Octaneratings

We’ve all heard the term ‘octane’ before, and many will even know that it’s a vital factor in spark ignition engines. But very few of us will know exactly what this means, and how it will affect our cars — especially when it comes to tuning them.

To put it in it’s most simple and relevant form, the higher the octane number, the more resistant the fuel is to what people call pre-ignition, knock, or detonation — basically the point at which the heat and/or pressure in the engine causes air/fuel mixture to ignite before it’s intended to.This pre-ignition can cause serious and often terminal engine failure.

Therefore, it’s desirable to use fuel with an octane rating as high as possible on a petrol engine. But, contrary to popular belief, unless that engine is mapped for the higher octane fuel, it won’t increase the power by any meaningful amount simply by adding it to the fuel tank.

Conversely, if an engine has been mapped for a higher octane fuel than the one you’re using, you really are asking for reliability problems.

However, using higher octane fuels does mean you can increase an engine’s resistance to pre-ignition – therefore allowing you to safely utilise higher compression ratios, higher boost pressures, and more advanced ignition timing. All of which will help the engine to produce more power.

Octane ratings are referred to in one of three main units of measurement: RON, MON, and AKI. These stand for Research Octane Number, Motor Octane Number, and Anti Knock Index. We use RON in the UK, as does most of the world, but in America and Canada AKI is the most common method of measurement. AKI is a mixture of RON and MON ratings, and as the MON test is much tougher (giving fuels an 8-10 point lower rating than RON), the overall rating of AKI is around 5 points lower than the RON system we use.

“THE HIGHER THE OCTANE NUMBER, THE MORE RESISTANT THE FUEL IS TO WHAT PEOPLE CALL PRE€IGNITION, KNOCK, OR DETONATION”