whats the bar size?
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Almost all petrol engines in the last 20 years are spark limited. They need 95RON + to not pull spark timing down low. In some cases they are tuned for 98RON and that's doubly so. Aggressive timing plus higher compression ratios for emissions and efficiency means they need high octane to work best. Basically if your engine has knock sensors then it wants high octane to work well. Sure it'll run on 91. No worries. It'll lose some low end torque and be a bit doughy but still drive fine. You won't even notice unless you change fuels and feel the difference. Modern electronics and a lot of calibration will see to that.
E10 is even cheaper than 91 and makes cars drive like they are on more expensive 95 PULP. In most cases the fuel economy difference is negligible and in all cases the cost per km is better on E10. What's not to like about that? The mythical 3% economy loss on E10 only applied to dumb old engines that can't retune for the fuel (technically it's around 2.5% by energy density). Anything with knock sensors will love it and give you that back, and more.
In the US they use AKI for their "octane" rating. Anti-Knock Index. That's MON + RON / 2. Simple as that, just an average. In Australia, and most of the rest of the developed world, we use RON. Research Octane Number. MON (Motor Octane Number) is lower than RON so AKI is also lower than RON. 91 AKI is about 95-96 RON, for example. The Yanks base fuel is 87-88 AKI which is equivalent to our base 91 RON. In much of Europe you can't even get anything less than 95RON. 91 is looked at as a poor fuel not worth using. I generally tend to agree...
There are good reasons to avoid E10 in outboard motors, but those reasons don't apply to most cars.
My SV21 Camry has done 4 or 5 hundred thousand kilometres on E10 with no problems.
I was once told I would get better power and economy with premium unleaded. I tried three consecutive tanks of premium and the fuel consumption was identical to the figures I got with E10.