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we've ran 87 octane into our xr500r bike iwth a 12.5:1 for a long time now, its never really had any objections, except one time it back fired and blew out acarb gasket.
anything higher than 11.5 :1 your octane should be increased or you run the risk of pre detonation (knocks or pings) and that could cause some big repairs. but you can run lower octane just might not run as well.
10:1 is the highest compression to go with 93 gas. any higher and your looking for trouble.
at the compression your at and the low octain your using your riding a ticking time bomb.
your going to have to mix race gas. octain booster wont work
sorry if that came off wrong. what does the ex come with? ive heard 1 point on compression 50/50 , 2 point race gas. i went up 2 points so 50.50 was not a option.