11:1 will sure give a good gain over stock compression, you might want to have your head or cylinder decked to have a good 40 " of squish... or you are mostly going to have in the 9.1 compression range.
ive ran 13:1 piston on pump gas actually 12:1 because of the deck height. with a good porting and cam was enought to keep with 450 on the track.. everything with stock stud, never pulled one at even torqued to 40 lbs ! BUT most 400ex have them pulling out. so i would go with gt or any thing with bigger botom threads.
you dont need to go new valve, and bigger valve spring, mostly going to stress everything more. you could even put a hot cam 3 witout any trouble.
ive ran 13:1 piston on pump gas actually 12:1 because of the deck height. with a good porting and cam was enought to keep with 450 on the track.. everything with stock stud, never pulled one at even torqued to 40 lbs ! BUT most 400ex have them pulling out. so i would go with gt or any thing with bigger botom threads.
you dont need to go new valve, and bigger valve spring, mostly going to stress everything more. you could even put a hot cam 3 witout any trouble.