I have heard this from many places. What is the difference between the old 'gross horsepower' and the new net horsepower. Also, how did the term 'brake horsepower' (bhp) come to represent hp at the flywheel?

Thanks
Jeff

 

Gross horsepower is the horsepower at the flywheel, Net horsepower is at the wheels. Old gross horsepower is when they used to dyno the engines without any accessories. Now gm rates their engine by what power they will make for a safe period of time (not just peak), that's why they rate the SS at 320 at the flywheel and it makes like 300 to 310 at the rear.

 

No, actually both Gross and Net HP are at the flywheel. Gross HP is w/ no accessories, running headers, aggressive fuel/spark curves, etc. Not how the engine would be installed in the car. Net takes into account all accessories, factory exhaust that the car will be installed with, etc. The change over between the ratings occured around '72-'73, I forget exactly. So basically old motors were not as strong as you would think from the HP ratings. But, GM underrated alot of their engines back then, espeically the top dog engines, so you can't draw direct conclusions about then vs now from the rated numbers. The true test is a dynojet run, IE getting rear wheel numbers.

GM likes to underrate the engines in the F-body, especially the LS1s of late. Basically they are installing the Vette engine in the F-bods and rating it lower in order to give the Vette a clear advantage on paper. Because rear wheel dynoing both a F-bod and a Vette will result in nearly identical HP numbers, thus they are either overrating the Vette or underrating the F-body. Since ~300 rear wheel hp doesn't exactly jive w/ a 305-320 hp crank rating I tend to believe the later is true. they used to do that in the old days as well (case in point, 375hp 396 in 67 Camaro was the exact same motor rated at 425hp in the Vette the year before...)

------------------
Ray87Z
-Vortec headed 350.
86 IROC w/ a cammed 305 TPI.
Formerly Ray86IROC.
www.inter-scape.com/Ray

 

Ray is correct

 

I'd vote for Ray's explanation, and I'll pick 1972 as the year the "installed" HP numbers were published.

I'll also agree that you cannot trust the HP numbers published by GM. There is the unwritten "golden rule" that no GM car will have a published performance number better than the Corvette in any given year. Insurance companies put additional pressure on manufacturers in those years, and the GM corporate lawyers were quick to jerk their knees and demand lower numbers.

The "energy crisis" of 1974 applied more political pressure to lower the published numbers. Remember when the world was supposedly going to run out of crude by 1978? These same "liberal people" were jumping on the Nostra-dumbass bandwagon in the eighties predicting the end of the world. For those types, I would be happy to oblige them by ending their world so the rest of us don't have to suffer listening to the bastards moaning incessantly. For the rest of us, it would be nice if the REAL HP numbers were published and let the chips fall as they may.

A prime ThirdGen example it the '89 TTA. It had published HP numbers of 250. Even running on camel-urine gasoline with fouled plugs and a flat tire, the worst one off the assembly line would make 305 RWHP on any dyno EXCEPT the official GM dyno...hmmm, I wonder.

Incidentally, am I the only one that finds it odd that the Corvette is rated at 340HP and the Dodge Viper RT/10 is rated at 450HP, but the 0-60 and top-end performance is nearly identical? I guess we can't trust Nopar, either. (Like we ever could.)

------------------
Later,
Vader
------------------
"Make Me Bad"
Adobe Acrobat Reader 4.0

 

Great explanations guys. Now I know. I knew better than "gross is at the flywheel and net is at the wheels", but the rest of his explanation was right on.

I didn't know that a Viper and a Vette were that comparable. I had always expected there was a taboo at Chevrolet about rating a car higher than the Vette. I read an article that said the LS1 isn't even "breathing hard" to produce 345 hp. But they won't up the ante because they are afraid of offending ZR-1 owners. What BS.

Jeff

 

The lower they rate the engine, the better.
The numbers they advertise make no difference in performance. It only makes a difference to "bragging rights." If they rate the power too high, the insurance rates will go "through the roof."

 

Ray is correct.

 

"Brake horsepower" has nothing to do with brakes, as such.

Most engine dynos these days use a thing to load the engine that works a little like a torque converter, except it has water in it. The engine's power ends up heating up the water. This device is called a "water brake". That's the origin of that term.