June '18

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JUNE 2018 THE SHOP 29 vacuum have been used as a source for the vacuum advance in both General Motors and Ford vehicles since the 1950s, based on the design engineer's preference. When manifold vacuum is used for the vacuum advance, the engine vacuum at idle must be at least 1 to 2 inches above the vacuum that the vacuum advance needs to be at its maximum advance position. If the vacuum advance used supplies the engine with spark advance when the engine vacuum is 4 to 5 inches of vacuum or lower, you could experience detonation or ping problems during medium-load operating conditions. Consider using a vacuum advance that does not supply any advance at a vacuum level where the power valve in the carbu- retor is adding extra fuel for high-load engine operation. If the power valve opens when the vacuum is below 4.5 inches, the vacuum advance should also not supply spark advance. It is not needed, since the engine is operating with the richer-power air/fuel mixture. VACUUM ADVANCE RATES Each vacuum advance unit has a min- imum vacuum required to get it to begin advancing the spark and a vacuum spec where it is fully advanced. When selecting a vacuum advance unit, consider the amount of vacuum that a given engine produces— especially when you are hooking it up to manifold vacuum. For example, consider an engine with a very radical cam in the 7-inch range at idle. The most common vacuum advance units we see start advancing the spark at about 8 inches of vacuum and are fully advanced when the vacuum reaches the 16- to 18-inch range. There are units that start advancing the spark at 5-inches of vacuum range and are fully advanced at the 8-inches of vacuum range. CHOOSING THE BEST VACUUM SOURCE Different tuners often have different opin- ions on which vacuum source is the best. My preference is ported vacuum, but an experienced tuner will select the vacuum source for the vacuum advance based on what the engine wants or needs for the best idle quality, throttle response and engine drivability. When you are using ported vacuum for the vacuum advance, the ported vacuum port should supply the full manifold vacuum when the throttle is opened just slightly above normal idle speed. Ported vacuum does not increase once its vacuum The readings from an exhaust gas analyzer can be used to determine how much spark advance an engine needs for maximum efficiency. This vacuum advance from a vintage Ford is adjustable for both the rate and amount of spark advance. Some aftermarket vacuum advance units can be adjusted with an Allen wrench, but the adjustment is often just for the rate of advance and not the amount of advance.

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