Since installing a set of Street 6X's on my 455 HO I have knocked off almost 4 tenths of a second. I replaced the #64 heads that were 10.75 to 1 with these 6X's and couldn't believe that I could get more power with lower compression. Pump Gas with more power how can you ask for more. I can run 91 octane at 34 degrees total advance. Don't take a chance from someone that doesn't know Pontiac Heads, Call Dave at SD Performance.