Thanks for the input guys. I'm obviously going back to 91 just to play it safe given the onwers manual requiring a "minium of 91 octane" Mike what do you mean by "pulling timing"?

 

The ECU is adjusting the parameters your engine uses to prevent damage. Spark timing is just one of those parameters, and one of the easiest ways to reduce power.

 

I have never seen any tests in a controlled lab environment that has shown any appreciable different in gas mileage between 87 and 91+ octane usage. Actually most testing will prove that the mileage between the different octanes remain identical. However, using a lower octane fuel in a car designed to run premium fuel will result in a decrease in power and performance. I hope this helps.

 

I may be wrong but it was my understanding that the 'premium' requirement was based on the japanese ratings, meaning these cars require premium japanese-equiv fuel and that the japanese premium fuel is comparable to our 89/90 US octane and this would be sufficient to use in our cars. Nonetheless, I still use 91 to 93.

 

The ecu doesn't have a way to read what type of octane you put in the car.
If the car starts pinging the ECU will retard the timing. It wouldn't know if you put 100ll, 110 race fuel, or 87 arco fuel in unless it saw a lean condition.

That being said.
I use supreme, but decided to do a test with the 87 octane, as I live at higher elevation, and have a lower risk of deto as opposed to a car at sea level.

I hand calculate my mileage, (the computer is almost spot on evertime for mpg)
Last tank of supreme I got 21.6 mpg. Filled up yesterday after running a tank of 87 and got 21.9 mpg. Being as the fuel mileage was almost identical, I'd say the only benefit to running the lower octane fuel is $5 at the pump.

I did fill up with 89 yesterday, so I'll calcualte that and report back (won't be till next week)

 

Here's an interesting article from UK's leading car mag. On the left, under contents, you can click through the pages. Keep in mind, UK's fuel grades are not on a US equiv. octane rating but on RON alone which is a higher number typically than our octane.

http://www.whatcar.com/news-special-...253&EL=3217470

 

Japanese gas is measured in RON, not (RON + MON)/2 like ours.

However, their premium, is STILL higher grade, than 91.

A prime example is the JDM F20C motor, which is 250ps, as oppposed to the USDM spec which is 237hp. It runs higher compression and requires higher octane than our 91.

 

I would only use premium as well... It's funny because when I bought the car it came with a full tank (as it should) and I seriously wondered with gas prices so high if the dealer put premium fuel or not.

 

Dealers absolutely do not phil the tanks up with premium fuel.

 

Right. They don't, and my dealer suggested I use 89 in mine, says that's what his salesman use in their cars and thats what the service team uses.