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I know it's recommended but can you just you mid grade, or even regular unleaded?

 
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use cheaper unleaded save you money

 
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Nope, you have to stick with premium. Its a civic, gas should not be an issue for you.

 
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Try the regular unleaded and see what happens, if the. engine knocks, you can have it retuned to burn reular unleaded

 
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What year? Newer cars have the ability to use lesser grades, as the ecu will automatically retard the timing to reduce detonation (aka knocking or pinging).































No harm in trying mid or regular, just pay more attention to the sound of your engine and switch back if you hear noticeable knocking. If its an older Si, i would guess pre-96, you can manually retard the timing.