The 3" inch downpipe adds around 15 hp and I have never heard if the the 4" adds much I could only tell the diff with the downpipe as I added them at different time. the 96 through 94 and a half's are rated at 215 IIRC, and I think on the dyno the 97's make around 180hp at the rear wheels stock. also Diamond eye makes a good one and can be bought for a 319.00$