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Chad, lynn probably has the best explanation of the posts to date on the topic. Your formula MIGHT work if the horsepower curve was FLAT and LINEAR...Then you PROPORTIONALITY-BASED formula would be correct. But have you looked at a horsepower graph from a dyno? Most show the curve rising from idle to a certain point, and then dropping off...and SOME are irregular BETWEEN points, due to perhaps the brake specific fuel curve (BSFC), or maybe even something as oddball as an airflow restriction that's only evident during a certain resonance range (such as exhaust reversion diluting the intake charge, making less efficient combustion, but ONLY at a certain RPM range...for example). Think about it for a second....if you could take the HP rating from the dyno at ONE rpm reading, and then just extrapolate it to whatever rpm you choose, then a horsepower or torque CURVE...WOULDN'T curve; it'd be a straight line. But the "upside" to this, if it was true, would be that you could save a lot of precious dyno time, as you'd only need to test at two rpm points to know the horsepower/torque output throughout the entire range. Unfortunately, it just don't work that way.
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