Big part of matching the 1320/mph=et formula is clutch/converter/gearing that allows the engine to operate between it's torque peak and it's horsepower peak. When you have that, it's usually pretty easy to come within a few tenths of the formula. It's those last couple of tenths that require quite a bit more effort.
The formula is a general yardstick for comparing efficiency, it doesn't care much about power/weight ratio or how expensive your engine is.
Grant
