Difference between gross and net HP
Thread Starter
Burning Brakes
Joined: Jun 2004
Posts: 1,240
Likes: 2
From: Cedar Rapids Iowa
Difference between gross and net HP
Does anyone know the difference between the gross and net methods of HP measurement? Sometime in the early 1970s, the manufacturers switched from gross HP to SAE net HP. There seems to be about a 20 - 25% difference. It is sometimes deceptive to compare HP ratings from cars in the 1960s to HP ratings of modern cars because of this measurement difference.
Le Mans Master
Joined: Jul 2004
Posts: 6,661
Likes: 1
Yeah.. I believe it was back in '72 when they changed from the gross hp rating to the net hp rating. What they used to do was dyno the engine without any accessories. In '72, they started dyno'ing the engine with the accessories that were going to be on the car including a full exhaust, all pumps, alternator, starter and any emissions controls. With this extra drag on the motor, the hp numbers were 20-25% less on average than without the accessories.
Just to let you know, they had a car tv show on recently that put a 426 Hemi Cuda 4 speed on a chassis dyno and the car only put out low 300 hp to the rear wheels. That's a far cry from the 400-500 gross hp rating at the crank that it had back in the hey day.
X
Just to let you know, they had a car tv show on recently that put a 426 Hemi Cuda 4 speed on a chassis dyno and the car only put out low 300 hp to the rear wheels. That's a far cry from the 400-500 gross hp rating at the crank that it had back in the hey day.
X
Last edited by xsiveone; Jul 28, 2004 at 11:15 PM.
Thread Starter
Burning Brakes
Joined: Jun 2004
Posts: 1,240
Likes: 2
From: Cedar Rapids Iowa
Originally Posted by xsiveone
Yeah.. I believe it was back in '72 when they changed from the gross hp rating to the net hp rating. What they used to do was dyno the engine without any accessories. In '72, they started dyno'ing the engine with the accessories that were going to be on the car including a full exhaust, all pumps, alternator, starter and any emissions controls. With this extra drag on the motor, the hp numbers were 20-25% less on average than without the accessories.
Just to let you know, they had a car tv show on recently that put a 426 Hemi Cuda 4 speed on a chassis dyno and the car only put out low 300 hp to the rear wheels. That's a far cry from the 400-500 gross hp rating at the crank that it had back in the hey day.
X
Just to let you know, they had a car tv show on recently that put a 426 Hemi Cuda 4 speed on a chassis dyno and the car only put out low 300 hp to the rear wheels. That's a far cry from the 400-500 gross hp rating at the crank that it had back in the hey day.
X
Last edited by slwhite; Jul 28, 2004 at 11:39 PM.
Racer
Joined: Aug 2001
Posts: 321
Likes: 2
From: Mobile AL
Echoing and reinforcing a previous reply which became quite relevant to me in my search for a muscle car I could afford in the mid 80's. 1972 was indeed the year the "hammer" fell. Check most any muscle car's net bhp output and it will fall dramatically for 1972 and then slowly rebound impotently. I ended up buying an EXTREMELY used 1973 Mustang Mach 1 with the 351 Cleveland engine (and a disconnected nitrous adapter I didn't recognize at the time) knowing that it really put out "much more power" than rated. The books forgot to tell me that's maybe 300 "extremely unreliable, with a tailwind and 147 octane fuel dumped via external fueling jet due to rate of consumption" rwhp, which only manifests in a straight line on odd numbered days in months containing the letter J and when the side exhausts burped "Mine Eyes Have Seen the Glory" at random.
Bottom line: comparison of pre-'72 engine and post-'72 engine impossible via paper. Very possible via dyno, provided multiple runs are done. The Mustang, on the other hand, will have to be pushed across the laser beam just for an RT.
:-)
(and yes, I still have it...)
Bottom line: comparison of pre-'72 engine and post-'72 engine impossible via paper. Very possible via dyno, provided multiple runs are done. The Mustang, on the other hand, will have to be pushed across the laser beam just for an RT.
:-)
(and yes, I still have it...)
