[DMCForum] Robert Sikorsky Ignition Timing Quote
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[DMCForum] Robert Sikorsky Ignition Timing Quote



(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

















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