What fuel should I run: 91, 93, or can I use 87?

Subaru's published WRX specification is premium unleaded, 91 octane or higher. That makes 91 the minimum rather than the recommendation, which is a distinction worth internalising. **Can you use 87?** No. The FA24 is a turbocharged direct injection engine running meaningful compression and boost, and its knock margin assumes premium fuel. The ECU will pull timing to protect itself, which means you lose power and gain heat while saving a few dollars a tank. Doing it once in an emergency because premium was unavailable is not a catastrophe - driving gently until you can dilute it with premium is the sensible response. Doing it routinely is asking the knock control system to compensate for you continuously. **91 versus 93.** If 93 is available at a reasonable price, run it. The gain is not dramatic on a stock tune and nobody should expect a transformation, but higher octane means more knock margin, and knock margin is what the engine spends when conditions get difficult - a hot day, a long climb, a heat-soaked intercooler after repeated pulls. That margin matters more than the peak power figure suggests. On a hot afternoon a car on 91 is pulling timing while a car on 93 is not, and that is felt as the car going soft rather than as a number on a dyno. **Once you tune, fuel stops being a preference and becomes part of the maintenance regime.** A map written for 93 running on 91 is asking the engine to tolerate conditions the tune did not assume. Match your fuel to your map, consistently, and if you cannot reliably get 93 then have your tune written for 91 rather than hoping. **Use top tier fuel where you can.** Octane rating tells you about knock resistance, not about detergent packages or consistency, and effective octane at the pump can fall short of the rating at low-volume stations with old fuel. Poor quality fuel is one of the most common causes of knock corrections that owners then misdiagnose as a tuning problem. **Ethanol is a separate conversation.** E85 is not simply a higher octane option - the VB is not flex fuel from the factory, and putting ethanol in an unmodified car means running dramatically lean for the fuel. With a flex fuel sensor and a proper tune it is the best value power on the platform. Without that hardware it is genuinely dangerous.