Quote:
Originally Posted by Cobalt
Long gone are the days of 104+ for Sunoco 260. And yes, it made a difference for the 11 to 1 or higher compression fire breathers roaming the streets.
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The bore and head design have a lot more to do with octane need than CR, IMO. The typical small airplane engine that needs 100 octane will have a CR of <9:1, but the bore is typically HUGE at 5" or more and the spark plugs are way off center (though there are two so, maybe not a big deal unless one magneto fails). I have a daily driver (rider) with 12.2:1 CR that runs just fine on 91 octane. It has a small bore (75mm) and a centrally locate spark plug, so it does not much spark advance.