Horsepower confusion

[try2pas]

The LT1 engine in 1970 in a corvette, which was one of the more powerful engines, was listed I believe as 370 hp and 380 lbs of torque with 11 to 1 compression ratio . Then in '71 it was listed as 330 hp and 360 lbs of torque with 9 to 1 compression. I believe this was all gross hp. Then in '72 the new hp ratings were changed to the flywheel instead of gross. The same '71 engine was now 255 hp and and 280 lbs of torque. So now a ZZ4 is advertised as 355 hp and 405 lbs of torque. Is that gross hp? And then are the new LS engines gross or flywheel or what? Like, again, the ZZ4 has and advertised 405 lbs of torque. The LS 2 has 400 lbs of torque. Does that mean the ZZ4 has more torque or are those advertised hp/torque ratings on different parts of the engine like gross for the ZZ4 and flywheel for the LS2? It doesn't make sense to me that the ZZ4 has more torque than the LS2 or the '70 LT1. Does anyone have any thoughts on this?

 

[DRTH VTR]

I think that the older "gross" HP ratings were as measured with an engine on a stand with no accessories attached and tuned for maximum horsepower. Power was measured "at the flywheel". The newer "net" HP ratings are also measure on a stand at the flywheel, but all the pumps, alternator, etc, etc are mounted. I am sure that it is a little more complex than that, but the main difference is that the older ratings were designed to yield the highest HP without any encumbrances, while the newer ratings reflect the engine as installed with typical accessory devices attached. The gross rating is on the order of 20% higher than the same engine rated using the net parameters.

 

[RVN]

Advertised HP ratings were sometimes measured and qualified at a lower RPM than @ redline. For instance (and possibly folklore), the aluminum '69 Z28 302 C.I.D. motor was advertised as producing 290 HP @ 4,200 RPM, for the sole purpose of making it possible for car buyers to obtain a lower insurance premium (which appealed to more consumers who were conscientious of lower insurance rates and affordability, thus selling more cars, more profit, the almighty dollar etc...) when that motor would indeed redline at 6,500 RPM and yield much more HP and torque than was "advertised".

 

[Hib Halverson]

The some of post above is...(I like RVN's choice of words)..."folklore"

The 302 in the 67-69 Z/28s was not aluminum. The engine was cast iron, block and heads.

The power rating for 302s was 290hp@5800 rpm but was generally believed to be making 310-320hp@6200-6500 rpm in production trim. The torque rating for 302 was 290 lbs/ft at 4200 and that is probably legit. Built and tuned for SCCA Trans-Am racing of that period, the engine made more like 400-450hp@6800-7000 rpm.

As for power ratings:
Up to 1971, all power ratings were gross. In general, that meant the engine on an engine dyno with no accessories except the water pump. Exhaust was a set of open headers. The engine's fuel and spark tuning were optimized for use with 103-octane gas. Coolant temperature and air intake temperature were optimized for best performance.

From 1972 to 2005, GM's engines were rated SAE net. In general, net ratings were taken with the engine fitted with all accessories and exhaust emissions equipment used in a production vehicle. Exhaust was the complete system from the car in which engine was to be installed. Fuel and spark were set at production calibrations and the engine was run on pump gas. Coolant and air temps were typical of road use. In the early 70s, when the changeover occured, it meant a significant loss in rated power, however, as time went on, the loss became not as much, ie: if you tested an LS3 out of an 08 Vette in both the old "gross" power and the current "SAE net" power ratings, the difference would not be as much.

From 06 to now, the ratings are still SAE Net, but the rating system works a little bit differently.

The power and torque differences between 1970 and 71, was due to the decrease in compression ratios that occured company-wide, to GM cars for MY71.

The engines sold by GM Performance Parts are rated using the old gross power rating.

The LS2 and other engines installed in production vehicles, are rated SAE Net and, thus, will have a comparatively lower number.

My guess is that if the ZZ4's torque was rated SAE net, it would be more like 375 lbs/ft.

If the LS2 was rated by the old gross system, it might be 425...maybe 435 lbs/ft.

 

[RVN]

I stand (stoop) corrected! Thanks Hib!

 

[Hib Halverson]

I stand (stoop) corrected! Thanks Hib!

You are welcome, RVN.
Glad to be of service.

 

[22229stingray81]

GREAT explanation Hib.

-Tatortot