From my FAQ's:
Q: What is the best fuel to use?
A: Ralliart only recommend the use of Super Unleaded (97RON) in all Evolution models in the UK. The Evolutions are built for the Japanese home market and as such the cars would normally use 100RON fuel (they do have up to 105RON). The engine management in the Evolutions can adapt to lower octane fuels but performance will be reduced slightly to prevent detonation or knocking. The higher the octane fuel you use the less chance that knocking will occur. There are new fuels coming on the EU market that are slightly higher rated (Shell Optimax at 98RON) so using those should in theory improve performance slightly over normal Super Unleaded.
The official advice from Ralliart is do not use 95RON Premium Unleaded unless in an emergency. If you have to fill up with 95RON Unleaded for any reason then drive carefully until you get a chance to fill up with 97RON Super Unleaded again. Ralliart do not recommended the use of Octane Boosters even in an emergency.
It has been suggested that driving for long periods on 95RON Unleaded can possibly lead to damage of the engine, however Evos are run in countries without major problems where the maximum rating of fuel available is only 93RON (e.g. Greece, China and the West Indies).
Do not use 4 Star or Lead Replacement Petrol, it may have a higher Octane Rating but it contains additives that can damage the Catalytic Converter and the Lambda (oxygen) sensor.
To find the availability and price of Super Unleaded in your area then look here: www.Ispypetrol.com (note not all petrol stations are listed).
Q: Should I use Octane Boosters?
A: The Evolutions are built for the Japanese home market and as such the cars would normally use 100RON fuel. The maximum rated fuel that is obtainable from filling stations in the UK (and most of the EU) is currently 98RON (Shell Optimax) or 97RON (Super Unleaded). Using a lower RON rated fuel than was intended by the manufacturer means the engine will not be running at it’s maximum potential and if the RON rating is too low it can actually be harmful (generally don’t use fuel rated less than 97RON unless in an emergency, see ‘What is the best fuel to use?’). You can use an Octane Booster to increase the fuels Octane Rating to 100RON (or more). Many people do use Octane Boosters, especially when using their cars on track days, however a lot of people do not use them, it is entirely up to the owner whether they feel it is worthwhile.
It is worth noting that Ralliart do not recommend the use of Octane Boosters, even in an emergency. Also note that your warranty maybe affected if you do use an Octane Booster. It has been said that Octane Boosters can actually leave traces of their use in the engine and fuel system and if you have an engine failure, a warranty claim may not be met if their use is evident (this is unconfirmed though).
(Thanks to Chunky for the link (I quickly added on to the FAQ
))
