I just purchased a GP GT2 2004, put a K&N FIPK
http://www.knfilters.com/search/product.aspx?Prod=57-3049 on and there is noticable gains.
Also with the help of guys on here I just purchased a PCM from ZZPerformance. Both are supposed to give 10 - 15 HP each. We'll see though....
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Grand Prix GT2 2004
ZZP Z7 Turbo Kit 62-1, BC, 42.5# Injectors, LT1 MAF, 160 tstat, A/A IC, BOV, 3" ZZP SS Exhaust /w cutout, GMPP Handling kit
339.2 WHP 358.6 WTQ @ 13 PSI (zzp's mustang dyno)