I recently gave some bad advice on this issue.

I have always believed that changes in timing, be it by vacuum advance, mechanical advance, or
by turning the distributor, do not affect or otherwise change dwell.

I had recomended that a gentleman change his (points type) distributor because the dwell changed when the vacuum advance was hooked up... his new distributor does the same thing....

I was sure that this should not happen, and I still don't think it should,

Am I ...............

 

Come on Tech guys!!!

 

You need a spot for "theoretically correct". Dwell is the length of time the points are closed, defined angularly. So, in a "perfect world", the vacuum advance should not affect dwell. However, anything that causes the breaker plate to move off center in comparison to the cam of the distributor shaft,will in fact cause the dwell to change. Thus, as the vacuum advance is connected to the breaker plate and causes it to move (in an imperfect arc) the dwell will change. More thana couple ofdegrees and I would look to "tighten up" the slop in the breaker plate.
Sometimes no good deed goes unpunished. Say 14 "hail Duntovs"and you will be absolved.

 

Yeah, what she said.

 

Dwell angle is how long (in degrees of distributor rotation) the points stay closed and allow saturation of the coil. I fail to see how rotating the distributor or changing when the vacum advance comes in can affect the physical relationship of the points to the distributor cam.

 

This condition is what we call a 'worn-out' or defective distributor.

 

I'm with Scott H. and Soaring. If the plate pivoted on the cam center like most aftermarket distributors, I would say no, but I seem to remember it pivoting off a pin on the side, if so, yes dwell will change. [>:]

 

Hello Mike, I was waiting for your answer..Everything I have always heard, and everything I "see" in my head tells me that if dwell changes with timing, then something is more loose than ideal, or the design is less than ideal.

It is my understanding that the breaker plate is supposed to move in a concentric circle around the distributor shaft in a precise manner so to not affect dwell when the breaker plates position is altered by either mechanical or vacuum advance.

The simple phrase I have always applied to this situation is "Dwell affects timing, timing does not affect dwell". ???

Thanks Mike, I was begining to doubt my most basic understanding of how a distributor is supposed to work.

 

I was not aware of the stock distributors construction, I guess this is an issue where theory runs smack into the middle of mass production reality...

This would be a case where a Petronia conversion would help to curb the built indwell changes.

Iwill assume at this point, that the early GM points distributors used a different manner of breaker plate attachment because I do not remember the dwell changing with advance on these distributors...

Just goes to show me that I should know what I am talking about prior to giving concrete advise.

 

I'm with you on this one too. But realistically, the breaker plates that I have seenhave a pivot point. The timing changes as a result points open sooner with Vacumn Advance but there has to be some dwell change due to the pivot action. I found a good article at the following url. (I know its chevy but the principles are the same):
http://www.chevyhiperformance.com/howto/97438