In a message dated 97-07-24 15:43:52 EDT, vwbob@JPS.NET (Bob Marshburn)
writes:

> I enjoyed the comments of Jim Davis on this subject.  Part of it is shown
> below.  For an 85 Waterboxer Westy is there any advantage (or need) of
> running the 89 or 91 instead of the 87?

> >In California typically each terminal rack will have tanks of raw 87
octane
> >and tanks of raw 91 octane gasoline.  These two are blended as they are
> >delivered to each truck to create the 89 octane midgrade fuel.  Almost all
> >gas stations in CA have 87, 89, and 91 octane fuels available.  These
octane
> >numbers use the (R+M)/2 method.

If there is any detection of pinging then the higher octane would be useful.
 Pinging usually exhibits itself under severe engine load like a heavily
loaded van going up a steep hill, especially at RPMs that are low for the
speed & gearing.  If there is no pinging under these conditions, you should
be safe the rest of the time.  The trouble is that if you can detect pinging
under the heavily loaded conditions then you MAY also have undetectible
pinging at less severe conditions.  Many newer engine designs (including
those in Golfs & Jettas) have knock (ping) sensors built into the engine to
detect pinging long before you can hear it.  When pinging (knock) is detected
the engine management computer adjusts the timing so as to eliminate the
pinging.  We have no such knock sensor on our Vanagons.

The basic rule of thumb I use is that if you detect any pinging, even under
severe conditions, then move up one level of octane number and try that.
 With the absense of pinging under severe conditions, I'd stay with the 87
octane.

By the way RON octane is typically about 4 points higher that (R+M)/2 octane
number.  So for a gas that has a (R+M)/2 rating of 87 the RON is about 91.
 Also the MON for the same gas would be about 83.

One more tidbit.  The (R+M)/2 octane number is sometimes called the CLC
method after the US governtment agency "Cost of Living Council".

Regards,
Jim Davis