RON, MON or PON?
Or how do you grade petrol/gas
I have seen many European/US conversations claiming that one petrol/gas is better than another or higher rated. This may not be the case as different rating systems are used in different countries and so not all numbers mean the same thing. You must be careful to also quote the measurement system used

To see why there are different numbers let us take a trip back in time to World War I. Aviators had a problem, many engines would suddenly self destruct through detonation, which is bad news when you're up in the air. An engine might run fine on one batch of fuel but blow holes in the pistons on the next batch. The fuels seemed the same, weighed the same and may have even come from the same factory.

The fuel companies tried to analysis and standardise the petrol, but were unable to weed out the bad batches. Therefore a standard test engine with a variable compression facility was built and the fuel to be tested run through it. This heavy duty, single cylinder engine would be warmed to a standard temperature and at a set rpm the compression increased until engine knock occurred. this would give its Highest Usable Compression Ratio (HUCR).

But with time it was discovered that different labs gave different results. So in an attempt to produce an unvarying standard, two reference fuels were chosen. The high reference was 2-2-4 trimethylpentane (iso-octane), while the low reference was normal heptane (n-heptane). Once the HUCR was determined a mix of these to fuels was made up that exactly produced the same results as the HUCR test. The result is quoted as the percentage of iso-octane. Hence a petrol that detonated the same as a mix of 90% iso-octane and 10% n-heptane is called a 90 octane fuel.

Since that time a number of tests have come into being to simulate a variety of engine conditions. Motor Spirit is usually rated using the Research or Motor test methods. Both use the same old engine but under different conditions

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 (The Molemobile has just had a particularly bad batch from Total, and has been pinking all week)

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

So now you know far more you wanted to about RON, MON and PON. Knowing this you might begin to wonder why certain garages are always cheaper than others, and experiment with different fuels. I'm lucky, my new job has just given me a fuel card for private use, so I'm moving up to super unleaded, that way I can be assured I'm getting at least 95 RON