Gross HP vs Net HP [Archive] - Chevelle Tech

: Gross HP vs Net HP


rshepard
Dec 2nd, 08, 1:59 PM
How do they compare, for instance a 1970 396/350 gross HP to a 72 402/240 net HP.

Mike
Dec 2nd, 08, 2:09 PM
Without accounting for the compression ratio drops across the entire engine line ('70 had 10.25:1 while '72 had 8.5:1) ,here's the description:

SAE gross horsepower

Prior to the 1972 model year, American automakers rated and advertised their engines in brake horsepower (bhp), frequently referred to as SAE gross horsepower because it was measured in accord with the protocols defined in SAE standards J245 and J1995. As with other brake horsepower test protocols, SAE gross hp was measured using a blueprinted test engine running on a stand with no belt-driven accessories, air cleaner, mufflers, or emission control devices and sometimes fitted with long tube "test headers" in lieu of the OEM exhaust manifolds. The atmospheric correction standards for barometric pressure, humidity and temperature were relatively idealistic. The resulting gross power and torque figures therefore reflected a maximum, theoretical value and not the power of an installed engine in a street car. Gross horsepower figures were also subject to considerable adjustment by the manufacturer's advertising and marketing staff under the direction of product managers. The power ratings of mass-market engines were often exaggerated beyond their actual gross output, while those of the highest-performance muscle car engines often tended to be closer in actual output to their advertised, gross ratings.

No pre-1972 engine in its unaltered, production line stock form, as installed in the vehicle, has ever yielded documented, qualified third party validated power figures that equal or exceed its original gross rating. Claims that such engines were "under-rated" are therefore dubious; for example, the 1969 427 ZL1 Chevrolet, rated at 430 bhp (320.7 kW), is frequently cited as an "under-rated" high performance engine, yet it produced only 376 horsepower (280 kW).

SAE net horsepower

In the United States the term "bhp" fell into disuse in 1971-72, as automakers began to quote power in terms of SAE net horsepower in accord with SAE standard J1349. Like SAE gross and other brake horsepower protocols, SAE Net hp is measured at the engine's crankshaft, and so does not account for transmission losses. However, the SAE net hp testing protocol calls for standard production-type belt-driven accessories, air cleaner, emission controls, exhaust system, and other power-consuming accessories. This produces ratings in closer alignment with the power produced by the engine as it is actually configured and sold. The change to net hp effectively deflated power ratings to assuage the auto insurance industry and environmental and safety lobbies.

Because SAE gross ratings were applied liberally, there is no precise conversion from gross to net. Comparison of gross and net ratings for unchanged engines shows a variance of anywhere from 40 to 150 horsepower. The Chrysler 426 Hemi, for example, in 1971 carried a 425 hp gross rating and a net rating of 350 hp, while the same company's 225 Slant 6 carried a rating of 145 bhp but 110 net hp.

source:http://en.wikipedia.org/wiki/Horsepower#SAE_gross_horsepower

curley8788
Dec 2nd, 08, 2:15 PM
look up gross in the dictionary and look up net in the dictionary

68chevelle533
Dec 2nd, 08, 2:44 PM
If I remember correctly 71 listed both ratings and I think the 402 you listed was about 330hp instead of 350.

Harold Sutton
Dec 2nd, 08, 3:12 PM
Net rating for a high (10.25) compression 402 would be about 280 net h.p. at the flywheel. They seldom made any more than that from the factory. When worked over for stock eliminator they can exceed the gross number usually because of better valve jobs and reground camshafts. I think Mike is right in that the '71s and later cars are rated lower because of the compression drop to 8.5 or therabouts. My low compression '72 Buick Stage 1 G.S. had a net rating of 270 h.p. and the '71 had a gross rating of 340 h.p. and was the exact same engine. That car ran 14.21 @ 99.5 mph, weighed 3670 lbs. with a four speed, no air conditioner and standard 3.42 rear gear.

CharlieCobra
Dec 2nd, 08, 6:23 PM
So, I'm guessing I oughta have been impressed with that little SC'd 281 CI 32 valve I had that put down 383.6 RWHP bone stock with a factory rating of 390HP? I miss that Cobra sometimes. Bolt on a grand worth the mods and put down 495 RWHP and still got 22 MPG. It wouldn't run with the 'Velle though, not now.

Schurkey
Dec 2nd, 08, 6:37 PM
And just GUESS which SAE standards aftermarket dyno operators use.

(hint: It's no mystery why GM crate engines--rated using the SAE Net procedure--easily surpass their "rated" horsepower on an aftermarket dyno test.)

curley8788
Dec 2nd, 08, 6:39 PM
So, I'm guessing I oughta have been impressed with that little SC'd 281 CI 32 valve I had that put down 383.6 RWHP bone stock with a factory rating of 390HP? I miss that Cobra sometimes. Bolt on a grand worth the mods and put down 495 RWHP and still got 22 MPG. It wouldn't run with the 'Velle though, not now.

this forum is for chevelles, not mustangs.

trmnatr
Dec 2nd, 08, 8:02 PM
this forum is for chevelles, not mustangs.

He just stated a comment about a car he had, Mike in a bad mood today, Cheer Up :thumbsup:

CharlieCobra
Dec 2nd, 08, 8:04 PM
this forum is for chevelles, not mustangs.


Ya think?

ericrickster
Dec 2nd, 08, 8:43 PM
mine went from 728 gross to 680 net the first time i dynoed the motor,dyno operator just plugged in an sae net hp factor on the 901 superflow.

rshepard
Dec 2nd, 08, 8:51 PM
Just wanted to know, side by side how do they compare to each other? Im supposed to have 396/350 HP found out I have a 402/240 HP. and feeling robbed.

SWHEATON
Dec 2nd, 08, 8:58 PM
Hi Butch,your definately onto something with Mike.

He jumped on me earlier today for no good reason that i could see then maybe my crappy typing & or typos in the post he started on Summits new carbs ,must be crabby.(LOL!!!!)

Scott

bikeron
Dec 2nd, 08, 9:02 PM
Just wanted to know, side by side how do they compare to each other? Im supposed to have 396/350 HP found out I have a 402/240 HP. and feeling robbed.

Was that at the rear wheels on a chassis dyno or at the flywheel on an engine dyno??

Ron

rshepard
Dec 2nd, 08, 9:09 PM
Got the 396 ratings from my build sheet. Knew I didnt have original motor Ran the numbers found out I have a 72 402 with 240 HP Info from nastyZ28

curley8788
Dec 2nd, 08, 9:44 PM
He just stated a comment about a car he had, Mike in a bad mood today, Cheer Up :thumbsup:

yeah i know. it must be because theres no more racing for me until next season. i still think this website should be all about chevelles, or just old gm cars in general. mustangs are the LAST thing i want to hear about.


Hi Butch,your definately onto something with Mike.

He jumped on me earlier today for no good reason that i could see then maybe my crappy typing & or typos in the post he started on Summits new carbs ,must be crabby.(LOL!!!!)

Scott

scott, you must be confusing me with the other mike. (big_red)

Mikeys69
Dec 2nd, 08, 10:14 PM
Hi Butch,your definately onto something with Mike.

He jumped on me earlier today for no good reason that i could see then maybe my crappy typing & or typos in the post he started on Summits new carbs ,must be crabby.(LOL!!!!)

Scott
More Mike's than you can shake a stick at..lol
I would be your Mike...I didn't know if it was your crappy typing "broken keyboard" or me looking at crappy stuff? Sorry if you took offense to my keyboard comment.:(

66 Buick Special
Dec 2nd, 08, 10:52 PM
Can't we all just... type along...:D

curley8788
Dec 2nd, 08, 11:14 PM
Can't we all just... type along...:D

hahahaha niceeee.

66 Buick Special
Dec 2nd, 08, 11:40 PM
thanks!:beers:

Bomber '67
Dec 2nd, 08, 11:59 PM
The dragstrip does the best job of measuring your car's horsepower - it really doesn't care about the different standards. if you know your cars mph through the traps and the car's weight you can calculate the horsepower.

Gross vs. Net is just the beginning of the horsepower standards. The 2007 Z06 Corvette ushered in the newest standards that are the most realistic yet. Among other changes, they now require correcting to reflect common weather and altitude. The new test requires that an independent lab test the engine - no monkey business from the manufacturer allowed. Several Japanese car companies were forced to lower their rated horsepower because the lab could not duplicate the manufacturers claims. Chevy had intended to release the LS7 ZO6 as a 500 hp engine, but the independent lab said it was 505.

Thomas

Mikeys69
Dec 3rd, 08, 1:22 AM
hahahaha niceeee.
I did say sorry..:cool: