Re: [alfa] Re: Quality of gas -- anybody use lower octane?

[James Montebello <jamesm@xxxxxxxxxxxx>]

On Wed, 24 Dec 2003 Alcon98340@domain.elided wrote:
> Someone most likely has answered this, but yes, we max out at 91 octane  fuel
> here in sunny (usually) California. Does anyone know why this is? I would

I don't know for certain, but I suspect it has to do with the "cleaner"
formula required by the state authorities.


> Usually if the compression is raised, one will  want to increase the
> octane of gasoline used in the vehicle. The reason for this  is that
> the higher the octane rating, the slower it is to ignite (cooler  burn
> as well?), which means it requires more heat to create
> preignition/detonation.

Sort of.  Octane is strictly a measure of how susceptible a fuel is
to self-ignition from heat and pressure alone.  Diesels do this as a
matter of course, so diesel fuel is formulated to self-ignite on purpose.
For a spark-ignition engine, self-ignition is bad.  It can happen under a
variety of circumstances, but it boils down to the fuel igniting all over
the cylinder at once rather than in a controlled (relatively) slow burn.
This causes shock waves to bounce around in the cylinder, which causes the
noise you hear as "ping", and the destruction of pistons in severe cases.
Diesels are overbuilt to withstand these shocks, at the cost of revability
and sheer weight.

A higher octane fuel will require more heat and pressure to self-ignite.
It *may* burn more slowly than a lower octane fuel, it may not.
Octane doesn't measure this.

A cooler burn would be self-defeating.  The measure of an engine's power
is essentially the average cylinder pressure over the course of the power
stroke.  In a closed cylinder heat = pressure, so a lower temperature =
a lower pressure = less power.  As far as I know, octane doesn't measure
the temperature of combustion, either.

With a higher temp/pressure required for self-ignition, a higher octane
fuel will allow for higher cylinder pressures in safety, which means
more power.  The fuel by itself will (usually) not supply these higher
pressures.  You have to do something else, which all of the usual tuning
techniques are geared towards.

jamesm
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