Specs HP ratings for their clutches on their site are very accurate if you consider flywheel hp like they do. An important disclaimer is that they will hold up to that point if not abused, IE if they are not slipped. For example, if you have a stage t2 clutch that is rated to 350hp and your motor makes 350hp at 3700 rpm, the clutch will slip there. This means you must granny shift, and make sure you have full engagement on every shift. If you do not, the clutch will fail at significantly lower levels. I guess this is true for every clutch out there, but it is a buzz kill when you have a 350hp clutch fail at 220whp because you launch it hard off the line a couple of times.
Oh and Spec does not stand behind anything, and buing used is asking for trouble.
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Hank