What Octane Fuel Should I Put in My Imported Car?
The first thing to understand with fuel, is that there are a number of different scales of octane ratings. So 100 octane, isn't necessarily the same 100 octane in different parts of the world. There are several scales that are in use. It would be like calling something 12 long, without knowing if that is 12 inches or 12 millimeter. Without the scale, the first number is pretty useless. In the US the octane ratings we use are AKI, or anti-knock index numbers. The AKI number is a calculated number from MON or motor octane number + RON or research octane number, divided by 2. RON and MON numbers are anywhere from 8-12 points different
What kind or octane gasoline should you use in your turbocharged imported car?
A. Premium pump gas.
What does that mean? Whatever is the highest octane fuel you can get locally. Here in the USA that can be anywhere from 94 octane (AKI) to 90 octane(AKI).
Research Octane Number (RON)
The most common type of octane rating worldwide is the Research Octane Number (RON). RON is determined by running the fuel in a test engine with a variable compression ratio under controlled conditions, and comparing the results with those for mixtures of iso-octane and n-heptane.
Motor Octane Number (MON)
Another type of octane rating, called Motor Octane Number (MON), is determined at 900 rpm engine speed instead of the 600 rpm for RON. MON testing uses a similar test engine to that used in RON testing, but with a preheated fuel mixture, higher engine speed, and variable ignition timing to further stress the fuel's knock resistance. Depending on the composition of the fuel, the MON of a modern pump gasoline will be about 8 to 12 octane lower than the RON, but there is no direct link between RON and MON. Pump gasoline specifications typically require both a minimum RON and a minimum MON.
Anti-Knock Index (AKI) or (R+M)/2
In most countries in Europe (also in Australia and New Zealand) the "headline" octane rating shown on the pump is the RON, but in Canada, the United States, Brazil, and some other countries, the headline number is the average of the RON and the MON, called the Anti-Knock Index (AKI), and often written on pumps as (R+M)/2. It may also sometimes be called the Posted Octane Number (PON).
Difference between RON, MON, and AKI
Because of the 8 to 12 octane number difference between RON and MON noted above, the AKI shown in Canada and the United States is 4 to 6 octane numbers lower than elsewhere in the world for the same fuel. This difference between RON and MON is known as the fuel's Sensitivity, and is not typically published for those countries that use the Anti-Knock Index labelling system.
There are a number of other things related to fuels:
91 Octane - In California our best normal pump gas is 91 AKI. You can see that on a MON scale this is about 95 octane fuel. This would be the lower end of a premium type fuel in most countries. So if you have an imported turbocharged car, and it has a tuned ECU or boost controller you always want to be careful on 91 octane fuel. Monitor knock, and make sure all the engine service and maintenance is up to date.
E10 - Most fuel in the US is E10, or has 10% ethanol content. https://en.wikipedia.org/wiki/Common_ethanol_fuel_mixtures#E10_or_less
E85 - E85 is a blend of up to 85% ethanol and 15% gasoline, although that blend changes through the year in most locations. Without going to much into E85, also called flex fuel, in a turbocharged car, when properly tuned, it is possible to see big horsepower increases due to its knock resistance. https://en.wikipedia.org/wiki/E85
Race Gas - Race gas comes in a number of blends an octanes. You don't need it unless you know you need it. If untuned for race gas, your car may make less power on a high octane race fuel. https://www.enginelabs.com/engine-tech/dyno-testing/the-big-fuel-test-part-4-race-fuels-fight-back/