![]() A thicker bushing limits the total amount of mechanical advance while a thinner bushing allows more.Ĭonversely, an engine with high static compression on pump gas might want a slightly slower advance curve to keep the engine out of detonation. For aftermarket distributors like an MSD, the pin is fitted with an interchangeable bushing. The distance the pin travels determines the total mechanical advance. The distance the weights travel is determined by a pin that travels through a slot. This twisting movement also moves the position of the spinning pole points that swing past the electronic pickup, which advances the timing. As engine speed increases off idle, centrifugal force swings the weights away from their pivot point. Most domestic distributors create this curve by employing a pair of weights and small springs that are placed on a pedestal attached to the distributor shaft. This is a typical street engine ignition curve. If you recall the timing graph from earlier in this story, you can see that mechanical advance starts from the initial timing point and adds more timing as engine speed increases up to just under 3,000 rpm where it should level out. As engine speed increases, the "lead time" increases up to a point. This is accomplished by creating a mechanical-advance system based on engine speed. In order to create maximum cylinder pressure at higher engine speeds, its necessary to start the combustion process sooner. As engine speed increases, there is less time for the combustion process to take place. Mechanical AdvanceHere's where most of the magic lies. So the rule of thumb is more initial timing for engines with big cams and less for stock-cammed engines. Engines with long-duration camshafts and low-idle manifold vacuum benefit the most from more initial timing, while near-stock valve timing responds poorly to excessive initial timing. So if you have an engine that prefers more initial timing, this may require some adjustments to the mechanical-advance mechanism to accommodate the additional timing. Initial timing is added directly to both vacuum advance and mechanical advance. If you're not sure which direction the distributor turns, you can remove the distributor cap and crank the starter and watch the rotor. To retard the timing, you would twist the distributor with rotation (clockwise). So to advance the initial timing, you must move the distributor against rotation (counterclockwise). For most GM engines, the distributor turns clockwise. The key here is to twist the distributor in the proper direction. This is set by loosening the distributor holddown bolt and twisting the distributor housing while watching the timing mark on the crankshaft using a timing light. For most performance street engines, that figure will generally be between 10 and 18 degrees BTDC. Now that we have that straight, initial timing is the setting for the number of crankshaft degrees BTDC that the spark plug fires.
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