Is there any torque loss with a lightweight flywheel?

When you install a lightweight flywheel, there is no torque loss from the engine. End of story.

A flywheel, by it's very nature, is an energy storage device. It's purpose is to either absorb excess energy from the engine, or average out the energy being put into the transmission, when an imperfect rpm vs. gear/wheelspeed clutch engagement happens.

The energy is stored in it's mass. When you rev your engine, energy is used to move that heavy flywheel... energy that is not put to the wheels. The mass of the flywheel stores that energy by it's motion.

Thus, a lighter flywheel requires less energy to be moved, allowing more of the net total force produced by the engine to go to the wheels, making the car quicker. The mathematics of this are covered on our website here:

But the important question is then, how does this jibe with "real world" experience?

Those who say their car feels quicker with a heavy flywheel are revving the motor above the correct gear/wheelspeed amount, and "dumping" the clutch. The heavy flywheel stored energy, and then transferred it to the transmission/wheels. Technically, during this engagement, the stored energy plus the engine input energy is more than the engine makes alone. The critical thing to realize is that this only holds true during the period of clutch engagement and is a "false torque" sensation.

That "false torque" sensation tricks human perception into thinking the car has more power, when the torque measurement of such an engagement style shows that the "false torque" jump up is immediately followed by a sub-normal torque dip as the heavy flywheel requires more energy input to re-accelerate than the lightweight alternative.

An example of this is quite clear in a dyno graph. This section shows a torque curve, specifically the initial clutch engagement at 2000rpm and up to 3500rpm.

The "false torque" effect of the green line clutch dump is obvious; during, and only during the clutch engagement up to 2500rpm, is the energy that the flywheel has stored being transferred to the drivetrain. After 2500rpm, it uses even more energy to re-accelerate. Contrast this with the red line smooth engagement which has more torque at the wheels everywhere above 2500rpm.

Even though that graph is showing two successive runs on the same car, the point regarding the "false torque" effect of the heavy OE flywheel is exactly the same.  This explains a few examples where magazine dyno tests are mistakenly showing torque loss.

So you see, this is the reason why a lightweight flywheel car will walk away from a heavy flywheel car every time, regardless of how the initial clutch dump may "feel" stronger with the heavy flywheel.

2004 UUC Motorwerks