Well, NHRA requires a SFI tagged flywheel on any car running 11.49 ET or quicker. Personally, I think age and condition are more a consideration than an arbitrary HP level. A 500 HP engine that spends its time cruising around on the road on sunny weekends would stress the flywheel much less than a 400 HP engine in a car that gets raced every weekend, at high RPMs. Keep in mind that most original FE flywheels are over 50 years old, cracks an not uncommon , and there may have been several cuts made to that old flywheel. I have seen a few stock cast iron flywheels come apart, and do considerable carnage in the process. Around 2004 I had a clutch disc, NOT a much heavier flywheel or pressure plate) come apart in my Fairmont. It was bad enough to break off the starter, bend the clutch fork, and deform my Lakewood scattershield so badly that the steel bulged up enough to prevent getting a wrench or socket on some of the bellhousing bolts. I am a pretty cheap guy, but I consider a good flywheel and clutch money well spent. My 59 will never run 11 second ETs,nor likely see 6000 RPM or more, but it did get a new SFI approved McLeod flywheel when I ordered my clutch.
