Date: Fri, 7 Feb 1997 19:41:10 EST
From: Russ Maak <russcm@JUNO.COM>
Subject: SPARK ADVANCE

            Here it is.


            Will have to do this in two postings it's kinda long.


                                                            THE SPARK
ADVANCE

                                                   By Petersons Basic
Ignition and Electrical,


           It's not enough merely to direct a spark to each cylinder; it
has to get there at precisely the right time. At low engine speeds the
spark plug fires at approximately 3 to 10 degrees of crankshaft rotation
before the piston reaches the top of it's compresion stroke. This gives
the fuil/air mixture an instant to " light up " before it starts to
expand and drive the piston down on it's power stroke. The fuil/air
mixture takes just about the same to get " lit " at high engine speeds ,
but since the engine is turning faster, the piston would have already
started down before the expanding gases could develop thier " push." An
engine operating under these circumstances would definitely have a steady
loss of power as engine speed increased. For this reason the spark has to
be fired earlier at high rpm than at idle  . Doing this is the job of the
distributors centrifugal spark advance mechanism.
          Before 1930 all cars had manual spark advances. This usually
took the form of a lever mounted " turnsignal style " on the steering
colume. When the driver moved out on to the highway, he advanced the
spark timing by pulling the lever downward on it's notched quadrant. For
starting, driving at low speeds, or ascending steep hills, the spark
lever was raised a few notches to retard the timing slightly. When
cranking the engine, it's spark control had to be placed all the way up
in the full-reyarded position.
           Although some all-out compatition sports cars and grand prix
racers retained thier manual spark controls untill the 1940s, such units
have subsequently been replaced by " automatic " advance mechanisms in
all modern automotive applications. A number of worthwhile reasons forced
this general changeover. Probably most important for the average driver
was that " broken arm " type accidents- which frequently occurred when
hand-cranking an engine without fully retarding the manual spark
control-were all but eliminated by automatic advance devices. Crank
starting, you may recall, was a feature retained on many cars for over 20
years after introduction of the electric starter.
          Another reason for abandoning manual spark controls was the
greatly increased compression ratios and higher rpm of later-model
engines. 35 years ago a real idiot could knock the tops of all four
pistons out of a Model A Ford by lugging the engine on a steep grade
without retarding the spark. Think of what his grandson could do to a 454
Corvette without the safeguards built into the automatic advance unit of
it's distributor.
          There are currently three spark advances in commen use. The
first is the pure centriugal type, found mostly in high-performance hot
rod distributors, Ford high-perfomance cars, 68 and 69 L88 427 Corvetts,
VW transporters, early VW Beetles, Porsches and the Formula Vee race
cars. This unit operates quite simply by just advancing the spark as
engine revs increase, and retarding it as rpm appoaches an idle. It's
chief disadvantages are relatively poor low-speed economy and the
possibility of excessive advance under certain full-thottle conditions.
       The pure vaccum-advance distributor has had wide use both in the
U.S. and abroad, but in recent years it's only signifigant application
has been on Fords economy 6s with Loadomatic distributors, and late VW
passenger cars. Vacuum advances on VWs function well enough under most
everyday driving conditions but tend to limit top speed and full-throttle
acceleration. This is the reason they are usually replaced with
centrifugal advance distrbutors when VW engines are installed in race
cars or dune buggies.



                             Here ends the first part of the article,
will post the second part later.


                         RUSS

                         56 oval
                         58 ragtop BAJA daily driver
                         63 convert. ( Snubs )
                         65 notch
                         and more
SPARK ADVANCE