I'm glad to see such impressive dyno results. I had the Stillen exhaust installed about a month ago and my butt dyno tells me that it did increase power, but I have not had the Z dyno'd. I should expect that my increase in power is similar to yours since my car was also stock before the exhaust. I agree with your assessment of the sound as well. It's is not offensive when cruising but has a great deep sound when the RPM's climb. I am very happy with mine. Congrats on the install!
AZ
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AZ Roadster
'09 Base|Chicane Yellow|TSW Nurburgrings|Stillen CBE|Eclipse AVN726E In Dash DVD/NAV|Clear Bra|H&R Spacers 15/20|Hankook Ventus V-12's 275/35 & 305/30
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