Wheel hp and Flywheel hp, doc's please

[Dan Juday]

Help me win a bet.

 

I am well aware that U.S. auto manufactures changed the way they rated and published horse power ratings for their new vehicles over the last forty years.

 

Back in the sixties and seventies the published numbers were taken at the flywheel and didn't take into consideration drivetrain losses. In some cases they also removed basic ancillary items like alternators, power steering pumps, and even water pumps to get the inflated numbers.

 

Today's numbers are now taken at the rear (or front for FWD) wheels.

Well this chuckle head buddy of mine insists I have it backwards. I need to straighten this character out big time before he farther embarrasses himself (and there's a lot of money riding on this bet: two lunches).

 

So, if you guys know of any handy, credible articles on this subject that I can wave in his face please, post them here.

 

Most helpful HybridZ member wins a free lunch with me...

 

next time you're in Rio Vista...

 

on a full moon.

 

Thanks guys, Dan

[dr_hunt]

Well,you are correct about the hp ratings of days of old. It was called SAE gross HP prior to 1972. Then there was SAE brake hp which came after 1972 and was dyno hp with all the accessories. I think what your looking for is wheel hp or effective hp, that which gets to the ground. I don't think any car manufacturers do that as of yet.

[pete280z]

http://en.wikipedia.org/wiki/Horsepower

 

SAE gross horsepower

Prior to the 1972 model year American automakers rated their engines in terms of SAE gross horsepower (defined under SAE standards J245 and J1995). Gross hp was measured using a blueprinted test engine running on a stand without accessories, air cleaner, mufflers, or emissions control devices and sometimes fitted with long tube "test headers" in lieu of the OEM exhaust manifolds. The atmospheric correction standard (e.g. barometric pressure, humidity and temperature) that was utilized in obtaining the Gross ratings was more idealistic than that which is used under the SAE net rating system. 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 times exaggerated beyond their actual Gross output, while those of the highest-performance muscle car engines often times tended to be closer in actual output to their advertised, Gross ratings. It should be noted that no pre-1972 "muscle car" engine in its unaltered, production line stock form and in its as "as installed" (SAE net) condition 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 highly dubious. It is therefore more accurate to say that the ratings of the highest performance engines from that period tended to be less exaggerated than those of more typical passenger cars, but still exaggerated relative to what could be obtained in the SAE net configuration. For example, the ultra-rare and exotic 1969 427 ZL1 Chevy (rated @ 430 Gross HP) is frequently cited as one of the most "under-rated" high performance engines from that period, yet it could only produce 376 SAE NET HP. [5] It should be noted that today's various "Stock" drag racing events (e.g. "Pure Stock Drags" and the "Certified Stock" sub-grouping) allow the engines to be fully blueprinted per NHRA technical bulletins, which yields ideal tolerances and can increase actual compression ratio by more than 2 full points. These series also permit various other performance-enhancing alterations (e.g. over-bores, wide-flank cams, forged internals, stiffer valve springs, adjustable push-rods and poly-locks for optimal valve train geometry, modern exhaust systems with mandrel bent, 2.5 inches (64 mm) pipes and low restriction mufflers, fully locked differentials, etc.). Therefore, the results achieved in those events often times don't reflect the performance potential (or engine output) of the car in its original, unaltered and production-line stock form.

 

Brake horsepower

Brake horsepower (bhp) is the measure of an engine's horsepower without the loss in power caused by the gearbox, generator, differential, water pump, and other auxiliary components such as alternator, power steering, and AC compressor. Thus the prefix "brake" refers to where the power is measured: at the engine's output shaft, as on an engine dynamometer. The actual horsepower delivered to the driving wheels is less. An engine would have to be retested to obtain a rating in another system. The term "brake" refers to the original use of a band brake to measure torque during the test (which is multiplied by the engine RPM and a scaling constant to give horsepower).

 

hp (SAE)

In the United States the term "bhp" fell into disuse after the American Society of Automotive Engineers (SAE) recommended manufacturers use hp (SAE) to indicate the net power of the engine, given that particular car's complete engine installation. It measures engine power at the flywheel, not counting transmission losses (or anything after the flywheel).

Starting in 1971 automakers began to quote power in terms of SAE net horsepower (as defined by standard J1349). This reflected the rated power of the engine in as-installed trim, with all accessories and standard intake and exhaust systems. By 1972, US carmakers quoted power exclusively in SAE net hp. The change was meant to 'deflate' power ratings to assuage the auto insurance industry and environmental and safety lobbies, as well as to obfuscate the power losses caused by emissions-control equipment.

SAE net ratings, while more accurate than gross ratings, still represent the engine's power at the flywheel. Contrary to some reports, it does not measure power at the drive wheels.

Because SAE gross ratings were applied liberally, at best, 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.[6]

 

SAE-certified horsepower

In 2005, the Society of Automotive Engineers introduced a new test procedure for engine horsepower and torque.[7] The procedure eliminates some of the areas of flexibility in power measurement, and requires an independent observer present when engines are measured. The test is voluntary, but engines completing it can be advertised as "SAE-certified".

Many manufacturers began switching to the new rating immediately, often with surprising results. The rated output of Cadillac's supercharged Northstar V8 jumped from 440 hp (328 kW) to 469 hp (350 kW) under the new tests, while the rating for Toyota's Camry 3.0 L 1MZ-FE V6 fell from 210 hp (157 kW) to 190 hp (142 kW). The first engine certified under the new program was the 7.0 L LS7 used in the 2006 Chevrolet Corvette Z06. Certified power rose slightly from 500 hp (373 kW) to 505 hp (377 kW).

[johnc]

That Wikipedia article has a couple small mistakes in it. Below is a thread where I post the official explanation from SAE (I'm a SAE member):

 

http://forums.hybridz.org/showthread.php?t=95385

 

EDIT: a simple search would have found this pretty easy.