Date:         Sat, 26 May 2007 14:23:40 -0700
Reply-To:     chrisrvanagon <chrisrvanagon@COMCAST.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         chrisrvanagon <chrisrvanagon@COMCAST.NET>
Subject:      Re: Octane smoctane
In-Reply-To:  <465882FE.5060105@qwest.net>
Content-Type: text/plain; charset="us-ascii"

Do any of us have an opinion as to the non-octane oriented benefits of the
higher grades of fuel?  E.g., Does "Techron" or whatever, have a benefit to
a stock, original 1990 WBX with 80K miles on it, besides "preventing"
pre-ignition?

ChrisR

-----Original Message-----
From: Vanagon Mailing List [mailto:vanagon@gerry.vanagon.com] On Behalf Of
Keith Hughes
Sent: Saturday, May 26, 2007 11:57 AM
To: vanagon@GERRY.VANAGON.COM
Subject: Re: Octane smoctane

>Date:    Sat, 26 May 2007 08:52:27 -0600
>From:    David Etter <detter@MAIL.AURACOM.COM>
>Subject: Re: Octane smoctane
>
>Higher Octane gas has flame front retarders in it, so it burns
>slower. No flash-over or at least less so than regular gas.
>        For some reason, although the total BTU's per liter are
>similar, published literature suggests that, in theory, a properly
>tuned engine should get better mileage from regular if you operate it
>within the optimum rpm range. ( near maximum torque levels).
>        Much like the old gun powder vs. the new gun powder where the
>new stuff burns slower allowing more energy extraction (muzzle
>velocity) for the same amt. of powder in a similar length barrel.
>
>YMMV..                  David (TDdsl82westy)
>
>P.S.: Better qualified List member advice is requested here.
>
>

There are quite a number of factors involved, but basically as you
state, the higher octane burns slower, and ignites at a higher
temperature/compression point.  So...you have several reasons for using
higher octane fuel in an engine designed for 87 octane:

    1.  Higher octane will allow for an increase in the timing advance,
with better high end performance, without pre-ignition.
    2.  In high temperature areas (like here in Phoenix) the higher
octane is often required to temper higher combustion chamber
temperatures, and alleviate                 pre-ignition.
    3. However, in an old WBX motor (i.e. after the compression has
dropped 10-30psig) there's little problem with pre-ignition, with any
octane, since the                 compression ratio has been lowered
significantly (as long as the timing is correct).

Basically, if you have no pre or post-ignition (dieseling or 'run-on')
with 87 octane, higher octane gas will gain you nothing.  Without
adjusting the timing to make use of the slower combustion front from the
higher octane, you may not extract as much energy from it as you would
from lower octane, thereby reducing mileage (although I've seldom seen
that actually happen).

As to comparing mileage between 87 and 91 octane tank fills, unless you
run several thousand miles with each, over roughly the same terrain, you
won't have any meaningful data.  There are too many variables such as;
driving style (changes with attitude), WIND (makes as much as 7 mpg
difference in my '86 Westy Tiico), road surface conditions, average
speed, average grade, traffic conditions, altitude, etc.   Unless you're
running the same course under the same conditions, it takes a lot of
miles to average out all the fluctuations in mileage these variables
will cause.

Keith Hughes
'86 Westy Tiico (Marvin)