Date:         Thu, 13 Mar 1997 14:09:01 -0500 (EST)
Sender:       Vanagon Mailing List <vanagon@vanagon.com>
From:         SyncroHead@aol.com
Subject:      Re: Gasoline Grade?

In a message dated 97-03-13 01:24:36 EST, hunkins@mbay.net (Thad Hunkins)
writes:

> What is the professional opinion on gas grade.  My manual (88 
>  Vanagon) says 87 octane is fine.  The guy I bought the vehicle from used 
>  nothing but premium.  Can I bump it down safely??

Please don't accuse me of being professional, but...

A higher octane number is an indication of the fuel's ability to withstand
compression detonation.  Too low an octane number will cause the fuel to
"pre-detonate".  That is, detonate from the effects of compression before the
spark ignites the fuel.  Having a higher octane number than the engine
requires  simply indicates that the fuel could have withstood higher
compression after the
spark already ignited it.  An overly high octane number is of no value to you
or your engine.

Be aware that one octane rating is NOT necessarily like another octane
rating.

Octane numbers are measured by two methods.  One called the Research Method
which consistantly yields a higher number and the Motor Method which yields a
lower number.  In the USA the gas pumps show the average of the two methods,
thus the little R+M/2 note on the octane number sticker.  Typically Europe
uses the Research Method only, and thus the VW (and others) manuals often
will state something like "use gasoline with an octane rating of 92".  This
would equate to something like an 87 rating using the R+M/2 method.

You can safely use the lowest octane number gas that doesn't cause
predetonation (pinging, also called knock or knocking) in your vehicle.  I'd
suggest backing down one grade from premium(R+M/2=92) and try a
midgrade(R+M/2=89).  Listen closely for pinging under hard accelleration,
like up a hill.  If you hear any, go back to premium.  If not, thry the
regular(R+M/2=87) grade and listen for pinging again.  If you hear any, go
back to the midgrade.  If not, continue using the regular and continue to
monitor for pinging.

Be a bit careful, since long term pinging can cause real damage.  A single
tank for test purposes with occasional pinging is not a problem.  Also
pinging begins well below our ear's ability to hear it.  Listening for it
under extreme load like attempting pedal-floored acceleration (relative for a
Vanagon) up a hill with relatively low engine RPM can typically give you
evidence of more general pinging.  If you can't hear any under these
circumstances, then your not likely to have pinging under normal driving, but
you may have some unheared pinging in these extreme circumstances.

Many newer engines have "knock" sensors built into the engine block to
"listen" for inaudible pinging.  The engine management system uses this
"knock" signal to adjust the timing of the ignition to eliminate the pinging
on a split second basis.

Regards,
Jim Davis
87 GL Syncro
88 GL Wolfsburg