Date:         Mon, 24 Jun 1996 20:31:46 -0400
Sender:       Vanagon Mailing List <vanagon@vanagon.com>
From:         SyncroHead@aol.com
Subject:      Re: Octane ratings

In a message dated 96-06-24 13:13:18 EDT, eugp@uclink3.berkeley.edu (Eugene
C. Palmer) writes:

>>> 91 octane?!  I beg to differ!  VW engines were built to handle cheap 
>>>low quality gas all around the world, like in places where they say 
>>>"octane?  what the hel

>>When the VW was designed, 93 octane fuel WAS cheap gas.
>>The cover to my '68 TII fuel fill says "91 Octane minium".
>>I feel that if the VW people tell me 91 is what I need, thats good enough 
>>for me.
>>
>>If you run cheap fuel, drop your timing back 2 or 3 degrees to keep from 
>>overheating.

Also, one octane rating is NOT necessarily like another octane rating.

A higher octane number is an indication of the fuel's ability to withstand
compression detonation.  Too low an octane number and the fuel will
"pre-detonate".  That is detonate from the effects of compression before the
spark ignites the fuel.  Having a higher octane number than needed simply
indicates that the fuel could have withstood higher compression after the
spark already ignited it.  

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.


Jim Davis
87 GL Syncro
88 GL Wolfsburg