Interesting little fact. The gasoline stream that is used to make optimax is actualy sourced by shell from BP.

MON is actually the better figure to use for performance. One thing I'm looking into at the moment is what they use for engine testing in Japan. In Japan 'Super unleaded' fuels are normally 95ROn with regular being 90 so wondering if something has got lost in the translation when they say to use 98RON in the book

 

MT was it Ian_km ?

 

Hers some more on RON PON and MON : stolen from another site .



3 main methods of specifying fuel octane content
Research Octane Number (RON),
Motor Octane Number (MON)
and the Pump Octane Number (PON).

Thc Research Octane Number (RON, or F1) simulates fuel performance under low severity engine operation.

The Motor Octane Number (MON, or F2) simulates more severe operation that might be incurred at high speed or high load. In practice the octane of a gasoline is reported as the average of RON and MON or R+M/2.

Classically, both numbers are measured with a standardized single cylinder, variable compression ratio engine.

For both RON and MON, the engine is operated at a constant speed (RPM's) and the compression ratio is increased until the onset of knocking.

For RON engine speed is set at 600 rpm and MON is at 900 rpm.

..................................... Motor Octane Test (MON) Research Octane Test (RON)
Inlet air temperature................ 148.9 C....................... 65.6 C
Engine jacket temp .................100 C........................... 100 C
Engine RPM .............................900.................. ............... 600

As you can see the Motor Octane Test employs a higher temperature and RPM and hence is probably a better indicator for today's engines. Of course the Research octane test gives a higher number and that's why the European manufacturers quote it (RON)

The spread between the two numbers is know as the fuels sensitivity, and it is very important. Because of the variety of engines it is possible for a petrol manufacturer to come up with a fuel that has a high RON, but a lower than expected MON. Hence although it looks normal on the pump it may perform badly. However on another day the same company may make its fuel out of a different blend to get the same RON but a different MON. This is done for profit reasons and is why you occasionally get bad fuel even though it is legally rated the same. In the past with high leaded fuels nobody noticed but nowadays high performance cars do notice.

In America the service stations use the Pump Octane Number or PON rather than RON. this is the average of RON and MON and gives a much better grade, and is also why the American gas always seems not as good as our when in fact it is is the same (and has better quality control). But even this system can be abused by adding octane boosters to poor fuel.

Below is an approximate comparison chart, these numbers can vary by as much as 2 grades

RON ..........MON............. PON
..90............. 83................ 86.6
..92............. 85 ................88.5
..95............. 87................. 91
..96............. 88 .................92
..98............. 90..................94
..100........... 91.5............. 95.8
..105........... 95 ................100
..110........... 99 ................104.5

 

This is PON isn't it? from my understanding from the first site posted.. and the terminology sounds American so that fits too

We have RON on our pumps, yanks have PON.

 

Looking at the figures form Shell and BP they look like they're playing off each other.

From experience, if you leave fuel long enough in your tank it will go off.
From my vehicle test work I've found that even after several days the fuel in the tank will not purge as much fuel vapour as when first put in. So it's not as volatile.
So its possible, if you get fuel from a station in the back end of nowhere the fuel may not be as fresh as that from a city or your local supermarket.

Stanza.

 

I know fuel goes off if you mix it for 2 stroke engines (one of the many, many reasons why outboard motors work only when they feel like it).