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MG MGB Technical - dwell

Anyone know what the dwell is on patronix distributor? IS it 51 degrees ? thanks , DOUG
dd doug46chief

60 degrees, the same as it is on a points style distributor. RAY
rjm RAY

Doug. Actually, it is not the same as a points type dizzy. The Lucas 25 type dizzy, with points, has a dwell of 60 degrees, as Ray notes. The Lucas 45 type has a dwell of 51 degrees. The various points replacement systems, as you have one example of, have their own various "dwell" readings, none of which have any real relationship to the "dwell" of a points type dizzy. /

With a points type distributor, the "dwell" is the percentage of the arc of a circle in which the points are closed and charging the coil. Thus, various degrees of dwell. With the various methods of points replacement systems, the "dwell", a reference to the points type system, is commonly much longer due to the quite different triggering system used. This tends to make the "dwell" system of measurement irrelevant. To the best of my recollection, the last time I installed a Petronix system, for a friend, the information package listed the "dwell" as being 89 degrees.

Dwell is a useful indicator with a points type distributor as in indicates whether the points have the correct gap. As the dwell increases, the points gap decreases. Thus, hooking up a dwell meter to the system tells us that the points are closing up, due to wear on the rubbing block, and we need to reset the points to a slightly larger opening. With a points replacement system, such as you are using, the dwell function becomes obsolete and has no real value.

Les
Les Bengtson

thanks Les, I got around 88 on the dwell after I timed it by the strobe at 10 deg at 1500 rpm. Doug
dd doug46chief

Doug. That is what you should see with your type of system.

Les
Les Bengtson

A lecture on Dwell.

Dwell is necessary with any points based ignitions. As usual these ignitions are a compromise. For optimum high RPM use the points need to be closed for a long time to allow the coil to charge, between firings. This would required a very large dwell (dwell being the time the points are closed, close to 90deg for a 4 cylinder (60 for a 6 and 45 for an 8). Now a large dwell translates to a small point gap, which causes a problem with arc quenching, despite the capacitor. One needs a larger gap to ensure the arcing is quenched, hence the compromise.
With electronic systems there is no need to quench an arc, as there isn't one. Thus electronic ignitions can turn back on, to charge the coil, once the sparking is finished, a relatively short time, hence the 80 plus degrees of dwell measured.

Herb
Herb Adler

I stand corrected. Never too old to learn new things. RAY
rjm RAY

thanks to all for the help
dd doug46chief

This thread was discussed between 19/02/2012 and 20/02/2012

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