jkoc

newbie question, please excuse...

i was browsing many WRX/STI forums as well as reading the STI website, and I came across a spec that says the STI 's minimum required fuel grade is 93oct???

is this true?

if so, wow, does this mean people who live in areas that do not have 93+ grade fuels readily available will suffer from knocking and overall poorer performance? will the ECU adjust for "poorer" 91 octane fuels?

where i live, most fuels are 91. the only place with better fuel is sunoco with 94, however sunoco stations are not as ubiquitous as the other stations.

did the 2005 STI's have the same requirement? any comments from you guys?

thanks

Egan

iTrader: (14)

As far as I know it has always been 91 octane. However, if you have access to something greater than 91 you should put it in every chance you get.

jkoc

in the canadian subaru website it says
http://www.subaru.ca

"Fuel: Recommended minimum octane (AKI) 93 "


it doesnt mention anything about fuel in the american site (www.subaru.com)

PyroManiac

Depends on the elevation. If you are like me, 5440 feet, 91 is all you have. Just use the highest grade available in your area.

meilers

No, this is NOT good advice! If he still has the 2 catalytic converters installed (a stock car) then running race gas or very high octane fuel can degrade the cats and cause them to fail. This was a more serious issue with the up-pipe cat, obviously; having a midpipe cat break up wouldn't be as dangerous. Running the grade the engine is tuned for is always the safest bet.

The higher octane a gas is, the less likely it is to burn -- I realize that is counterintuitive, but it is definitely the case. Thus, to burn higher-octane fuel requires advance timing for the spark, or much of that fuel will pass into the chamber unburnt and you get incomplete combustion, and gas (as vapor) makes it into the exhaust. Raw fuel hitting the cats is a bad idea! The only way to have that timing advance is to have the engine tuned for it; the WRX and STI are tuned for 91 (US fuel) and 93 (Canada and Europe) in anticipation that the owner will use that gas. Using 100 or 96 in an engine not tuned for it is literally throwing gas away (and at $4.00 a gallon, no less.)

Here's some links about the issue:

http://auto.howstuffworks.com/question90.htm

http://science.howstuffworks.com/gasoline3.htm

Sorry to drop a big lecture on you, I just feel strongly about the issue (having known several idiots I won't name who have toasted engines and exhaust systems by dumping octane booster in their tank).

Uncle Scotty

.......NO....not.

Higher octane fuel is more resistant to NON-spark ignition......that is totally different from 'less likely to burn'. A leaner mixture can often be used with higher octane fuel and not detonate, as well.....but this is not connected to 'burn rate'.

Also, there are other more complicated issues(RON VS MON being one), depending on fuels available and race fuels more specifically.....but you should use 93 or 94 octane fuel if available and running higher octane fuels will IN NO WAY damage your catalytic converters UNLESS it is LEADED fuel which will quickly clog and destroy converters.

Subaru ECU's have been shown to benefit from using fuels in the 95-96 octane range....no timing pulled, as well.

meilers

We're talking about the same thing using different terminology. I'm trying to simplify the issue because, as you note, it can get complicated quickly. High octane fuel resists combustion from compression alone; it does so by being less likely to flash into combustion by just pressure/heat, and thus it is less likely to burn as an aerosol without a direct spark. You are absolutely correct that race gas is a whole different issue, I should have left that to the side. I didn't want to get into the whole heptane/pentane/octane hydrocarbon stuff...

Whether it was from bad tuning or high EGTs or fuel, I know of two cars with toasted cats, both of whom added "octane boosters" to their fuel to bring it to the 95-96 range. Perhaps they were getting actual backfires as the unburnt fuel hit the cat. I've also seen stock WRX and STI models pull timing on just normal 91 octane -- that ECU is a sensitive beast (04 STIs knock like crazy on California 91 at WOT). In any case, you're still throwing the $4.50 gas away unless you've been tuned for it; why else would Cobb and many other tuners create specific maps for specific grades of fuel?

Uncle Scotty

Using fuel at a certain octane rating and adding 'octane booster' are totally different.

Using 94 octane fuel and a 91 octane map will have no detrimental effect what so ever..... while using 91 octane fuel and a 94 octane map almost certainly will cause det issues.

Egan

iTrader: (14)

Thanks for the lecture, but we were talking about 93 octane, not race gas.

WRX-pilot

What do you consider high octane or Race gas? Your info isn't really true as 100 is considered Race Gas by some and is perfectly fine for your car. If you go to certain race tracks you may find something around 114 or 116 and that will contain lead. Leaded fuel is what destroys O2 sensors and Catalytic converters, however 95,96.... 100 octane will not hurt anything (because in realistic cases there is no lead in the fuel).

No, not counterintuitive, nor is that correct. Its not less likely to burn, nor is more likely to burn. You made need to take a closer look at those links you posted, the point of a higher octane is the amount of compression the fuel can take without combusting on its own. I know this is what the Uncle Scotty said but you need to realize what you are saying and compare that to the info you are trying to back yourself up with... they conflict.

I can agree with you that using higher than the recomended octane is throwing away money... after a certain point (I though it was 94 but maybe Uncle Scotty has a better idea) you are paying for better gas but not getting a better result.


As far as the Original threadstarter

I thought on the fuel door it has a message saying use 93 otherwise use 91... That might be a place to check. Maybe Ill check mine when I get the chance.

jkoc

ok thanks.

i guess 91 is fine

hey , i'm not a WRX nor STI owner yet. i'm doing my homework and reading a bunch of faq/stickies and using Search feature to find out my answers heh.

WRX-pilot

Well I went ahead and looked and there is nothing on the fuel door. There is a thread on Nasioc though that has word for word what is written in the manual of my 2005 STi:

I realize that the information being sought was for an '06 but considering there haven't been any drastic revisions to the STi's 2.5L engine the information should still be completely relevant.

And on another note, I live in California and with an exception of a few specialty 100 octane pumps scattered throughout the state 91 octane is the best you will find.

meilers

...you need to read my follow-up post, not the one you quoted. We're in perfect agreement here.

Well, my real point is to caution against "homebrew high octane" and pump 110 (which you can get at some select stations; I know of two here in AZ) in a car not tuned specifically for it.

Stargazer

That's the exact same info in my 2006 manual: designed for 93 but OK to use 91. Where I live, we don't have 93--it's either 92 or 94 unless you're out in the suburbs where they only have 91.

WillisW555

iTrader: (1)

You could mix them.