I think the alternator pulley gives you 2 hp, maybe 3.5 hp max.
With my music at full blast (2 amps, subs) my meter goes down to the line before the last one when the bass hits, so I don't play it much.
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1993 Mustang GT - March Ram Air, K & N filter charger, Pullies, Super Competition Hooker Headers, 2 1/2 in. mendrel bent cat back, High Flow H-Pipe, Pro 5.0 Power Tower