Continued from this link

This way we can keep all the related info together for future reference. smile

The book I have been reading refers to what they call Octane Requirement Increase (ORI). Logically, as carbon builds up in the combustion chamber (from petrol remnants, oil past rings, etc), it marginally increases compression ratio. With every increase of ratio by 1, they state octane requirement increases by a minimum of 4-6 points.

However, they also state that these carbon and other deposits also do not carry heat as well as the regular steel does - and that this is in fact the main reason for ORI. Hot spots cause the preignition/knock.

From new, the deposits build up and progressively increase over several thousand miles. Levels then plateau when they can't build up any thicker without being burnt off. Overall they stated that there is normally around a 3-5 octane point increase requirement for most cars within 10,000 miles from new.

Some engines tend to have the deposits form near the intake - making ignition from these possibly red hot deposits even more likely.

Fuels high in aromatics, along with city stop start driving, will cause the worse deposits.

The lead also made it worse - presumably leading to having to add the lead scavenging chemicals that often rotted the immediate exhaust system.

Fortunately, there are anti ORI additives - ranging from boron, a 'polyetheramine detergent' commercially known as 'Techron' was noted to reduce ORI by 30-40%. They also mention a 'PDP5400' additive that was launched in 1985.

Caltex used to distribute techron, however I emailed them 2 months ago and they no longer stock it.


With the above information, it makes sense that using 98 octane in a car designed for 91 will probably only result in a power restoration to near (but below) the manufacturers specified levels.
The knock sensor automatically retards timing to prevent the more prevalent knock caused by these deposits. With 98 octane, it will be able to advance back to BTDC degree closer to that of a new engine without the deposits - hence return to original power.

This is just my lamans understanding and I am really interested to know other peoples views on this interesting topic



The book is 'Motor Gasoline' by E.L. Marshall and K. Owen, 1995 ISBN 0-85404-409-4