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So I'm trying to understand what octane rating I really need here in the great white north:

My manual says:

1. "Your engine is designed to provide optimum performance and fuel economy using unleaded premiuim fuel with an octane rating of 98 RON (93 CLC or AKI).

Porsche also recognizes that these fuels may not always be available. Be assured that your vehicle will operate properly on unleaded premium fuels with octane numbers of at least 95 RON (90 CLC or AKI), since the engine's "Electronic Oktane knock control" will adapt the ignition timing, if necessary."

2. On the fuel filler door of my Box it says "Minimum octane RON+MON/2 = 93"

Some quick net research tells me that AKI (Anti Knock Index) is the same as (R+M)/2.

Therefore the manual and the fuel filler door don't agree, and I'm left wondering if I really can run 91 or 92 instead of the much pricier and rarer 94 I'm currently using.

It sounds some of you are running 91 without any issues, but I'd like to know if that's more universally the case before I switch.

And I'm also curious if USA cars have the same label on the fuel filler door that I have.

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yes, you can use 91/92.

however, you will not extract the most amount of power that the engine can deliver, and your mileage will not be as good as if you were using 93 (which means you will have to buy more gas more often). the engine was built to use 93 and likes 93 the best.

so, in the end it's a wash. use cheap gas = buy more gas. use expensive gas = buy less gas. ;)

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Strange my 01" S says 96 RON under the fuel cap

I have never used anything less than 95 Ron mainly due to 98 not being available, worked ok i guess, didnt really notice any fuel economy or power decrease.


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The thing to remember here is terminology. RON (Research Octane Number) is used in most of the world (Europe, Australia, etc), and ((RON+MON)/2) is used in the US. Since MON ratings (Motor Octane Number) have a lower numerical number, it reduces the overall numerical value. So "93" in the US is about the same as "96" (actually probably closer to 97) in Australia. The main difference between RON and MON is the way it's tested. The RON number is tested at 600RPM and the MON number is tested at 900RPM (with a few other differences, including inlet/mixture temperatures).

It is generally thought that the R+M/2 method is better, since it gives an average of two tests - and since the MON number tests fuel under harsher conditions, it does appear to be a better test. So even though your fuel may be well within the RON spec, you could still theoretically destroy your engine if the MON number is too low. Considering how improving fuels is a costly exercise, I do worry that when using the R+M/2 method, that fuel manufacturers could skimp a little on the MON rating, increasing the RON rating to make the combined average look high. Most cars on the US roads wouldn't really care (low performance automatics), but a high-performance engine could easily be destroyed. Then again, maybe I'm just being paranoid...

Living in California though, I generally only have access to 91 R+M/2 (theoretically equivalent to around 95-RON), and haven't too many issues - I also have to use this sub-standard fuel in my turbo-charged Evolution, which means I'm definitely not getting the performance I should... Unfortunately, California simply has too high of a demand on premium fuels, so they have to reduce the quality to have enough to go around.

Bottom line is that your car will be fine on 91 R+M/2 - that's what almost everyone in California use.



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