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Question on the Road & Track Forum:
For the manufacturers advertised HP, is it SAE Gross or Net, or some other method ?
Measured at the flywheel, with or without accessories, exhaust, etc., or at the rear wheels ?
When compared to after market engine builders/assemblers, same procedure or each builder/assembler may use a different method making comparisons to each other and manufacturers engines inconsistent?
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Reply from Road & Track Associate Engineering Editor :
Over the years the measurement of horsepower has changed. It also differs in other countries.
Currently in the USA manufacturers are required to test their engines to an SAE certification J1349(
http://www.sae.org/certifiedpower/brochure.pdf ). The specifications have changed over the years, but they've always measured horsepower at the crank, not at the rear wheels. And thus are listed as bhp, indicating that the measurement was taken with a brake-dynometer.
The current SAE test takes into account all the accessories on an engine, as well as ambient air conditions. The idea being that this is a very close approximation of the power being delivered from the crank in the production car at the dealership.
Hot rod shops use a chassis dyno and measure hp at the wheels, sometimes referred to as rwhp (rear-wheel hp). This will be less than the SAE measure because of the energy lost in driving the transmission, differential and wheels.
Typically chassis dyno results will be corrected to standard conditions, but you are right, it is difficult to compare results taken at different times, locations and dynometers. For best comparison results, the cars would have to be tested at the same time, place and machine.
-Shaun
Shaun Bailey
Associate Engineering Editor
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Comment:
Correct or ? The 2011 ZR1 6.2L Corvette is rated @ 638HP @ the " crank " , RWHP for the engine would then be in the range of approx. 575 + or - , for comparison to high performance after market engines, that do not have a supercharger ( four-lobe Eaton® Twin Vortices Series™ supercharger with intercooler )