(I will try to use the English language to the best of my abilities in this post). I had to re-read about one half of Robert Sikorsky's book "Drive It Forever", but I finally found the quote I remembered: "If you advance the ignition timing to take advantage of a higher octane gasoline or perhaps alcohol injection, remember that an over-advanced ignition can cause combustion chamber temperatures to soar. Over a prolonged period this additional heat can spell disaster for the engine. Be prudent when advancing the ignition; do it only a degree at a time. In no case should it be advanced over five degrees total." The last sentence is the one I remembered. I wonder if the first sentence is a typographical error because higher octane fuel is usually associated with high compression engines, which typically are timed closer to TDC (Top Dead Center). For example, the "high test" grade of the 1960's, which averaged about 97 octane, was designed for 10:1 to 11:1 compression engines which disappeared from ALL domestic manufacturers' lineups in 1971 and 1972 (the same cubic inch blocks were re-engineered in low compression versions. All four companies did this -- even AMC). The copyright date of this book is 1983, which is about the same time gasoline companies were coming out with ever LOWER octane -- and ever less expensive -- fuels (does anyone remember the controversy over "super economy" grade that still lives on in Ohio?). I suspect that first sentence is a historical anachronism from those days when Americans were trying anything possible to lower fuel costs, and should read "to take advantage of a LOWER octane gasoline". There obviously is an ignition timing window because specifications on the sticker in the engine compartment usually read "+/- 2 degrees", or some value similar. While looking in Ford's 1978 factory literature for comments on ignition timing, I came across this startling admission: "A vacuum switch sensing intake manifold vacuum provides an automatic retard signal to the distributor under heavy engine load, while allowing spark advance under light to moderate engine loads" Basically Ford is admitting it over advanced specified ignition timing the point the engine will now regularly knock at higher RPM's. This is the gradual movement away from TDC in timing specifications I mentioned before. To counteract it a second chamber (the 1978 spark advance unit had vacuum nipples on either side of its diaphragm) was energized to negate any effect on the rotor. Replacement dual chamber spark advance units are not available from parts houses, by the way -- traditional spark advance units can be used IF ignition timing is moved closer to TDC. Bill Robertson #5939 To unsubscribe from this group, send an email to: DMCForum-unsubscribe@xxxxxxxxxxxxxxx Yahoo! Groups Sponsor ADVERTISEMENT <http://rd.yahoo.com/SIG=12c186s9q/M=266841.4316200.5507732.1261774/D=egro upweb/S=1705126215:HM/EXP=1071990316/A=1911856/R=0/*http://www.lifescapein c.com/picasa/landing.php?capid=222&caId=1985> click here <http://us.adserver.yahoo.com/l?M=266841.4316200.5507732.1261774/D=egroupm ail/S=:HM/A=1911856/rand=570846128> _____ Yahoo! Groups Links * To visit your group on the web, go to: http://groups.yahoo.com/group/DMCForum/ * To unsubscribe from this group, send an email to: DMCForum-unsubscribe@xxxxxxxxxxxxxxx <mailto:DMCForum-unsubscribe@xxxxxxxxxxxxxxx?subject=Unsubscribe> * Your use of Yahoo! Groups is subject to the Yahoo! Terms of Service <http://docs.yahoo.com/info/terms/> .
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