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On 11 Jan 2004 at 17:35, David Yaghoubian wrote: > I was wondering if someone could explain to me the (mathematical?) > relationship between timing and dwell. Meaning, if dwell is reduced, does > the timing advance or retard? What the heck is "dwell" anyway? I did an > archive search but nothing came up (which means the archive/search tool > might be broken, as this is a pretty basic search term!) Timing determines the moment that the spark fires. Dwell is the amount of time (expressed in degrees of distributor shaft rotation) that the points (or their equivalent) are closed. The spark is triggered when the points open. In a standard coil ignition system the points need to be closed for some amount of time before they open so that sufficient energy can be stored in the coil. This is because the coil is an inductor and the current thru it rises only slowly. If the dwell were too short, the current would still be small when the points opened and the spark would be weak. The dwell is generally set at a number that means that the points are closed a little more than half the time. In a 4 cyl engine this 50% duty cycle gives us a 45 deg dwell. In a 6 cyl this is a 30 deg dwell, and in an 8 cyl this is 22.5 deg. On a car with points it is important to keep close to the 50% point, so that the points are opening when the cam is at it's fastest rate of rise, but with an electronic switch the dwell can be longer because the points are no longer a consideration. In general, if you need to set the dwell, you must do it first, because changing the dwell changes the timing. Once the dwell is set you set the timing. On your <other> car you make no allowance for the changed dwell. You set the timing exactly where it is supposed to be. Do not attempt to correct for a different dwell by changing the timing; no correction is necessary or desirable. -- ******************************* Jim Adney, jadney@vwtype3.org Madison, Wisconsin, USA ******************************* ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ List info at http://www.vwtype3.org/list | mailto:gregm@vwtype3.org