Several answers on this board are correct.

The manual says 91, so you are pushing the warranty. Does it harm the engine for a bit, probably not. But this is a fairly high compression motor and the timing pull has limits. So if the timing reduction is not sufficient (maxed out) you will get knock and the potential for engine damage. I can't tell you where that point is because I don't have the maps.

One of the real differences in high octane gas is the additive package. Every analysis I have ever seen shows VASTLY better additives in premium gas. Those additives reduce carbon buildup on the valves and head. That carbon raisese the octane requires of the engine AND creates preignition hotspots.

Don't get low octange gas.

 

I see what you did there....

 

huh?

 

you'll start knocking at higher rpm.

91 is crap as it is. this car should be running 98 octane just like how Evo owners manual suggest running 101 octane, but 93 octane is ok.

 

Every once in a while, this question comes up, and then we have to calculate how much he's gonna save each year by filling up 89. If you bought the S and find yourself wanting to save money by not filling 91, then you bought the wrong car and instead should've gone with a miata. I wouldn't even think of feeding the car 89, occassionally I even feed it 94. Seriously, the damage you're gonna cause will be dramatically greater than the pennies you save to get yourself a few Macdonald meals which make you an obese hoe resulting in lower mileage.

 

I have a friend that fills 4 gallons of 87 and 6.5 gallons of 93, he claims that with this mix he gets 91 octane and saves money and the car doesn't ping.

 

that is the most ridiculous thing i have ever heard

 

 

I'd congratulate him for passing his Gr. 1 Mathematics.

 

From wat I have read, the map has a higher limit for dynamic timing to above 93+, so all of you adding 1 bhp $1,000 toys could just use better gas...