June '18

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30 THE SHOP JUNE 2018 That Old Spark port is exposed to vacuum as the throttle is opened, and it will drop to near zero during high-load engine operating conditions and wide-open throttle. Since there is no spark advance from the vacuum advance when you are using ported vacuum it is easier to select and tune the vacuum advance system, since you only need to tune the amount of advance from the vacuum advance to match the needs of the engine. Manifold vacuum allows the actual mani- fold vacuum to the distributor vacuum advance unit whenever the engine is run- ning, while ported vacuum supplies no vacuum to the distributor vacuum advance We installed a cotter pin to limit the amount of advance to 10 degrees in this Ford distributor. We used a cotter pin to limit the amount of advance from the vacuum advance on this 1960s-era Chevrolet 6-cylinder distributor. The General Motors vacuum advance on the right is a 10-degree unit but is no longer available, while the B28 vacuum advance on the left is still avail- able but has 16 to 18 degrees of spark advance. Ole's Auto Parts offers a vacuum advance limiter that can be used to limit the advance from the vacuum ad- vance that is used with most MSD and most pre-HEI General Motors V-8 distributors to 10 to 12 degrees of spark advance. when the throttle is closed (normal engine idle). Once the throttle is opened, it sup- plies the manifold vacuum to the vacuum advance unit. Many stock or mild camshaft engines may not want the additional spark advance from manifold vacuum, assuming the ini- tial timing has been properly tuned, but if the engine has a high-performance cam- shaft it may respond well to the additional spark advance. An engine should produce 18 or more inches of manifold vacuum at 2,500 rpm without any load on the engine and about 1/2-inch of vacuum at wide-open throttle, but the vacuum it has at idle can vary from the 6- to 8-inch range if the engine has a high-performance camshaft to the 18-inch range for an engine with a stock cam. When working with an engine that has a high-performance camshaft, the additional timing at idle that you get from a vacuum advance connected to manifold vacuum may help the engine idle better. However, you need to select a vacuum advance con- trol unit that is fully advanced at a vacuum level that is 2 inches below the manifold vacuum that is present at idle (in gear if the vehicle is equipped with an automatic transmission). The engine vacuum at idle will also be lower at high altitude than it would be at sea level. Many of the various calibrations of vacuum advance units have been discon-

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