Date: Fri, 7 Feb 1997 19:41:10 EST From: Russ Maak 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sports cars and grand prix racers retained thier manual spark controls untill the 1940s, such units have subsequently been replaced by " automatic " advance mechanisms in all modern automotive applications.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