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Old 07-09-2009, 02:15 AM   #1
Strobe Media
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Default HP: wheel vs. crank?

Well since there is no sticky and I myself need some education on this maybe I could get some help. I hear talk about horse power to the wheel vs horse power to the crank, what's the difference? I also read somewhere that when talking about wheel vs. crank, there's a 15% loss of HP when talking about HP to the wheel vs. HP to the crank. So any info on this subject would be very helpfull.

Thanks.
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Old 07-09-2009, 02:42 AM   #2
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Default Re: HP: wheel vs. crank?

the HP that is mentioned for the mazda 3 in the magazines and brochures is the power at the crank. that power is basically a measurement of work over time. crank power is measured with an engine connected to a dynamometer and tested (with out the rest of the car attached, just the engine by its self). wheel HP is measured with a chassis dynamometer, and is measured with the car on a rolling road type of machine. wheel HP is typically lower because the engine has to use some of its power to spin the transmission and wheels (among other things). there are rough estimates for drivetrain loss for front wheel, all wheel, and rear wheel drive vehicles. those estimates are used so people can compare the power their car is making at the wheels to the power the car was rated at the factory (from the crank).
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Old 07-09-2009, 03:03 AM   #3
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Default Re: HP: wheel vs. crank?

^ perfect explanation.

As a concrete example, the 2004 2.3L Mazda3 engine is rated as having 160hp @ 6500rpm -- this is the power measured directly at the engine's crankshaft in a lab, without any other parts attached to the engine. When owners (like us) get their car dynoed, it's pretty much always done on a chassis dynometer -- the car is strapped down on a platform with rollers (kind of like a huge glorified treadmill made for cars) which measures how much power your actual drive wheels produce. The average "at the wheels" power made by the same engine mentioned above is usually around 125hp. That's 22% less power than the engine makes directly in a lab with nothing else attached. This is due to all the power needed to run accessories and turn all the moving parts of the drivetrain between the crankshaft and your front wheels.

"At the wheel" dyno measurements are seen by many as a more accurate gauge of how much power a car makes in the real world. However, you can't compare results between different models, and often even between different physical machines even of the same model, since there's so many environmental factors and calibrations involved which drastically affect the final numbers. This makes it hard to compare results between different dynos. Strictly as a tuning device, though, they are very useful.
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Old 07-09-2009, 03:10 AM   #4
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Default Re: HP: wheel vs. crank?

when you add drivetrain modifications, is that adding power to the crank as well or just to the wheel?
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Old 07-09-2009, 03:22 AM   #5
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Default Re: HP: wheel vs. crank?

if you mean drivetrain modifications such as light weight wheels, lighter flywheel, etc. they dont actually add power to the engine, but will free up power lost in the drivetrain and will show up on a chassis dyno as a HP gain.
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Old 07-09-2009, 03:29 AM   #6
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Default Re: HP: wheel vs. crank?

so unless I'm adding a turbo in my car the crankshaft Horsepower will remain the same regardless of the mods, but the wheel HP will change?
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Old 07-09-2009, 03:40 AM   #7
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Default Re: HP: wheel vs. crank?

[quote author=Strobe Media link=topic=149973.msg3217995#msg3217995 date=1247124557]
so unless I'm adding a turbo in my car the crankshaft Horsepower will remain the same regardless of the mods, but the wheel HP will change?
[/quote]

No, most mods (Intake, exhaust, etc...) will increase both. But a percentage of it will always be lost in the drivetrain. Most rotational stuff like lighter wheels, flywheel, rotors, pulleys does not add to crank power but will increase the dyno number at the wheels.
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Old 08-03-2009, 11:43 PM   #8
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Default Re: HP: wheel vs. crank?

less weight you gotta turn, more to the wheels pretty much
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Old 08-11-2009, 08:56 AM   #9
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Default Re: HP: wheel vs. crank?

Think of crank hp as the gross hp (before any deductions) and the power to the wheels as the net hp (with all deductions).
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Old 08-11-2009, 08:58 AM   #10
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Default Re: HP: wheel vs. crank?

pretty much if the car is off or still, you have 160hp (2.3) and as soon as you turn it on or start moving it is now 125hp
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