SAE gross horsepower

Prior to the 1972 model year American automakers rated their engines in terms of SAE gross horsepower (defined under SAE standards J245 and J1995). Gross hp was measured using a blueprinted test engine running on a stand without accessories, air cleaner, mufflers, or emissions control devices and sometimes fitted with long tube "test headers" in lieu of the OEM exhaust manifolds. The atmospheric correction standard (e.g. barometric pressure, humidity and temperature) that was utilized in obtaining the Gross ratings was more idealistic than that which is used under the SAE net rating system. The resulting Gross power (and torque) figures therefore reflected a maximum, theoretical value and not the power of an installed engine in a street car. Gross horsepower figures were also subject to considerable adjustment by the manufacturer's advertising and marketing staff under the direction of product managers. The power ratings of mass-market engines were often exaggerated beyond their actual Gross output, while those of the highest-performance muscle car engines often tended to be closer in actual output to their advertised, Gross ratings. It should be noted that no pre-1972 "muscle car" engine in its unaltered, production line stock form and in its as "as installed" (SAE net) condition has ever yielded documented, qualified third party validated power figures that equal or exceed its original Gross rating. Claims that such engines were "under-rated" are therefore highly dubious. It is therefore more accurate to say that the ratings of the highest performance engines from that period tended to be less exaggerated than those of more typical passenger cars, but still exaggerated relative to what could be obtained in the SAE net configuration. For example, the ultra-rare and exotic 1969 427 ZL1 Chevy (rated @ 430 Gross HP) is frequently cited as one of the most "under-rated" high performance engines from that period, yet it could only produce 376 SAE NET HP. [6] It should be noted that today's various "Stock" drag racing events (e.g. "Pure Stock Drags" and the "Certified Stock" sub-grouping) allow the engines to be fully blueprinted per NHRA technical bulletins, which yields ideal tolerances and can increase actual compression ratio by more than 2 full points. These series also permit various other performance-enhancing alterations (e.g. over-bores, wide-flank cams, forged internals, stiffer valve springs, adjustable push-rods and poly-locks for optimal valve train geometry, modern exhaust systems with mandrel bent, 2.5 inches (64 mm) pipes and low restriction mufflers, fully locked differentials, etc.). Therefore, the results achieved in those events often don't reflect the performance potential (or engine output) of the car in its original, unaltered and production-line stock form.