mar 7 '17 at 23:18
• If, instead, your comment is arguing that wheel horsepower, crank horsepower, and "actual power" (what is "actual power" anyway?) have no real relationship with each other, then I now ask: what is the purpose of a dynamomete if what it measures can't be related to anything else? – Ellesedil Mar 7 '17 at 23:18
• Two purposes 1) marketing, which incidentally does not require actual use of a dyno or honest reporting of the results 2) evaluating the effects of tuning changes on individual cars. If you dyno a car and it makes 190 hp and then you make some changes and retest and it makes 220 hp, you know you've made progress and how much. – Jim W Mar 7 '17 at 23:30