I'm thinking about getting a Pro Design cool head, mostly to bump up the compression.  I'm wondering if anyone knows the smallest cc dome I can use without having to run race gas?  I expect to run 91 octane.  The engine doesn't have any mods, except an FMF pipe and Boyessen rad valve.  It will be running at about 1300 feet above sea level.  I just put a new piston in and I'm getting around 175 psi.  I wish I knew the volume of the stock dome, I'd just go from there.  I'm thinking about getting a dome between 22 and 20 cc's.  Any help is appreciated.