Ahhhh, fun stuff. Dyno games
Gears do not actually affect power. They will multiply torque, which is why "Rear Wheel Torque" is always quoted wrong, but that's a different issue, so I digress lol.
What is REALLY happening is that todays dyno tests are mainly done as a "sweep". Ie, the car accelerates the rollers.
Since the rate at which you accelerate a rotating mass is proportional to power, the faster you accelerate something (ie, your wheels, tranny, driveshaft, gear set), the more power you "use" doing so. Soooo, the faster you are accelerating (due to gears), the more power you "used" before it hits the rollers (ie, used more power to accelerate your entire drivetrain).
Now, dyno's "know" this, and most have some sort of "inertia compensation". Since output it not really decreased, they try and add in some power in the software to compensate so that you get the same power whether you have 3.07's or 3.55's.
This is a best guess since they dont know what car is on the dyno. Inertia change of the rollers is of course factored into the power output calculation, and their mass and 'inertia' is precisely known.
A correctly calibrated dyno for THAT particular car SHOULD show the same power whether you do the pull in 2nd, 3rd or fourth gear, but no one does this.
If the dyno shows MORE power in say fourth than in third, the inertia "value" is too low. If it show more power in third than in fourth, this inertia "value" is too high. It is JUST right when they both result in the same power.
But this is a tremendous pain in the ass, and dynos are TUNING tools.... we don;t race dynos
SO no one bothers, and most operators probably have no clue how to do this.
To 'cheat', I frankly use a standard ramp rate.... ie, on my dyno, I simply tell the dyno to allow the motor to accelerate at 500rpm/sec as an example. In this case, a gear change will show no significant change in power output since the acceleration rate of the run is the same.
Now, if you want the "real" power that car is making? Set the load type dyno to hold STEADY your peak hp rpm so it takes losses to inertia out of the equation, and let the load cell MEASURE your output.
