Vanagon EuroVan
Previous messageNext messagePrevious in topicNext in topicPrevious by same authorNext by same authorPrevious page (May 2007, week 4)Back to main VANAGON pageJoin or leave VANAGON (or change settings)ReplyPost a new messageSearchProportional fontNon-proportional font
Date:         Sat, 26 May 2007 11:57:02 -0700
Reply-To:     Keith Hughes <keithahughes@QWEST.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Keith Hughes <keithahughes@QWEST.NET>
Subject:      Re: Octane smoctane
In-Reply-To:  <20070526182537.C0DC0763C14@mpls-mf-13.inet.qwest.net>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed

>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)


Back to: Top of message | Previous page | Main VANAGON page

Please note - During the past 17 years of operation, several gigabytes of Vanagon mail messages have been archived. Searching the entire collection will take up to five minutes to complete. Please be patient!


Return to the archives @ gerry.vanagon.com


The vanagon mailing list archives are copyright (c) 1994-2011, and may not be reproduced without the express written permission of the list administrators. Posting messages to this mailing list grants a license to the mailing list administrators to reproduce the message in a compilation, either printed or electronic. All compilations will be not-for-profit, with any excess proceeds going to the Vanagon mailing list.

Any profits from list compilations go exclusively towards the management and operation of the Vanagon mailing list and vanagon mailing list web site.