View Full Version : Octane

Chia Wei
03-01-2001, 04:25
Ok, haven't gotten my EVO just yet, but was concerned about fuel octane. The highest we have here in Bolivia is 85, and that is the premium gas. We also have leaded at 79. I know this sucks. Any side effects besides perfomance? I also live at 2500 meters above sea level, so performance wise, it is bad. My Eclipse felt sooooo slow when it got here. Thanks.

03-01-2001, 11:04
I guess the first thing to find out is what octane measurement system the fuel companies are using.

In most of Europe and Japan, octane is measured in RON (Research Octane Number). However, in the US, the measurement scheme is known as R|PLS|M/2
(RON |PLS| MON / 2), which is the average of the RON and MON (Motor Octane Number).

Understanding the measurement system is important, as 95 Octane measured using RM/2 is equivalent to about 98.5 RON.

For more information on this stuff <!--http--><a href|EQU| http://www.refiningonline.com/EngelhardKB/crep/TCR4_29.htm target|EQU| _blank >look here</a><!--url-->.

So worst case, you only have 85 RON fuel, best case is 85 RM/2 (or about 89 RON).

Whatever it turns out to be, the fuel is really not good enough for running an Evo 6. The car is designed to run on 100 RON in Japan, so dropping to say 97 RON is okay, as the ECU can compensate by retarding the ignition to prevent detonation.

Under light loads, you might be able to get away with it on your gas, but under heavy loads/fast driving I don't know if the ECU will be able to compensate enough, and detontation might occur.

I know that it is possible to get the ECU remapped to run on lower octane fuel, but this is usually only down to about 95 RON. Even then, there are questions over how reliable/safe this is.

If you are really serious about doing this, then you probably need to do all of the following:

1) Always buy 85 octane fuel.

2) Get ECU remapped to run on as low an octane fuel as possible (probably 95 RON).

3) Always use some kind of octane booster with every tank of fuel e.g. Millers.

All of these combined might be enough to get the car running okay, but I doubt if it will ever perform as it really should.

Hope this helps,


03-01-2001, 14:27

Great reply! Can you tell me what's the RON equivalent of a 93(R|PLS|M)/2?

I live in the U.S. but my Evo VI TM is still in Europe, where I use 98RON. The highest available octane I find here is 93 (R|PLS|M)/2. I'm thinking about bringing it to the U.S. as a temporary import, although I'm afraid I will never be able to enjoy it here as I do when I drive in Spain.


03-01-2001, 15:19
Hi Rallyman,

As a rough rule, simply add 4 to the RM/2 value to get RON.

In actual fact, I believe that 93 is equivalent to about 96.7 RON.

The Evo 6 runs quite happily on 97RON fuel here in the UK, so I don't think you would have too many problems.

One thing that would be worth doing before importing the car is running the fuel down to almost zero, then having the ECU reset so that it forgets all of it's existing settings.

When the car comes into the US, fill it up with 93 octane, and then take it for a run.

This should then ensure that the ECU is properly retarding the ignition for the fuel.

Alteratively, I know that 95 RM/2 fuel is available in the US. This is the equivalent of about 99RON, so should make the Evo6 go like a rocket. It is difficult to find, but if you contact the fuel companies, they should be able to give you a list of stockists.

If you can't find any information, then drop me a line, and I'll find out where my friend in Boston gets his.