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Fine tuning spark advance with dwell angle changes

 
Old 12-17-2016, 10:41 AM
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SWCDuke
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Default Fine tuning spark advance with dwell angle changes

A couple of weeks ago I evaluated a local NCRS SoCal Chapter member's '65 coupe with a rare option combination - L-76 with C-60... did the normal spark advance map/idle mixture evaluation, and we took it for a ride on my "test course" behind LAX.

It's solid car, and we plan to drive it over to the Laughlin NV Regional in April. The AC is still R-12, and it works! I used this opportunity to verify the dwell angle - timing relationship that I knew to be 2:1... or is it 1:2? I forgot!

Turns out it's 1:2. A one degree change in dwell changes initial timing by two degrees. Increasing dwell advances timing and decreasing dwell retards timing.

Since the 365 HP engine won't idle stably below 700 where the centrifugal curve starts I checked the dwell-timing relationship at over 2500, which is above the 2350 revs that centrifugal is all in with the OE spark advance map.

This one actually didn't have the OE weights and springs and total centrifugal was more than the OE 24 degrees, but it was all in by 2500.

This is a useful relationship to know. You can easily experiment with finding the detonation limit or retard timing if you get into a situation such as hot weather or too little octane fuel that might get the engine into detonation, like on a road trip. All you need is the allen wrench and a dwell meter... no need for a timing light or loosening the distributor hold down bolt to adjust the timing.

Being as how the recommended dwell angle range is 28-32, if you set it at the nominal 30 degrees and know the initial and/or total WOT advance you can easily advance or retard 4 degrees with just a dwell angle change and know exactly where it is without the need for a timing light, and you can go beyond the OE recommended range, if necessary in an emergency.

Back in the sixties the Royal Pontiac (Detroit) "Bobcat Tune" included quickening the centrifugal curve and setting the dwell angel at 35 degrees. This keeps the points closed a little longer, which increases the engine speed that the coil remains "saturated" for a little more spark energy.

Also, since a higher dwell angle means the points don't open as far it can add a few hundred revs to the point bounce speed, which is good if you're using standard 19-23 ounce breaker arm tension points on SHP engine.

Magazine road tests showed that the Royal Pontiac Bobcats were quicker than the OE tune.

Back in the sixties I would increase the dwell angle a couple of degrees when I went out looking for some Saturday night action along with adding a few gallons Chevron Custom Supreme (about 103 RON) to the SWC's nearly empty tank.

Duke

P. S. if you replace points with an electronic switch you no longer have the above option.

Last edited by SWCDuke; 12-17-2016 at 11:56 AM.
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Old 12-17-2016, 11:28 AM
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Originally Posted by SWCDuke View Post
Increasing dwell advances timing and decreasing dwell retards timing.



Duke
You sure about that?

Last edited by Critter1; 12-19-2016 at 08:35 AM.
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Old 12-17-2016, 11:43 AM
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MelWff
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increasing the dwell means the spark occurs later which would be retarding the timing in my mind.
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Old 12-17-2016, 11:51 AM
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SWCDuke
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I can be tricky to intuit, but increasing dwell reduces point gap, so they close later and open earlier and vice versa for decreasing dwell. It's points opening that triggers the spark.

If you have any doubts, just run the same test I did. Anyone with a timing light and dwell meter can do it.

Duke

Last edited by SWCDuke; 12-17-2016 at 11:55 AM.
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Old 12-17-2016, 12:27 PM
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Not getting it either: In my mind, more dwell=less point gap=open later=retard timing

I will try it.
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Old 12-17-2016, 12:31 PM
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tuxnharley
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That's an interesting relationship that I was not aware of, but makes sense when you think it through.

I guess that is why the old tune up proceedures were to set the dwell before the timing.

Thanks for posting!
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Old 12-17-2016, 12:45 PM
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I always set the dwell on the low side of the spec and timing on advanced side of spec if there was a range. My thought was as the points wear the dwell increases and stays with the recommenced range longer. For example the corvette spec I think is 28-32. I would set the dwell at 28 and the dwell would stay in spec as the points wear through 4 deg of dwell change.

Tom

Last edited by Sky65; 12-17-2016 at 12:46 PM.
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Old 12-17-2016, 12:46 PM
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Sounds to me like just another reason your hood release cable will come off in your hand when the cable wears out.



The point block wears down. This reduces the point gap which means they open later and this in turn retards timing. That's why a tuneup on a high mile engine always felt like the engine ran better. And it did.
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Old 12-17-2016, 01:45 PM
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Originally Posted by SWCDuke View Post
P. S. if you replace points with an electronic switch you no longer have the above option.
But the Pertronix 3 has "Adaptive Dwell" for this exact purpose. And it does not alter the timing.

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Old 12-17-2016, 01:46 PM
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[/QUOTE]"Bobcat Tune" included quickening the centrifugal curve and setting the dwell angel at 35 degrees. This keeps the points closed a little longer, which increases the engine speed that the coil remains "saturated" for a little more spark energy.[/QUOTE]

Hi. Coil saturation question. OK. It's been many years since I played around with points type ignition. I thought coil saturation only happens the instant the points OPEN cutting off the current path to ground through the capacitor. Which then let's the coil build current until it can jump the spark plug gap to ground, which then drains the coil.

Do I have this wrong?
Dave
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Old 12-17-2016, 01:48 PM
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I respect you guys but now you are giving conflicting information. It's a simple mechanical concept. The later the points open, the later the spark, which means retard. Longer dwell equals later open (and earlier close), equals retarded spark. How is this difficult? Duke?
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Old 12-17-2016, 01:55 PM
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Originally Posted by survivor66 View Post
I respect you guys but now you are giving conflicting information. It's a simple mechanical concept. The later the points open, the later the spark, which means retard. Longer dwell equals later open (and earlier close), equals retarded spark. How is this difficult? Duke?
I though everybody but the OP was agreeing with you?
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Old 12-17-2016, 01:56 PM
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Originally Posted by SDVette View Post
But the Pertronix 3 has "Adaptive Dwell" for this exact purpose. And it does not alter the timing.

indeed it does have adaptive dwell, but all the adaptation is on the switch close end of the dwell interval. The switch open time within the dwell interval does not change, so the coil firing time does not change either.
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Old 12-17-2016, 02:41 PM
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"Bobcat Tune" included quickening the centrifugal curve and setting the dwell angel at 35 degrees. This keeps the points closed a little longer, which increases the engine speed that the coil remains "saturated" for a little more spark energy.[/QUOTE]

Hi. Coil saturation question. OK. It's been many years since I played around with points type ignition. I thought coil saturation only happens the instant the points OPEN cutting off the current path to ground through the capacitor. Which then let's the coil build current until it can jump the spark plug gap to ground, which then drains the coil.

Do I have this wrong?
Dave[/QUOTE]

Respectfully, it's the opposite.

Coil saturation is kinda like saturating a sponge. You let the water run on the sponge for a while, and eventually the sponge soaks up all the water it's going to. Any additional water is just a waste of time and water. A coil gets saturated when the current through it has gone on long enough to input all the energy that it's going to store. Any additional current level above that point just wastes electricity and heats the coil up like a toaster.
Once the points open, the current stops, the magnetic field collapses, the coil output voltage increases a bunch until the plug arcs over, and then the stored energy in the coil passes into the plug gap.

There's an ignition sticky in C3 Tech that explains everything in more detail.
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Old 12-17-2016, 02:51 PM
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69427 Thanks. Dave
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Old 12-17-2016, 04:31 PM
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Somebody who has a good old fashioned oscilloscope combo engine diagnostic machine ought to be able to test this by playing with the dwell and watching the results.......

Last edited by tuxnharley; 12-17-2016 at 05:02 PM. Reason: Typos
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Old 12-17-2016, 04:51 PM
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Since the coil is an inductive element (in addition to having resistance) the circuit has "inertia". An inductor is equivalent to mass in a mass-spring-damper system. The mechanical analog of a spring is a capacitor, and a resistor is the mechanical equivalent of a damper.

So when the points close, it takes a few milliseconds for the circuit to achieve steady state DC current, otherwise known as "saturated", but above about 2000-2500 on an eight cylinder engine they're not closed long enough, and less than steady state current means less than maximum open circuit secondary voltage and energy, which is usually measured in millijoules.

You don't need a scope to verify my 1:2 dwell-timing relationship and polarity. Like I said in the original post a timing light and dwell meter is all you need.

Some of you guys need to run the test. Then have fun figuring out why.

Duke

Last edited by SWCDuke; 12-17-2016 at 04:55 PM.
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Old 12-17-2016, 05:07 PM
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It's just easier to watch it all in one place on the machine, that's all......
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Old 12-17-2016, 06:50 PM
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I always understood it was important to set dwell before timing but I didn't realize dwell affected timing that much. Do GM distributors maintain steady dwell when the vacuum advance is disconnected and reconnected?

The reason I ask is that on my Ply GTX I noticed that my dwell changed 2 deg when I disconnected the vacuum advance to time the engine. When I asked about this on the B-Body Mopar forum, I was told this was a fluke of the design of the points plate and where it was hinged which cause the points to shift slightly when the vacuum can pulled it across the arc. So that means my initial timing is changing as much as 4 deg simply by hooking the vacuum hose up after setting timing. Great.

http://www.forbbodiesonly.com/moparf...-dwell.100081/

Last edited by DansYellow66; 12-17-2016 at 06:55 PM.
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Old 12-18-2016, 12:21 AM
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Struggling here some. I've always seen it as "increase gap/decrease dwell/increase timing".

I used the manual switch on a dual point distributor trick years ago to increase the dwell and retard timing a mile with a tunnel ram/street -racer. Ran around on the single set of points...with a ton of timing (55* used for street cruising and through 2nd gear at the track) and then flip the switch to bring in 2nd set to increase dwell and retard timing under load. You could vary the amount of retard based on dwell settings of 2nd set of points. Grumpy Jenkins did it way back when and it still worked in the 80's!

JIM
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