Re: gas

[Scott Fisher <sefisher@domain.elided>]

Peter F. in LA asks, "Does high octane gas give a noticeable difference
or would I just be throwing my money out the window?"

No and yes, respectively.  The octane number on gas represents its
ability to resist pre-ignition -- also called detonation or "pinging." 
Pre-ignition means that, for a variety of reasons, gasoline ignites some
time before the spark plug fires; in extreme situations this can burn or
break pistons.

One factor that increases an engine's likelihood for pre-ignition is
high compression -- the relationship between the volume of the cylinder
with the piston all the way down, and the volume with the piston all the
way up.  As the piston compresses, it heats up the air-fuel mixture.  If
the compression is high enough, the heat can actually cause the mixture
to ignite even without the spark plug firing.  (This is of course how
diesels work, but let's not go there just now.)

In racing cars, the compression ratio is often increased in order to get
more power from the engine.  For such cases, the car needs to run on
higher-octane gas, but the high-octane gas itself doesn't give more
power -- the engine modification does.  The 103-octane gas just means
you are less likely to find that the face of the #2 piston has broken
off, from the ring land up, through an arc of about an inch in length
(been there, done that, kept the piston as a reminder...)

The octane rating required in your owner's manual reflects the
manufacturer's recommendations, based on the compression ratio of your
car's engine.  If you're not getting detonation now, then the octane
rating of the gas you're using is sufficient.  

So in short: unless you've shaved your Milano's heads to give a 12:1
compression ratio, 103-octane gas will do nothing for you but drain your
checkbook that much faster.

- --Scott Fisher

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