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Old 09-06-2003, 09:08 AM
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Distributor Gap vs Dwell

Ok so I don't understand the diff. between gap and dwell. Well I know dwell is how long the gap is closed but from looking at my stock chevy dist. I see that turning the screw through the window on the dist it moves the gap. So is gap and dwell the same adjustment? How do you do one without doing the other? Never did get that part.

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Old 09-06-2003, 09:48 AM
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They are kernels on the same cob of corn. Dwell is the important parameter. The goal is to get as much dwell (closed points) between spark plug discharges as possible. The longer the dwell time, the more charge built up in the coil and the stronger the subsequent spark. Unfortunately, with mechanical points systems, there is a physical limitation to how long the points can dwell closed and this is where the 'gap' specification comes in. The ideal would be to have an infinitely small gap so when one of the points cam lobes bumped the moving point open, it would be an infinitesimally small opening, just long enough to collapse the magnetic field in the coil.

In real life, that is an impossible goal with mechanical points. First, it is impossible to machine the parts with that precision. Even if you could machine to that precision, ANY wear or thermal expansion would destroy spacial the relationship of the parts. Add to that the nasty tendency of all that inductive electrical load to arc across a small gap and you are left with needing to set the points with the equivalent of a grand canyon sized gap. Any sized gap isn't a good thing whit respect to hot sparks, just a physical necessity. That is why some high end mechanical distributors are fitted with dual points. The two sets of points are installed slightly out of phase so when one set opens with it's necessary big gap, the other set is still closed. When the second set finally opens with it's also large gap, the first set of points is just closing. Thus, this is a mechanical way to achieve the 'infinitely small' gap, longest possible dwell, the highest possible charge in the coil, and the hotest possible spark.

Electronic, points-less ignitions achieve the goal of infinitely small gap, maximum dwell by replacing the crude mechanical points with a transistor switch. Thus the 'gap' is the size of an electron and opens and closes at the speed of light. Add to that unrestricted engine speed capability since there are no mechanical parts to accelerate open and closed, ZERO maintenance, relatively low cost and why would one even give a points system a second thought?
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Old 09-06-2003, 10:45 AM
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You don't change one without changing the other. You adjust dwell by changing the point gap. On GM V8's, they make it easy to adjust with an allen wrench. Otherwise, you have to manually change the point gap, tighten it back down, & then check the dwell. Trial & error, in other words. A change in gap causes a change in dwell. More gap = less dwell, & vice versa. Changing dwell also changes ignition timing, just to throw in another frustration factor. More dwell = retarded timing, & vice versa.
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