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Old 06-04-2005, 05:15 PM   #1 (permalink)
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Horsepower! Crank vs. Rear Wheels

If the SSR has 390 HP at the crank, what is that at the rear wheels? Also, if, for example, 390 HP at the crank equals 300 HP at the rear wheels, does that mean that if I get it up to 410 HP at the crank that it will also convert to 320 HP at the rear wheels?
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Old 06-04-2005, 11:21 PM   #2 (permalink)
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Hey Matjow,

Heres the way I always figue it. I know that there are many variables between auto trans and standard, also in which dyno you use and what temperature, elavation and humidity is at the time of measurement. But ON AVERAGE when you are measuring (dyno) rear wheel h.p. if you allow 15% loss in the drive train you will be pretty close to accurate flywheel h.p. The automatic loses a little more and the stick loses a little less. For example, my LS1 Camaro dyno'd @ 323 rwhp. If I divide that number by .85 (15% loss) I come up with 380 flywheel h.p. My S.L.P. Camaro was rated at 345h.p. and as you can see it is a little under rated. All LS1's are under rated from the factory and that is a good thing. If the 6.0 liter vette engine is rated at 400 h.p. then dyno numbers should be around 340 rwhp. I think when you start seeing more '05 and '06 SSRs getting dyno'd this is the average number you will see. 340rwhp divided by .85 = 400 fwhp.
Just an interesting side point, remember Brake H.P. (bhp). Thats the way all the muscle cars in the 60's were measured. You would have to use a 25% adjustment to come up to accurate rwhp. Let's say you had a 427/425 b.h.p 1966 vette (which I did). This car would come in at around 320 rwhp on a dyno to be equivilent to 425 bhp. 320 divided by .75 (25% adjustment) = approx 425 bhp.
Anyway that's the way I recall it and I hope that I didn't confuse you.
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Old 06-05-2005, 06:47 AM   #3 (permalink)
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I agree mostly.

The loss of power on a manual drivetrain ranges from an absolute lowest of 13% to as much as 16% from what I have seen, but the loss on an automatic varies a lot more. I have seen factors ranging from an absolute low of 15% to a high of 25%.

The stock 03 and 04 5.3 L SSR powertrains, which come ONLY as automatics, appear to deliver a net of 235 to 250 hp at the rear wheels, which translates to 17% to 22% loss. I donot believe that the engines vary this much in output, but rather that individual dynos are not calibrated correctly and identically, and that individual trucks have cleaner or dirtier air filters for example.

However, there is another factor that affects things. General Motors appears to use a more conservative rating standard than most dyno shops use. Dyno shops using Dynojet dynos usually use STD horsepower, which corrects the measured horsepower to a 66 degree standard temperature, whereas GM uses a stadard that corrects to temperature in the mid 70s (I can't remember the exact temperature, as I read about this some time ago). The difference is larger than you might think: a full 5%.

So, when GM says "300 hp" GROSS at the crank for the 5.3L, a dyno shop would say 300 x 1.05 = 315 hp GROSS at the crank. Those same dyno shops measure SSRs (and Silverados using the same engine) at 235 to 250 hp net at the wheels, which implies an even larger loss factor.

When I had my Z06, I noted that the 405 GM gross rated Z06 engines delivered 335 to 360 hp at the rear wheels on most Dynojet dynos, with a MANUAL transmission (only way the Z06 came), for a loss of 11 to 17% against the 405 hp rating, BUT after allowing for the 5% difference in correction factor, that loss becomes 15% to 21%, even with a manual transmission!

Best advice: Use the SAME dyno operated by the same technician each time you dyno your car, and look for the DIFFERENCES between runs.

To show you how crazy this can get, if you look at my dyno curve for my very mildly modified 04 SSR published on this board recently, you would conclude that my new GROSS ratings for hp and torque can be anywhere from:

Horsepower: 331 to 354 hp gross
Torque: 363 to 386 ft lb gross

Jim G

Last edited by JimGnitecki; 06-05-2005 at 08:01 AM.
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Old 06-05-2005, 07:44 AM   #4 (permalink)
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Thanks guys!

Now I finally understand it!

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