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Date:         Mon, 20 Dec 1999 22:02:01 -0800
Reply-To:     David Marshall <vanagon@VOLKSWAGEN.ORG>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         David Marshall <vanagon@VOLKSWAGEN.ORG>
Subject:      Re: Unleaded Regular or Unleaded Premium?
Comments: To: Dana Morphew <kdm@WHIDBEY.COM>
In-Reply-To:  <385EE9E3.86F1101B@whidbey.com>
Content-Type: text/plain; charset="us-ascii"

I would have to somewhat disagree with that statement - under "normal" driving conditions you could get away with 87 octane in these 10:1cr engines. I ran my 1980 Rabbit PU with a 2.0L ABA engine for an entire summer without a knock sensing ignition. I used 94 octane Chevron fuel and set the timing to 6 BTDC at 850rpm. On your typical summer day (low 20s C) there will be no knock. If the temps got to +30C there would be some knock if you are stuck in stop and go traffic. Also at higher altitudes (Rockey Mountains) there is knock if you push it, even with 94 octane. This was all done using a 1982 Rabbit Distributor with vacuum advance and vacuum retard - generally considered the best performance non-knock sensor distributor. If there was a knock sensing ignition present, there would have been no knock as it would have retarded the timing until it disappeared - thus giving you less power. With 87 octane fuel in a high compression motor (with a knock sensor) you will not see any knock while driving (unless something is not adjusted correctly). With the car at idle and the oil at 80C static timing of 6 BTDC will result in no knock. Bring the engine under load and you run the risk of knocking, especially if it is hot outside. The ignition will compensate and retard the timing - a loss of power will result. Running higher octane fuel will reduce the frequency of knock and allow your timing to be more advanced most of the time. The factory setting for most of the 10:1cr engines is 6 BTDC. You *can* set these engines to 10 to 14 BTDC and see a 5 to 7% increase almost the entire RPM range. The theory behind this is the knock sensor WILL retard the timing under load at the lower RPMs (to around the factory +6 BTDC), but as the engine starts to go up in the RPM band it is less prone to knock - the computer advances the timing due to increased RPMs, now there is no knock and the computer will increase the timing to the fullest (6 more degrees than normal) to gain more power. Some ignition systems up to 30 degrees advance can be seen (total of +36 with a static timing of +6), on systems with +12 BTDC timing total advance goes to +42 hence 5 to 7% more power and torque. I like to compair 10:1cr engines to fine wine - the experience is better if you drink it out of a wine glass as opposed to a tupperware container! Same goes for the gasoline you put into the engine, the experience is better with more expensive gasoline. There is one catch though... high octane fuel has less poential energy than low octane fuel, so you need to burn more of it to do the same work (especially if there is alcohol in the blend)... that is another thread!

At 06:45 PM 20/12/1999 -0800, Dana Morphew wrote: >List, I followed Doktor Tim's advice and called Campbell/Nelson, a local >VW dealer in our area, about what octane fuel the RV, HT, RD, PF, 3A or >ABA engines from VW (all with high compression and knock sensor) >require. Their service dept. said these engines will all do fine with >87 octane with no degradation in performance. So there we have it...you >can have your cake and eat it also. > >-Dana- > >

-- David Marshall - - Quesnel, BC, Canada -- -- 78 VW Rabbit, 80 VW Caddy, 84 VW Westie, 85 VW Cabriolet -- -- 87 Audi 5000 Quattro, 88 2.0L VW Syncro Double Cab -- -- David's Volkswagen Home Page http://www.volkswagen.org -- -- Fast Forward Autobahn Sport Tuning http://www.fastforward.ca -- -- david@volkswagen.org (pmail) or vanagon@volkswagen.org (list) --


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