Domes?

[Fastashell69]

Hey there i got a prodesign cool head with im not sure what size domes but im running 170 compression in each cyclinder right now and im at sea level in santa rosa , california. but i go to sand mountain and there cause of the elevation my compression drops to about 140 and i want that compression back for up there so i was thinkin about gettin some domes for up there when i go and i see something about 68mm domes and some other size like that what is the difference. Also im not sure if my crank is trued and welded so would the only way to find out be to take it apart cause i bought the quad as it sits still a 350 but maxxed out full dune drag racing port with cpi inframes and everything done to it with bigger carbs and the whole setup, how much compression should i run up to with out knowing if the crank is trued and welded? hope to here from you guys soon! Thanks