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HP & Torque; How was your vehicle rated?
By: Derek Martel
How many of you know what your vehicle was rated at for horsepower and torque? If any of you actually had your vehicle Dyno Tested in its stock configuration, you may have wondered why the numbers didn’t match up. The next question is, how was your vehicle rated and how was it tested?
As most of us are aware, we can measure horsepower and torque at the crankshaft of the engine using an Engine Dynamometer, (this is known as Brake Horsepower: BHP) or we can measure the power output at the wheels using a Chassis Dynamometer. But how was your vehicle tested when it was rated by the manufacturer and how did they perform the test? The answer will depend on the year of the vehicle, but all ratings were performed using an Engine Dynamometer to measure brake horsepower. But which rating was used on your vehicle, SAE Gross or SAE Net?
Vehicles that were Pre-1971 were rated using Society of Automotive Engineers (SAE) Gross ratings (standards J245 and J1995) at the crankshaft. What made these results unique was that the test was performed using an Engine Dyno, but the engines had no accessories on them for the crank to power, they had free flowing exhaust (usually no exhaust installed), ignition timing could be set to optimum settings, carburetors could be jetted for performance, etc. As you can imagine, the horsepower and torque numbers that were produced were very desirable, much like what we would do if we had our engine on an Engine Dyno.
With little to no regulation of the SAE Gross ratings, manufacturers could easily inflate the horsepower and torque ratings and use them as a marketing technique or they could understate the engine’s performance to reduce its reported power numbers. This understated rating may have been used for a few reasons; drag racing and automotive safety lobbyist are two examples. A vehicle Drag Racing class was determined by the power to weight ratio by using the advertised (gross) horsepower rating and its posted vehicle weight. When it came to safety, automotive insurance companies were making it difficult to near impossible for someone to insure a powerful vehicle.
As of Post-1971, vehicle ratings were now rated using SAE Net standards (standard J1349). The difference was that this was a more “realistic” rating of the engine as it was installed into a stock vehicle. The test procedures have now changed. The test was still performed using an Engine Dyno, but now the accessories were installed on the engine (power steering pump, alternator, air pump, A/C compressor), ignition timing was at factory stock settings, fuel metering (carburetor or fuel injection) was stock, and a stock exhaust system was installed. All of these differences took power away from the engine and what was left was the horsepower and torque measured at the crankshaft. The change in reporting power ratings combined with the installation of smog reduction components on engines in the 70’s caused a common Chevy 350 cid engine having a Gross SAE rating of 325-350 hp to only have a SAE Net rating of about 160 horsepower.
As this process has been used since 1972, the ratings which have been reported by U.S. Auto manufacturers is a more realistic number of what is powering the drivetrain, but like many systems there may be loopholes. In 2005, the Society of Automotive Engineers (SAE) issued a new standard, J2753. This new standard required an independent observer to be present while the tests were being performed to determine the Net ratings. For some auto manufacturers this caused their reported ratings to increase, where as some ratings decreased.
So if the horsepower and torque are measured at the crankshaft, brake horsepower and the crankshaft turns the drivetrain, what is at the wheels for power? Less. How much less? That also depends. Watch for my follow up article, BHP vs. WHP. Where did the power go?
For information on Automotive degree programs offered by New England Institute of Technology, please contact the Admissions Office at 401-467-7744 or visit www.neit.edu/motorhead.