Question: Dwell vs. Points Gap

[Porsche-O-Phile]

Subscribing.

This is great information. For one, I'm still trying to learn/understand the nuances of points ignition (I have points/CDS on my '74).

[RoninLB]

this is not rocket science



I had points with Bosch grease take 1,000 miles to seat.

Easy to eyeball the gap and just set timing. They shouldn't pit.

[RWebb]

Originally Posted by Flieger

I am trying to wrap my head around all of the great information here.

Can someone please post a photo of the distributor internals setup in the car?

none yet & I don't have points -- it works just like a rocker arm on your camshaft

looking down, there are small cam lobes on the distr. shaft.
- the points arm extends past the shaft with the pivot pin on one side & the points themselves on the other side
- in the middle of the arm is the rubbing block (brown phenolic 'plastic' or maybe they are black now) -- that is what is pushed up by the cam lobe & makes the points open at the end of the arm

clear as mud now?

[Peter Zimmermann]

Originally Posted by RoninLB

eyeball the gap and just set timing. They shouldn't pit.

Correct, points combined with a CD ignition will not pit, like, say, points in a 356...

[Flieger]

Originally Posted by RWebb

none yet & I don't have points -- it works just like a rocker arm on your camshaft

looking down, there are small cam lobes on the distr. shaft.
- the points arm extends past the shaft with the pivot pin on one side & the points themselves on the other side
- in the middle of the arm is the rubbing block (brown phenolic 'plastic' or maybe they are black now) -- that is what is pushed up by the cam lobe & makes the points open at the end of the arm

clear as mud now?

Thanks, that clears things up quite a bit.

[RoninLB]

[Flieger]

Great photo

Thanks

[RWebb]

there is a slot beneath the slotted screw*
you move the points back & forth along that slot to adjust the dwell -- there is a small round tab (hidden) that is the pivot


* apparently, slots were quite popular when these things were originally designed

[T77911S]

Originally Posted by Peter Zimmermann

One must always assume that a tuner/technician will set the dwell within the allowable range calculated by the factory. At that point the ignition timing must also be set, because one degree of dwell is equal to one degree of timing. As the points wear (close up) from their correctly installed setting, the dwell, and timing, will change, creating a retarded condition that will also cause backfiring, maybe tach needle bounce, and generally sluggish running (more easily recognizable when the dwell reaches 43-44 degrees). If, at install, the points are set too wide (smaller dwell number), let's say 30 degrees, if the timing is reset to factory spec at that dwell there will be little evidence that something might be wrong.

you did not understand my question. if i set the points to either 30 or say 45 or even 50 deg. and set the timing, will it have an efffect on how the car runs now, not in a 1000 miles. after all, the points dont have voltage on them and are not actually creating the spark, they just provide a ground trigger.

[Paulporsche]

And, can anyone confirm if there is a diference between an early car ignition system in which the change in dwell angle does change the timing, and a later CDI car, in which some have stated the dwell change does not affect the timing?

[Bill Verburg]

A point set is just an electric switch, just like in your house. There is a voltage and current through it. Older conventional systems had the full 12v dc current used to saturate the coil going through them and needed a condenser to reduce the spark generated when the point faces had a small separation. In these systems the point faces will pit and burn themselves out in short order, longer if the condenser is doing it's job.

CD ignition reduced the voltage and current so that the point faces last far longer, in a CD the points act like a relay, the small point voltage/current controls the much larger secondary voltage/current.

In both systems rubbing block wear has an effect on timing, as the block wears gap closes up and timing is proprotionally retarded.

The length of dwell(point closed) determines the saturation time of the coil. The spark plug fires when the points open, then the coil is resaturated when the points close. Too little coil saturation caused by too little dwell reduces available spark energy,

[Scott Douglas]

Seems to me that a CDI car's timing would change, as the points are acting as a triggering device. As the block wears down the timing of when the points open changes by default, thus the timing of the CDI changes too.
I will admit that I hated the idea of changing points in my '67 912 so I went the Pertronix route and never looked back. I think I set the timing once in the three years I had that car.
Scott

[FrinkFrog]

Thank you Bill for your informative explanation. Analogies always work well for me.


Kirk

[Peter Zimmermann]

Originally Posted by T77911S

you did not understand my question. if i set the points to either 30 or say 45 or even 50 deg. and set the timing, will it have an efffect on how the car runs now, not in a 1000 miles. after all, the points dont have voltage on them and are not actually creating the spark, they just provide a ground trigger.

Actually, I didn't misunderstand anything. Why even bother trying to work outside of design parameters? Little ridiculous, eh?

Bill: Thank you!

[bkreigsr]

not to worry
the car probably won't start at 30, 45 or 50 deg anyway.
I know mine won't at 45 or 50.

Bill K

[drauz]

many thanks for this discussion. it took me right back to '73, learning how to tune & time (both incorrectly & correctly) my BMW 1600. I am tempted to drag out the old dwell meter & hook it up, just for fun!

[Paulporsche]

OK, thanks to Bill V for clarifying.

This now brings me at least back to square 1. If the point dwell angle goes from 35 to 41 degrees, and if each increase in dwell by a degree retards the timing by a degree, then if your timing was set @ 0TDC @ 35 then it would be retarded to 6ATDC @ 41.

If that's the case, then why not check both timing and dwell every so often, say 3000 mi, and reset either one or both to maintain the correct optimal timing? Maybe this is self evident and others have been doing this. I have always just let the points go through their wear cycle and then replaced them @ 41. Points aren't too expensive, as Bill K pointed out, but why waste them? Who knows how much longer they will be available?

[Bill Verburg]

Originally Posted by Paulporsche

If that's the case, then why not check both timing and dwell every so often, say 3000 mi, and reset either one or both to maintain the correct optimal timing?

That's what many DIY's do, it would be crazy to pay a shop to do it