Only way to really tell is with a before and after dyno pull. Most people have seen gains without a tune though. I'd personally hold on any under drive pulley upgrade. Marginal gains (+/-5 hp) with a reasonable amount of risk involved to the motor. Plenty of argument from both sides, but why take a chance for a 5 hp or less when combined with other mods.
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[09][MB][6-Spd MT][Touring][Stillen Gen III][K&N][Borla CBE][Evo-R]
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