8.5:1 vs 10.1 Compression

[Jim]

I wrote some questions in the Carb section, and you have helped me make up my mind on which carb and mufflers to get. But I am having second thoughts if this motor will have enough power for me without a Turbo on it. If it does then I might consider raising the compression up a little. It's 107 cu in S&S Super Sidewinder Evo w/ S&S 600 Cam, OEM Bagger exhaust with SE mufflers and 44 SE carb, and 8.5:1 compression. With your Dyno experience, just guessing, about how much Rear Wheel HP and Torque can I expect from this motor with only 8.5:1 compression? And how much with 10.0:1?

[Mike]

A well setup S&S motor typically gets 1hp/cubic inch and slightly more on torque. The computer model shows 8hp difference between 8.5 and 10.5 compression, experiance has shown this to be more like 10-12. It also shows corrected pressure (based upon when the intake valve closes) to be 7.27:1 for 8.5 and 8.94 for 10.5 using the S&S 600 cam. 9.1-9.4 is what I like to see for a street motor using 92 octane pump fuel. To achieve this would require 10.7-11.0 mechanical compression.
My larger concern would be that the 8.5 comp version would not support the 600 cam. The torque may be flat and low with the HP curve shifted beyond the rev limiter (still climbing at max rev and never reaching peak, typical for an overcamed motor).
Another, less complex, option would be a narrower cam that would make more cylinder pressure as a function of the earlier intake closing. Overall HP would not match more cam/compression but the torque curve would be better than the 8.5/600 combo. The Wood 6h or S&S 561 look good for this. Last the low compression version will run cooler, be more forgiving on fuel quality, less sensitive to ignition timing and theoretically last longer.

OK, thanks for the input Mike. I was thinking about a 10 HP loss myself. I also have a Redshift 559 cam that closes the valve, I think at 46 ABDC, compared to the 55 ABDC of the S&S 600. I may stick with the 8.5:1 for now! Only because I "may" go back to the Turbo next winter. For what it would cost to change my compression, I could get a Turbo (since I would have to have pistons made, or buy a new set of shorter 107" cylinders to replace the 113" cylinders). I will just have to see how the bike runs on the road first before making any long term decisions. IF it feels OK, and I know I can get about another 10 more HP out of it, then I may trash can the Turbo idea completly. I'll have to bring it in for a Dyno tune anyway so you can rejet the carb. That will show me how much power I am making. Thanks for all your help and advice.