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shihan969
By shihan969 | AA Member | 2 Posts | 1 September 2011 at 8:17pm

My Mitsubishi Diamante (1999) has a 3.5ltr engine. Contrary to what I understood before, this car has printed on its petrol tank access flap to "use 91 octane"... I thought for big engines 95/98 octane was more suitable. Does the size of engine have a direct relation to the fuel grade?
Thanks!

jbiddle
By jbiddle | Community Member | 1657 Posts | 9 September 2011 at 1:04pm

The recommended octane rating can vary not on engine size, but on the compression ratio within the engine. This is the basically the amount of pressure inside the combustion chamber that the fuel can tolerate before ignition takes place.

The lower octane fuel (91) is used when those tolerance levels are low.

For example, using 91 octane fuel in an engine which has a high compression tolerance (and a higher octane fuel is recommended), can cause the fuel to pre-ignite which in turn can cause internal engine damage.

Having an engine which can run the lower octane fuel is an obvious advantage when it comes down to fuel costs and it certainly does not mean an engine will lack performance because of it.

However, the higher the recommended octane rating, the more performance orientated the engine will be.

Adding a higher octane fuel (95) to an engine designed for 91 octane is never going to do any harm and for some owners, who don’t know what fuel they should be adding, it is a safe option.

In summary, there is no advantage in you switching to the higher octane fuel, you are best to stick with the manufactures recommendation and save your money.

shihan969
By shihan969 | AA Member | 2 Posts | 10 September 2011 at 5:36pm

Thanks! That makes a lot of things very clear.

Of course now another question comes to mind: how/where does one find out the optimum octane level for an engine? My present car is the only one that I have come across so far with a recommended octane level printed on the petrol tank lid.

jbiddle
By jbiddle | Community Member | 1657 Posts | 15 September 2011 at 10:49am

It’s a very good question and one which we can't really answer with any certainty. The franchise Dealer for a particular brand is the best place to start however they often make a call based on caution more than anything else especially with older models or used imports.

In most cases they will recommend 95 octane as it is a safe option.

If we are given an engine code we can often search various data bases to try and find a engine compression ratio which helps determine the required fuel.