Bill
Crude was a poor choice of words. Basically F=MA translates to F/M=A. We know F at any given rpm for both power plants, and we can calculate the M for either power plant to give the equivalent A. From this we can determine a starting point for a weight adjustment.
Power lose estimates are important to consider because in our model they are a constant, not a variable. Simply put a 10 HP difference at the wheels is a greater percentage of available power than a 10hp difference at the crank and as such requires a slightly large mass adjustment.