Octane follies

[Mike Arman <armanmik@n-jcenter.com>]

>
>	Uhh, How do you figure.  Higher Octane allows the motor to run
>better so you should see better Fuel Economy with the 93 octane.  I do on
>my car by a 1-2 mpg.  With Higher Octane you get more power per gallon so
>you use less gas.
>	Right???  
>
>	Todd Phenneger

No, wrong (gong!).

Higher octane does not allow the motor to run better - nor does it give
more power.

The purpose of the additives which raise the octane is to prevent
pre-ignition and detonation in high compression engines. Low compression
engines don't need high octane gas because they don't raise the combustion
chamber temps as quickly or as far as high compression engines. High
compression engines need high octane gas to keep them from
self-destructing. (Incidentally, pre-ignition and detonation are not the
same thing, but have similar disastrous results.)

Raising the compression ratio is an easy way to get a little more HP from
an engine, but there is a price - you then need higher octane gas. As the
engine gets older and carboned up, the effective compression ratio rises
even further and eventually, you get knocking no matter how high the octane
of the gas is.

Turbochargers also raise the effective compression ratio - that's why a
turbo engine's actual compression ratio is lower than the N.A. version of
the same engine - 11.5 to 1 plus turbo would need probably 400 octane fuel
. . . which we can't buy.

The highest octane fuel commonly available is 100/130 (at your local
airport) - but if you need this, there's something seriously wrong with
your car, unless it is an out and out dragster.

Best Regards,

Mike Arman (retired from songwriting by popular request)