a dynapack dyno in particular uses a hydraulic pump attached to each wheel. restricting the outlet on the pump turns it into a brake, and measuring the pressure the pump builds up turns out to be an accurate measurement of how much torque the engine is exerting to turn the pump. the amount of restriction at the outlet of the pump controls the rate of rpm increase and the length of time it takes for you to hit redline, not your horsepower.

my question is with a brake applied to the drivetrain (dynoing on a dynapack), would mods like a lightened flywheel or UDP show an increase in power?

i don't think the dyno would show the increase as the engine is being mechanically bogged, i guess you could say, to measure the hp so the effects of any lightening shouldn't be noticeable. i guess you could say the engine is turning at its full capacity but not accelerating at it full capacity making the lightening of components almost impossible to see.

 

You are correct.

As you know, lightened flywheels and crank pulleys reduce the rotational moment of inertia of the engine. It will take less torque to accelerate these components to the same angular speed. As you pointed out, the slower the engine is allowed to accelerate, the less torque "savings" you realize from the lightened components. If held at a constant rpm, there is no difference (except for negligible windage losses)

This is why lightened components are most noticeable under no load (ie engine free revs a lot quicker) and in first gear

The only measureable dyno gain would be from the reduced parasitic loss from the underdriving.

 

i thought so. i just dynoed at 179 hp and 191 torque and way rich. i was figuring i'd get alittle more cause of the flywheel but once i found out how the dynapack works i figured i wasn't seeing the gains. thanks for the info!