Gross HP vs Net HP - Chevelle Tech
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post #1 of 22 (permalink) Old Dec 2nd, 08, 12:59 PM Thread Starter
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Ronnie
 
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Gross HP vs Net HP

How do they compare, for instance a 1970 396/350 gross HP to a 72 402/240 net HP.
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post #2 of 22 (permalink) Old Dec 2nd, 08, 1:09 PM
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Re: Gross HP vs Net HP

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:

Quote:
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).
Quote:
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/Horsepo...oss_horsepower
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post #3 of 22 (permalink) Old Dec 2nd, 08, 1:15 PM
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Re: Gross HP vs Net HP

look up gross in the dictionary and look up net in the dictionary
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post #4 of 22 (permalink) Old Dec 2nd, 08, 1:44 PM
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Re: Gross HP vs Net HP

If I remember correctly 71 listed both ratings and I think the 402 you listed was about 330hp instead of 350.

Mike
70 chevelle
420 sb combo - 10.93@120.76mph
405 sb combo - 11.87 @112.5
307 combo - 12.91@105.33
68 chevelle 533 almost there
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post #5 of 22 (permalink) Old Dec 2nd, 08, 2:12 PM
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Re: Gross HP vs Net HP

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.

Last edited by Harold Sutton; Dec 2nd, 08 at 2:26 PM.
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post #6 of 22 (permalink) Old Dec 2nd, 08, 5:23 PM
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Re: Gross HP vs Net HP

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.

1970 Chevelle, ZZ502 with 262/262@.50 .686 108 LSA Solid Roller, 1050 DOM, NW DP 400 shot, Super Victor, TH400 with 3000 stall, Moser 12 bolt with 4.11's, 33 spline and spool. No numbers yet.
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post #7 of 22 (permalink) Old Dec 2nd, 08, 5:37 PM
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Re: Gross HP vs Net HP

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.)
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post #8 of 22 (permalink) Old Dec 2nd, 08, 5:39 PM
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Re: Gross HP vs Net HP

Quote:
Originally Posted by CharlieCobra View Post
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.
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post #9 of 22 (permalink) Old Dec 2nd, 08, 7:02 PM
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Re: Gross HP vs Net HP

Quote:
Originally Posted by curley8788 View Post
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
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post #10 of 22 (permalink) Old Dec 2nd, 08, 7:04 PM
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Re: Gross HP vs Net HP

Quote:
Originally Posted by curley8788 View Post
this forum is for chevelles, not mustangs.

Ya think?

1970 Chevelle, ZZ502 with 262/262@.50 .686 108 LSA Solid Roller, 1050 DOM, NW DP 400 shot, Super Victor, TH400 with 3000 stall, Moser 12 bolt with 4.11's, 33 spline and spool. No numbers yet.
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post #11 of 22 (permalink) Old Dec 2nd, 08, 7:43 PM
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Re: Gross HP vs Net HP

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.

69 camaro ex-NHRA superstocker,548" 93 octane,g-force 4speed,9" w/ 4.86's and a lenco billet locker,NEVER to be trailered.9.63 at 141.80,1.38 60',126 mile round trip[3000rpm's at 60mph]
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post #12 of 22 (permalink) Old Dec 2nd, 08, 7:51 PM Thread Starter
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Re: Gross HP vs Net HP

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.
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post #13 of 22 (permalink) Old Dec 2nd, 08, 7:58 PM
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Smile Re: Gross HP vs Net HP

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
1969 CHEVELLE SS396,ORIGINAL #'S MATCH,GOT IN 1978,(In 2001 rblt/bored original 396 .030 to 402)/M20/12BOLT/3:31'S
2002 MAXIMA (DAILY DRIVER/1 owner,GOT 3/2013 w-44k miles)
2009 HD ELECTRAGLIDE CLASSIC ULTRA (GOT 11/14 W-9,700 miles)
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post #14 of 22 (permalink) Old Dec 2nd, 08, 8:02 PM
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Re: Gross HP vs Net HP

Quote:
Originally Posted by rshepard View Post
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
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post #15 of 22 (permalink) Old Dec 2nd, 08, 8:09 PM Thread Starter
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Re: Gross HP vs Net HP

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
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