I realize it is a high performance motor and the manual calls for 91 octane (which is high test in the US). I've been running regular (87 octane) and the car runs fine. The first time I tried it, I expected knocking but there doesn't seem to be a problem.
Is there a good reason to spend the extra money to get 91 vs the 87 or even the 89?
These cars are designed for high test. If you take care of them according to manufacturer specs, they can last a looong time. Never put regular in mine but once when the station had no high test. Ran fine, but I would recommend you use high test.
I am sure the techies here, which I am not, will share their scientific microscope.
As above, it will run fine on normal fuel but performance will be slightly reduced.
It's all about pre ignition when pushing on, the higher the octane the less chance of that happening. Pre detonating is bad for your engine and performance, I would recommend that you use what it is recommended in the manual. :x
You'll also find that you will get poorer MPG ie: what you save on cents / gallon you will lose by having to use more fuel because as @i860 said, the ECU will compensate.
If there are maintenance/repair issues down the road, I don't know.
You also need to know that the computer will take some time to adjust for higher octane. It retards the timing very quickly upon ping from low octane fuel, but does not advance as quickly. You might have to run two tanks before you’ll see a performance increase.
Everyone's said what I would say except you should check out the prices here in the UK. But we do get 99 Ron. Years ago we had what they called 5 star which was 105 Ron as I recall, but then gas was peanuts by comparison... Showing my age!
Just copied and pasted the instructions as a word doc. I'll try it first thing in the morning - there is automotive get together in the morning at Michelin. They do it every month but it looks like heavy rain tomorrow so I may miss it.
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