I was curious about actual horsepower gains from a lightened flywheel, 2.7hp/lb (horsepower per pound) being the common throw around answer in various circles. I found this article which applies actual physics (imagine!) and breaks it all down. I'm too tired to summarize, if you got this far take it one step further and click HERE then scroll down for the discussion on lightened flywheels.

Reading the whole article is worth it, but here's the summary. Essentially, lightening the flywheel DOES NOT increase horsepower, but it DOES allow your car to get to its horsepower faster, which would lead to faster acceleration, so it may have the same affect for most people. Every post I've read on this site raves about lightened flywheels, I would assume because of the fact of increased acceleration.

Summary from article:

"Finally, a flywheel will not show much of a gain in horsepower on a dyno because it doesn't increase fuel or air and can't increase horsepower. [B]Remember, you're not actually improving your car's power, just how quickly it can get to that power.[/B] An inertial dyno will show gains by the engine reaching a predetermined rpm sooner with a lightened flywheel."