99 yz 400f bore

so i have a quick question. i bored it my 400 to the 420 and im thinking about runing the 13:5:1 piston ration. some people have told me i need to run race fuel or higher octane with this ratio is that true? and would i be able to just buy 91 and add more octane to it? anything helps thanks :thumbsup:

13.5:1 is usually manageable on pump premium, but with the 400 you may have to just try it and see. Part of it depends on whether you ride at 2-3000 feet or above most of the time and/or whether you like to lug the bike around or rev it up some.

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