Depends on what you have to tune. With a standalone system like hondata/neptune/etc and a good tuner, I've seen 13:1 run. With no ECU mods, I've seen around 11.5:1 run on premium. On regular, I wouldn't go above 9.5:1. If you want to bump the compression, you're at least gonna have to run premium. To get the most potential out of the B18A all motor, you're going to need a VTEC head.
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2015 Ford Mustang GT Fastback - Ingot Silver - 6M - Performance Package - Gibson Catback, JLT CAI, FR 47lb injectors, BAMA E85 tune, Eibach Sportline, BMR wheel hop kit, UPR oil separator, Steeda shifter bushing/bracket
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