Re: 100 Octane Fuel

["John I Gray III" <jira@xxxxxxxxxxxxx>]

As I understand it, octane is a measurement of a fuels volatility. Those 
with lower octane (AKI) ratings are more volatile and are more likely to
combust on their own under pressure and heat generated by the compression
stage a.k.a detonation or pinging. Therefore, a higher octane fuel is a more
stable fuel, but also one requiring more compression and heat to ignite
properly. If the engine is not designed to produce this kind of compression,
you can suffer incomplete combustion resulting in hesitation and drivability
problems especially while the engine is cold. Without an engine designed to
take advantage of the higher octane there is no performance advantage
achieved. In fact, the engine management system in todays fuel injected cars
will try to lean out the mixture to such an extreme that you risk losing one
of the advantages of gas in that it helps cool the combustion chamber,
helping to avoid the creation hot spots within the engine. The only
situation where a higher octane is helpful, not harmful, is in those
applications where the engine utilizes an anti-knock sensor and compensates
for lower octanes by making the appropriate ignition adjustments to prevent
pinging. In this example there is a distinct performance advantage in using
a higher octane as the engine is designed, but the advantage diminishes
greatly as you exceed the  manufacturers recommendation. If you still want
to "clean out the system", stick to the BMW recommended Techron made by
Chevron every 6K miles or so to reduce the formation of deposits on the back
of your intake valves and fuel injector pintles. Also note that the big
number you see on the gas pump is an AKI rating, an average of its research
and motor octane rating. Two fuels with the same AKI rating can therefore
perform very differently in your engine so stick to name brand gas from
reliable distributors, which also tend to have gas formulated with better
detergent/additive packages.

Regards,

John Gray
'97 528i
'91 318is
'97 Land Rover (sort of a BMW)
'90 Mazda Miata (BMW in spirit)

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