I believe the clutch will typically give out ahead of the flywheel...if you're using the HD or KC I assume that would be somewhere in the ballpark of 400bhp. So unless you're doing some heavy forced induction, the iron flywheel should be able to withstand the power. I never really saw the benefit of the billet flywheel when you could just move to aluminum for 60-70 dollars more. Somebody correct me if I'm wrong
600+fwhp thats what DDperformance.com said IIRC. It was high before they recomended using something other the the ford flywheel. Since the Walshes are drivtrain specialist I listen when they speak. .02
I needed a new flywheel for my combo since its a 28oz balance. I went aluminum for the obvious benefits.
If you have the money go with a fidanza aluminum flywheel. They come with different weights so you can interchange it with different engine imbalances.
There isn't a hp # that tells you its time for a new flywheel.