I felt a difference at high rpms. The engine feels smoother above 3500. I find myself staying in 4th gear sometimes when I should be in 5th because of the decreased vibrations. There could be other factors at play too. Maybe my engine mounts are worn, so I felt more vibrations with stock crank pulley. I don't know. But I did feel a difference. If you decide to get one, get if from Bill Jenkins at Signature Lincoln/Ford in Las Vegas. He sells them for about $50 if I remember correctly. Cheapest I have seen anywhere.
06 Mustang GT - stock
96 SE V6 5spd Midnight Red - 15.9 @ 86.8
150K miles
Mods: Bilstein struts, H&R springs, Borla Cat-Back, Brullen y-pipe, Gutted Precats, SVT UIM+LIM+TB, K&N FIPK, , DMD, Magnecor KV85
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