Sorry to hijack the thread, but was the 13.5 a big change? I think this thread needs hijacked....hahaha Yes, the 13.5:1 was a big difference. It makes the bottom end and mid-range real strong. The top end stayed about the same. I liked it, but I thought the big bore kit with the 13.5 would be too much, so I went back with the 12.5. Sometimes I kind of wish I had the 13.5 w/ the big bore, but its got more power than I can manage now.