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Rev · 02-23-2001, 05:57 PM

From what I can gather, gross was used 1970 and before. Net has been quoted since 1970. When gross was used, manufacturers wanted to inflate HP numbers as much as possible for advertising purposes. Gross numbers were without any accessories at all. Only fuel, oil, and water pumps and no fans or anything else. Headers were dyno headers and no mufflers. The headers didn't even need to fit in the car. These engines also were tuned especially for the dyno test by someone like Smokey Yunick or some other factory racing guru. These engines were well broken in with fairly loose tolerances, thus minimizing friction. The idea was to maximize the numbers. Factory engineers referred to these numbers as the "A" curve.

At some point in the late '60's, muscle cars were criticzed by insurance companies and by the Feds. In '68, Ford started advertizing the "B curve". That was just an off the line engine with factory exhaust system, but no accessories. This was to appease insurance companies and the Feds. This is how the 428 CJ was rated.

After about 1970, manufacturers used net HP or the "C" curve. That is just net HP that we use today. It's just as the engine comes in a new car with all accessories except PS and AC. Factory exhaust and no special tune. Also a tight new engine with no break in.

Basically, Manufacturers tried to make the HP numbers be what suited them at the time. It created a lot of confusion. The above mentioned numbers are flywheel dyno numbers (hence "brake"). Rear wheel numbers are about what Unit said they are percentage-wise.

I know this is long, but thought it might help a little.

Rev

------------------
'66 Coupe, 306, 300 HP, C-4, 13.97 e.t., 100.3 mph
1/4 mi.

02-23-2001, 05:57 PM	#10
Rev Registered Member Join Date: Nov 1998 Location: Houston, Tx. Posts: 3,887	From what I can gather, gross was used 1970 and before. Net has been quoted since 1970. When gross was used, manufacturers wanted to inflate HP numbers as much as possible for advertising purposes. Gross numbers were without any accessories at all. Only fuel, oil, and water pumps and no fans or anything else. Headers were dyno headers and no mufflers. The headers didn't even need to fit in the car. These engines also were tuned especially for the dyno test by someone like Smokey Yunick or some other factory racing guru. These engines were well broken in with fairly loose tolerances, thus minimizing friction. The idea was to maximize the numbers. Factory engineers referred to these numbers as the "A" curve. At some point in the late '60's, muscle cars were criticzed by insurance companies and by the Feds. In '68, Ford started advertizing the "B curve". That was just an off the line engine with factory exhaust system, but no accessories. This was to appease insurance companies and the Feds. This is how the 428 CJ was rated. After about 1970, manufacturers used net HP or the "C" curve. That is just net HP that we use today. It's just as the engine comes in a new car with all accessories except PS and AC. Factory exhaust and no special tune. Also a tight new engine with no break in. Basically, Manufacturers tried to make the HP numbers be what suited them at the time. It created a lot of confusion. The above mentioned numbers are flywheel dyno numbers (hence "brake"). Rear wheel numbers are about what Unit said they are percentage-wise. I know this is long, but thought it might help a little. Rev ------------------ '66 Coupe, 306, 300 HP, C-4, 13.97 e.t., 100.3 mph 1/4 mi.