June '18

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28 THE SHOP JUNE 2018 EDITOR'S NOTE: In Part 1 (THE SHOP, January 2018) of this three-part series of vintage carburetor-equipped engine ignition spark timing articles we covered optimizing the spark timing when the engine is at idle. In Part 2 (THE SHOP, February 2018) we looked at tuning an engine's mechanical or rpm- based ignition spark advance. In this final installment, we look at vacuum-based spark advance. T he main purpose of a vacuum advance system is to give the engine the additional spark advance it needs (in addition to the initial advance plus the mechanical advance) when the engine is operated at low-load driving conditions. At low-load/part-throttle operating con- ditions the engine will need more spark advance to fully burn the slower-burning, leaner air/fuel mixture from the carburetor. Plus, the cylinders of the engine are not as full as they would be during wide-open throttle operating conditions. In addition, the scavenge effect in the exhaust port needed to expel the spent exhaust gases from the combustion chamber is not as efficient as it would be during high-load engine operating condi- tions—thus the spent exhaust gases left over in the cylinders dilutes the incoming air/fuel mixture charge and slows the burn rate. Additional spark advance from the vacuum advance system can help increase the fuel efficiency of an engine and allow the engine to run cooler, because more of the energy from the combustion process is converted into energy and less of the energy goes out with the exhaust as wasted heat. MANIFOLD OR PORTED VACUUM There are two choices for vacuum sources for the vacuum advance system that is used with a distributor in a mechanical advance system. They are ported vacuum, which gets its vacuum signal from a port above the closed position of the throttle plate; and manifold vacuum, which is the actual vacuum the engine has in the intake mani- fold when it is running. The use of manifold vacuum to actuate the vacuum advance will cause the vacuum advance to increase the spark timing at idle an additional 10 to 24 degrees above the initial timing, while the ported vacuum option will not add in extra timing at idle. Many people think that ported vacuum for the vacuum advance was only intro- duced for smog/exhaust emission reasons, but actually both ported and manifold Old Spark That By Henry P. Olsen A look at vacuum-based spark advance. Part III A dial-back timing light can allow you to observe the amount of advance from a vacuum advance. A hand vacuum pump is used to observe the amount of spark advance and the vacuum it takes to activate the vacuum advance.

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