On an inertial dyno you will record a horsepower gain.
On other types of dynos you will not record a horsepower gain.
It depends on which type of dyno you use.
The main things is that with a proper aftermarket lightweight flywheel your car will rev much more freely. Allowing you to get up in revs quicker to reach the power you have, and a lightweight flywheel will facilitate heel-and-toe rev matching much easier.
Originally Posted by
How"Solving for horsepower, the (stock OEM) flywheel consumes 5.8 horsepower in 3rd gear. If we hold all the other variable constant but swap in a a 9-pound lightened flywheel, the flywheel only consumes 2.9 horsepower, freeing up 2.9 horsepower in 3rd gear. A lightened flywheel frees up even more power in the lower gears since it is harder to begin momentum than it is to continue it.
Solving for 1st and 2ond gears, the lightened flywheel frees up 16.6 horsepower in 1st, and 5.9 horsepower in 2ond. Gains in the higher gears are negligible, about 1.7 horsepower in 4th and just over one horsepower in 5th." Author Sarah Forst