Donna and John: Not correct.
It is not the rpm of the roller that gives the dyno computer the raw data for calculating the horsepower, but rather the RATE OF CHANGE of the rpm of the roller.
To keep the roller turning at any specific rpm onan INERTIA type dyno takes almost no power - just enough to overcome the friction within both the dyno roller system and within the vehicle's drivetrain. That is nowhere NEAR the 240 to 325 hp that the 2 versions of the SSR can generate.
What the whole idea is on an inertia dyno is to measure the RATE at which the roller is ACCELERATED. Because the computer on the dyno knows the moment of inertia of the roller, and it also knows (thanks to sensors on the roller) how quickly the roller is being acclerated by the vehicle, it can calculate the instantaneius torque being applied to the roller. Knowing the instantaneous torque, and knowing the instantaneous rpm, it can then calculate the instantaneous POWER being applied. By doing this in small increments over the entire acceleration run, it can build a power curve. It then plots this curve versus the ENGINE rpm, not the dyno rpm, so that you know the power available at each engine rpm.
Running the dyno test in a lower gear, whether via using a lower gear than the 1.0 to 1.0 ratio gear in the tranny, or because the rear axle ratio has been changed, does NOT change the power reading just because of the gear ratio change. If you follow the mathematics out in detail, you will see why.
It is rather the fact that the vehicle drivetrain is being ACCELERATED from starting rpm to peak rpm in a shorter time period, when you have numerically higher gearing (i.e. "lower gearing"), that causes the apparent "reduction" in power. The drivetrain has an overall moment of inertia, whose magnitude depends on the weight and radius of the various driveline components that are being accelerated. Those components include the wheel & tire assemblies, the rear axle rotating components, the driveshaft, all the rotating components within the transmission, the flexplate or flywheel. the complex total assembly of crankshaft with rods and pistons hung on it, and all the rotating accessories attached to the engine.
When that engine works to accelerate the roller, it also has to work to rotationally accelerate all these rotating components. With shorter (numerically higher) gearing, it has more leverage against the roller and accelerates it more quickly (check with any experienced dynovoperator - he will tell you that a vehicle with shortened gearing completes its run more quickly).
It takes WORK to accelerate those rotating components. It takes MORE work in LESS time to accelerate them on a vehicle with shorter gearing. More work in less time = more horsepower. So, more of the available engine horsepower is being diverted to spinning up these rotating components. THAT's why the reported net horsepower that is accelerating the roller is lower with shorter gearing.
That does NOT mean EITHER that (a) the engine is producing less power than before the gear change, or (b) that the fricitonal losses are higher (although they will in general be a BIT higher with shorter gearing - but not anywhere near the magnitude of change seen on the dyno). It just means that more of the available engine power is being diverted towards spinning up the rotating components instead of towards spinning up the ROLLER.
This IS manifested on the street as well. With stock gearing, and my set shoift points, my SSR accelerates the rotating drivetrain mass through ALL of 1st gear and again through ALL of 2nd gear to get to 89 mph. With 4.56 gearing, it accelerates that drivetrain mass through ALL of 1st, again through ALL of 2nd, and then through about 800 rpm of 3rd gear as well to get to that same road speed. Furthermore, it accelerates that mass more quickly than with stock gearing, because it is a full SECOND faster from 0 to 60 mph for example. It IS diverting more of its power to spinning up the rotating mass. However, that unrecoverable "diversion" is MUCH smaller than the GAINS made by (a) providing more leverage in the critical 0 to 60 mph speed range and (b) spending proportionately more of the total acceleration time in rpm ranges where there is higher horsepower being produced.
I am looking for that difference in dyno results in order to determine the amount of power being DIVERTED, and therefore then being able to estimate the moment of inertia of the entire drivetrain. (GM doesn't think we need to know that, so doesn't provide it
Hope that explains it for you.