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Originally Posted by guyryan100
is this based on compression ratio? Is there a rule of thumb here? I would prefer to burn the 87 octane if possible.
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yes it is based on CR. The higher octane helps prevent autoignition or knocking in high compression engines. Stay under 10.5:1 CR if you want to stay on 87 octane. Any higher Compression and you will have to increase your fuel octane. As for a rule of thumb <10.5:1 87 octane, 11:1 89 octane, >12:1 92 octane.