mtx28

Ok, I've been under the impression that legally, cars hp ratings have to be at the wheels, not the flywheel. This rule came into effect in '74, no? Well, lately I've seen many people saying that hp ratings are at the flywheel.. ie. LT-1 camaro's dont have the 275hp at the wheels like magazines say, its actually aruond 230hp(at wheels). What gives? here is another example. go to http://www.jovesoft.com/Steve/Cars/ then click on 'see the results' on the left. He says again the LS-1's are rated at the flywheel, not wheels. I've been going around telling poeple they are wrong, and the power rating is at the wheels because the law says it has to be.. but I'm beginning to think I'm the one thats wrong. Can someone clear this up?

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• 83 Z28-- not a pretty site to look at, 200k miles and im sure it was treated like a rental car every day of its sad life(before i got it).. sold, thank the lord
• 74 Z28-- 383/400, green on black. pretty clean. FOR SALE!!!

Apeiron

The HP ratings you see for old muscle cars are "gross" HP. This was obtained by putting the engine on the dyno stand without a water pump or alternator or any other accessories. Sometimes they would tweak the timing and jet the carb just right to get the maximum horsepower out of the engine, and that would be their advertised number. Sometime around '72 or so, the SAE changed the way they made HP measurements. The new "net" HP was made with the engine on the stand again, but with all the accessories, and timed and tuned exactly as it was when the car was delivered to the customer. This is why net ratings are lower than the gross.

Both ways measured at the flywheel though, the rear wheel HP will always be less due to losses in the transmission and drivetrain.

TransamGTA350

iTrader: (2)

The other difference in gross and net horsepower ratings was that gross horsepower was rated at 60 degrees, while net is at 77 degrees.

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89 Transam GTA, 350TPI, auto, 3.27 rear, dual cats, gray.
Magnaflow cat-back,
K&N open element,
Best E.T. 14.63@95mph

mtx28

i thought i read somewhere that the net hp was the hp at the wheels, and the gross was, like said before, with the engine on the dyno. so thirdgens are supposedly, what? 210 at the wheels?

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• 83 Z28-- not a pretty site to look at, 200k miles and im sure it was treated like a rental car every day of its sad life(before i got it).. sold, thank the lord
• 74 Z28-- 383/400, green on black. pretty clean. FOR SALE!!!

Apeiron

Probably more like 180.

Guest

Whoever wrote what you read that said net hp is at the rear wheels didnt know what they were talking about.