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Re: [T3] OT: Timing/dwell relationship and Pertronix

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

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