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Old 05-08-2006, 07:51 PM   #1
richard glickel.
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Default Difference between rear wheel/flywheel HP

I have heard that the difference between horsepower measured at the engine (viz., flywheel?) and the HP @ the rear wheels is something like 15% to 19% less (at the rear wheels).

Would it be fair to estimate that an engine that dynos at 208/209 HP (rear wheels) is at 250+HP measured @ the engine? This estimate is based upon a 17.5% loss from engine to rear wheels.

Any thoughts? Thanks.
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Old 05-08-2006, 07:53 PM   #2
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Automatics tend to have greater power losses due to torque convertors and the like (15-20%)

Manual transmissions tend to have less power losses (10-15%)

I'd use 12.5% as a nice middle ground for manuals, 17.5% for auto boxes.
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Old 05-09-2006, 01:09 AM   #3
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Quote:
Originally Posted by richard glickel.
I have heard that the difference between horsepower measured at the engine (viz., flywheel?) and the HP @ the rear wheels is something like 15% to 19% less (at the rear wheels).

Would it be fair to estimate that an engine that dynos at 208/209 HP (rear wheels) is at 250+HP measured @ the engine? This estimate is based upon a 17.5% loss from engine to rear wheels.

Any thoughts? Thanks.
Richard,

I dynoed my track car (968 engine) at 200-202 rwhp. If the stated crank HP is 236, then 15% seems to be appropriate. Of course different dynos will give you different numbers and the same dyno will give you different numbers under different conditions.
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Old 05-09-2006, 07:45 AM   #4
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Depends on the dyno. On a dynojet 248 I was seeing 205 before any mods, but it read 180-190 on the dynapak . . . and mustang dyno's show low number too.
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Old 05-09-2006, 12:01 PM   #5
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lemming is right - it is highly dependent on wheel weight, tire presure, differential temp and strap tension - it can easily vary as much as 20hp from car to car with those variables, so comparing anoyone's runs is indeed pointless

but, yes, 15% is the standard calculation for transaxle cars and in line with rwhp results and measured losses i've seen
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Old 05-09-2006, 12:29 PM   #6
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Thanks for the reponses.

Measurements are Dynopak so tire/wheel friction isn't in the equation as the rear wheels are removed and the hubs bolted to the dyno. Basically, the only "engine" mod is a Michael Mount header. HP was 209 @ 5500 RPM (after which it flattens out).

I guess an 8 - 9 HP gain from the header is about right?

Thanks again.
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Old 05-09-2006, 12:37 PM   #7
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I guess you had your's done in Nyack; when I had my car tested there (twice) the numbers were way lower, showed torque and hp bumps where they shouldn't be, and the redline was off-I went back and forth with them but I guess it was a software issue.
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Old 05-09-2006, 12:37 PM
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