Fuel Octane Ratings for Recreational Engines
How important is it to pick the right one?
When you drive into your local petrol station, something most of us do on about a weekly
basis, you have a choice of 2, 3, or even 4 different grades of petrol (gasoline) to choose from.
But itís rarely a difficult choice.
When it comes to road vehicles, most of us are driving a locally made vehicle, or an imported
vehicle made specifically for Australian and New Zealand conditions. Therefore the ownerís manual,
and labels on the vehicle, will be written so the specifications match local terminology. Fuel
requirements will be something "like use ULP or unleaded petrol". Hereís an example:
(Holden Commodore VX Series) -
||Premium (95 octane) UNLEADED petrol
||Regular UNLEADED petrol of 91 octane or higher
Petrol station fuel pumps are marked with similar words, thatís why itís not such a
difficult choice, but what if you are using imported equipment that may not match the local
terms? Can you easily get into trouble by picking the wrong fuel? Unfortunately, the answer
Youíll notice the word Octane is used a lot when describing fuels. Of course, that refers to a
rating given to the fuel for its resistance to detonation. Higher octane ratings mean greater
resistance, which is what higher output, more highly stressed engines usually need. The name
comes from one of the laboratory fuels used when measuring octane ratings, isooctance (see the
following article on octane rating history).
Ever since the octane ratings were first established back in the 1930ís, that word has been
used around the world, but (and hereís where we can get into trouble) there is more than one
type of octane rating method and not all countries advertise the same one.
There are two basic octane measuring methods, the Research and Motor methods, plus some derived
versions like AKI (Anti Knock Index) or Pump Posted octane numbers (also in the following article
on where these names came from).
The Motor rating came first, is a more severe test and represents engines under high load, full
throttle conditions. Itís usually abbreviated to MON (Motor Octane Number).
The Research method was developed to represent engines under lighter load, part throttle
conditions more like those commonly found in road vehicles. Itís abbreviated to RON (Research
Octane Number) and is usually a higher number than the Motor rating. The AKI or Pump Posted
numbers are derived by averaging the MON and RON ratings for a given fuel. Hereís how the
currently available Australian automotive fuels rate in the various systems -
||AKI or PP (3)
|Premium Unleaded (PULP)
|Lead Replacement Petrol (LRP) or Super
|High Octane (BP Ultimate, Shell, Optimax, etc.)
(1) - Minimum specified in Australian Standard, where applicable.
(2) - Typical average at Australian refineries.
(3) - Anti-Knock Index or Pump Posted octane, as used in USA based on
Typical octane numbers and rounded up to whole number.
You will see that the MON, RON and AKI numbers for each fuel type are
quite different and this is where the potential problems arise. You need to know exactly which
octane rating you are looking for when the vehicleís specifications says something like "use
Australia and New Zealand oil companies generally show the RON (Research Octane Number) when
advertising local fuel octane numbers. This is partly because the RON method is intended for
light load applications like road vehicles and partly because (from an old fashioned marketing
viewpoint) the large number look better in print. However, other countries are different.
Various government legislation around the world requires other ratings to be advertised and of
course, some petrol powered equipment is used in high load, high rpm conditions where the RON
rating is not the most appropriate. The recreational petrol powered equipment market is not as
big as it is for road vehicles, so often just one or two varieties of a vehicle are used all
over the world. Same with the Owners Manual or Operators Guides, one version for several
countries. That means the English language version we see will probably be written for the
largest English speaking region, North America. >>>>