i had 2:73 gears on my 93 Camaro Z28 6 speed manual tranny. I dynoed at 278.00 rwhp with every bolt on. I dynoed a second time with those same gears again at the same numbers pretty much. I changed to 3:42 gears and only that mod i dynoed lower to 266.00 rwhp lost 12rwhp. Ive heard from several people that you lose rwhp on the dyno due to gears but why ? and do you really lose rwhp ? why cant the dyno compensate for the gear swap and give you correct rwhp numbers. Can anyone help please ! ! !

 

There was probably a change in ambient temperature, barometric pressure and humidity.

Also, the dyno may not be correct.

The gear change will produce more torque at the rear axle, but will not affect RWHP other than negligible changes in internal differential frictional loses.

 

I know what your sayin, but i dynoed twice around 278 before the gear install and twice around 266 after the gear install. R&D and Haddadmotorsports, plus countless other people have said you will lose rwhp on the dyno but not actually lose it, best explanation i got is that your car isnt working as hard to spin the wheels kinda like a longer wrench gives more torque but less effort is what ive been told so on the dyno comes up less, but explanation wasnt very specific.

 

A Dynojet determines HP by measuring how rapidly the car accelerates the drum. This is a quite accurate way to determine HP, as long as something else isn't limiting acceleration.

One thing that can limit acceleration is flywheel mass. A light flywheel allows an engine to accelerate much more quickly than a heavier one. The reason is that to increase a flywheel's speed requires the engine to change the flywheel's angular momentum. This takes power, power that becomes unavailable to the rear wheels.

Ok, this is well known. What may not be so obvious is that to accelerate a flywheel more rapidly requires more HP. When you change the car's gearing so that the motor has to spin up more to reach the same rear wheel speed, a flywheel of the same weight requires more power to change its angular momentum in a given amount of time. Thus, the rear wheels won't have as much available HP to spin up the Dynojet's drum as it did when the gearing was milder.

On a paddle brake dyno, which can apply a load at constant RPM on the engine, you'd still read the same rear wheel HP regardless of gearing. But the Dynojet doesn't impose a speed independent load. It depends on the inertia of the drum for its load, and that makes its measurements vulnerable to the effects of other rotating inertia loads in the system. So it'll read lower with stiffer gearing.

 

i dynoed it at 278 without the flywheel just stock and twice at 278 the third time i dynoed it was still with stock flywheel just the rear change from 2:73 to 3;42' no other mods and dynoed at 266 then i dynoed it again and still with the same steel flyweehl and 266 again i haven dynoed it yet with the aluminum flywheel and the clutch and the 4:10 gears so these three mods have not been dynoed yet. So does the lightweight flywheel rob rwhp from what i understood.

 

I think you misunderstood Shopdog's explanation. A lighter flywheel would have less rotational inertia, not more. Therefore, if you had dynoed the car before and after the gear change, but with the lighter aluminum flywheel, you could expect to see less HP drop. Also, if you swapped out the steel flywheel for a lighter aluminum flywheel, with no other changes, you could expect to see a HP gain.

Aside from the rotational inertia issue, shorter gears (higher numerically) do have more internal friction, and this also results in a slight power loss.