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Old 02-05-2005, 12:39 PM
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Crank HP to rear wheel HP

Ive been wondering for a while how much horsepower from the crank actually gets transferred to the rear wheels. I know there are several factors to consider such as transmission, torque converter, rearend and gearing. Is there a common formula that would help me get a good estimate. For example if i have a 500 horse, 500 ft lb 383 with a 3000 stall, a rebuilt TH400 tranny with a shift kit and aftermarket high energy clutches. and a Moser 12 bolt rearend with 3.73 gears,,,, in a heavy 79 camaro, how much of that power could i expect to actually get to the ground? Thanks for any help.

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Old 02-05-2005, 08:18 PM
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HP loss

Unless its changed over the years, the rule of thumb has usually been a loss of 50-60 hp with automatic trans and 35-45 with a stick.

On modern cars, Differential hp is figured with all the accessories running, which further saps the engine.
With an honest 500 hp engine, you can expect a good 425-435hp at the rear wheels. If you have electric fan, you can add 10-15 hp for that loss of load.
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Old 02-05-2005, 09:22 PM
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Our AutoTech teacher told us usually between 20-30% of flywheel hp is lost by the time it gets to the wheels. Dont know if that's true....





Mike
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Old 02-05-2005, 09:26 PM
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HP loss

That would be a rather large amount on some vehicles as drivetrain loss is a relatively constant dynamic, and losing 20-30% of hp of a 400 hp engine would be a rather drastic pericitic load.
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Old 02-06-2005, 01:56 PM
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I agree with Max. Horsepower represents energy. 80 hp loss from 400 at the FW is a lot of friction (heat).

That said...if you hooked up a ricer of about 80 hp to your trans it might not turn the rear wheels!!

I don't know that I've ever seen a direct comparison of the same motor on the engine dyno vs. it in the chassis. Just getting the calibration between the 2 would be difficult.
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Old 02-11-2005, 12:10 AM
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Net HP

1965 289 High-Performance Automatic
Actual Rear Wheel:
141 hp/254 lb-ft
Estimated at Flywheel:
176 hp/318 lb-ft
Factory Rating:
271 hp/312 lb-ft

1970 Boss 302 Four-Speed
Actual Rear Wheel:
179 hp/209 lb-ft
Estimated at Flywheel:
209 hp/245 lb-ft
Factory Rating: 290 hp/290 lb-ft

1967 Shelby GT500 Four-Speed
Actual Rear Wheel:
240 hp/354 lb-ft
Estimated at Flywheel:
281 hp/415 lb- ft
Factory Rating: 355 hp/420 lb-ft

Roughly, it is usually 75 to 80% of flywheel hp (net hp)., but this is a very rough estimate. Manuals usually run 5 to 10% better than automatics, but all vehicles suffer a significant loos between flywheel and true rear (or front on front wheel drive) wheel hp.

Last edited by Scorpio Shaping Flow; 02-11-2005 at 12:18 AM.
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