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Originally Posted by Damon at NastyZ
GMPP crate motors are rated the same way they do for production line engines. It's SAE NET at the flywheel. However, the CONFIGURATION of the motor as-tested is not necessarily the same as a production line engine.
In a production line engine HP is rated "as installed" with all accessories, full factory exhaust system, factory air box, etc. Since a crate motor will have unknown as-installed configuration, they sorta "pick and choose" the configuration of the engine for the test. In the case of the popular 260HP 350 they chose headers, high flow exhaust, aftermarket dual plane intake, 4bbl carb, low-restriction air cleaner with few front end accessories (just water pump, I think) and, of course, tuning optimized for power.
That's why that engine (a veritable clone of the LM1 in a late 80s Z28) is rated much higher than the factory installed engine was. The factory installed engine had cast iron manifolds, single exhaust, cat converter, factory cast iron intake with a QJet topped by a quiet air cleaner assembly and a bunch of emissions equipment along with a full complement of front end accessories. Tuning was also designed to meet emissions, not necessarily maximize HP. That's a lot of differences.
Please note that "260 HP" crate engine has been rated at 195, 205, 225 and 250HP over the years. The engine has been almost unchanged since the 1980s, however. Only the rating has changed and only becuase GM used different configurations for the testing. More conservative in the past, more liberal in recent times. That "260 HP" engine is the one that GM has most stretched the believability of the rating.
However, almost every other crate motor GM just carries over the production line HP rating to the equivalent crate motor. Which means if you slap some headers on them and a free flowing intake system they often put out considerably more power than their official rating.
LAST DIFFERENCE...... Only the mother ship consistently rates using SAE numbers. Just about everything aftermarket is quoted in STP (Standard Temp and Pressure) numbers. SAE is more "stingy" while STP is more "liberal". You can imagine why the aftermarket would tend to gravitate to the more "liberal" STP numbers- it makes for higher power numbers which is good for business.
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