Don't Dwell On It?

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Myrtles Man
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Don't Dwell On It?

Postby Myrtles Man » Tue Sep 03, 2019 12:28 pm

In the far reaches of my personal man-cave, among the long-since replaced spark plugs that 'seem to be still OK so may come in useful one day as a spare' (or is that just me?) and sundry ignition leads, dizzy arms etc, is a dwell-meter with which I used to fine-tune the ignition of my long-departed Rover 3500S. Could this be of any worthwhile use in setting up the Moggie engine too (which, I hasten to add, is running perfectly well as it is, having had nothing more sophisticated than the benefit of a carefully-inserted feeler gauge between the points)? If so, what would be the correct angle of dwell to aim for? I assume it would be around the 45 degree mark. Any thoughts, or is it a case of 'if it ain't broke don't fix it and in any case, the A-series engine is too simple and undemanding to require any of that fancy-pants electronic jiggery-pokery'?

PoolGuy
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Re: Don't Dwell On It?

Postby PoolGuy » Tue Sep 03, 2019 12:49 pm

In the good old days I used to set my Mini to 60deg, but iirc different distributors require differing amounts?

geoberni
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Re: Don't Dwell On It?

Postby geoberni » Tue Sep 03, 2019 12:54 pm

Well yes, the Dwell would be around the 45 degree mark, but not something I've ever bothered with.
Be interesting to see what others say, especially those that tune and race their Moggies with A Series engines.
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geoberni
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Re: Don't Dwell On It?

Postby geoberni » Tue Sep 03, 2019 1:00 pm

PoolGuy wrote:
Tue Sep 03, 2019 12:49 pm
In the good old days I used to set my Mini to 60deg, but iirc different distributors require differing amounts?
Interesting angle.
From what I remember it's all about the collapsing of the magnetic field in the coil as the points open/close. Too little Dwell and the field doesn't build up enough, too much and the points close too soon before all the energy of the coil has gone to the plug.

So on a 4 cylinder engine, the cams are 90 deg apart, therefore the Dwell is around the 45 deg mark. Being 15 deg over that I would have thought would have reduced your spark power a lot. :-?
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philthehill
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Re: Don't Dwell On It?

Postby philthehill » Tue Sep 03, 2019 4:33 pm

The dwell angle for a Morris Minor 25D4 distributer is 60 degrees plus or minus 3 degrees with the contact breaker gap set to 0.014" - 0.016".

Setting the contact points gap/dwell angle is much easier with a dwell angle meter and I would recommend using one every time the points are replaced or adjusted.

Setting the points gap with a dwell angle meter allows any wear in the distributer to be compensated for.


PoolGuy
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Re: Don't Dwell On It?

Postby PoolGuy » Tue Sep 03, 2019 5:05 pm

geoberni wrote:
Tue Sep 03, 2019 12:54 pm
Be interesting to see what others say, especially those that tune and race their Moggies with A Series engines.
Sorry, I missed this earlier. we never used points on the (Edit: Mini Miglia) racer, even in the late 80's we relied on a programmable ignition system. The distributor was present (as required by the regs) but all it did was distribute the spark, it was empty aside from the shaft and the rotor arm. It made for a very drivable engine even with the regulation 649 cam.
Last edited by PoolGuy on Tue Sep 03, 2019 5:17 pm, edited 2 times in total.

geoberni
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Re: Don't Dwell On It?

Postby geoberni » Tue Sep 03, 2019 5:14 pm

philthehill wrote:
Tue Sep 03, 2019 4:33 pm
The dwell angle for a Morris Minor 25D4 distributer is 60 degrees plus or minus 3 degrees with the contact breaker gap set to 0.014" - 0.016".

Setting the contact points gap/dwell angle is much easier with a dwell angle meter and I would recommend using one every time the points are replaced or adjusted.

Setting the points gap with a dwell angle meter allows any wear in the distributer to be compensated for.
I stand corrected then; it's a lot more than 45 Degrees.
Perhaps I'll think about getting a meter sometime. :)
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oliver90owner
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Re: Don't Dwell On It?

Postby oliver90owner » Tue Sep 03, 2019 6:28 pm

So on a 4 cylinder engine, the cams are 90 deg apart, therefore the Dwell is around the 45 deg mark. Being 15 deg over that I would have thought would have reduced your spark power a lot. :-?


Exactly the opposite. The dwell angle has nothing to do with the spark duration - spark occurs fairly instantaneously at timing point. What the dwell does is provide sufficient time for the coil current to build up to a maximum, ready for the next spark. What is also important is that the points open quickly, so collapsing the coil current (and magnetic field) as rapidly as possible - without undue sparking at the points (lost spark energy).

As the coil is an inductor, the current rises relatively slowly due to the rising induced magnetic field resisting the current change. 45 degrees dwell is adequate for a ‘cooking’ engine up to about 6000rpm for a 4 cylinder engine. Extra is good. Not so for an eight cylinder engine, or a highly tuned, high revving power unit which needs rather more sparks per minute - this is where the dwell angle is far more important, to avoid misfires at high engine speeds (high spark rates), where the coil may not have time to reach a sufficient current magnetic field before the next spark.

geoberni
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Re: Don't Dwell On It?

Postby geoberni » Tue Sep 03, 2019 7:32 pm

I didn't say Spark duration, I said spark power.
The Spark is the spark, it'll jump as determined by a number of things, including the energy available to it.
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oliver90owner
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Re: Don't Dwell On It?

Postby oliver90owner » Tue Sep 03, 2019 10:14 pm

geoberni wrote:
Tue Sep 03, 2019 7:32 pm
I didn't say Spark duration, I said spark power.
The Spark is the spark, it'll jump as determined by a number of things, including the energy available to it.
What did you mean when you wrote “too much and the points close too soon before all the energy of the coil has gone to the plug.

The energy goes to the plug long before the points close!

geoberni
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Re: Don't Dwell On It?

Postby geoberni » Wed Sep 04, 2019 9:16 am

oliver90owner wrote:
Tue Sep 03, 2019 10:14 pm
geoberni wrote:
Tue Sep 03, 2019 7:32 pm
I didn't say Spark duration, I said spark power.
The Spark is the spark, it'll jump as determined by a number of things, including the energy available to it.
What did you mean when you wrote “too much and the points close too soon before all the energy of the coil has gone to the plug.

The energy goes to the plug long before the points close!
Well trying to remember back a very long time, I thought the relationship between the Dwell and the point gap was that an excessive Dwell meant the points closed too soon after opening, cutting off the collapse of the Magnetic field before it's delivered it's full 'Ommph', but I may be wrong, it's over 40 years since I've worried about such things.
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Myrtles Man
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Re: Don't Dwell On It?

Postby Myrtles Man » Wed Sep 04, 2019 9:26 am

Oh dear! I seem to have inadvertently opened something of a can of worms here. Re the current (no pun intended - honest!) mild spat/constructive disagreement on the subject, a little Google research revealed the following which, to my reading at least, appears to confirm geoberni's view:- https://www.liveabout.com/dwell-and-tim ... re-4059400

PoolGuy
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Re: Don't Dwell On It?

Postby PoolGuy » Wed Sep 04, 2019 9:36 am

Far from it, it’s good to understand it. If you look at how ECU’s deal with it, you can adjust the ‘coil pulse width’ which is essentially the dwell angle, it’s around 2-4 milliseconds.

philthehill
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Re: Don't Dwell On It?

Postby philthehill » Wed Sep 04, 2019 1:33 pm

Thee dwell angle of a Minor distributer is 60 degrees plus or minus 3 degrees. Not approx. if I read it correctly 45 degrees as outlined in the link above.

The 60 degree plus or minus 3 degrees setting is stated in the factory workshop manual.

I have set enough BMC/Minor distributers in my time to know that 60 degrees dwell angle is correct and have set the dwell angle using both feeler gauges and Crypton / SUN engine analysers. Setting the dwell angle with a electronic dwell angle meter is always the preferred option.

Phil


Myrtles Man
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Re: Don't Dwell On It?

Postby Myrtles Man » Wed Sep 04, 2019 2:33 pm

Game, set and match then. Thanks Phil.

JOWETTJAVELIN
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Re: Don't Dwell On It?

Postby JOWETTJAVELIN » Thu Sep 05, 2019 7:09 pm

More important is the ignition timing. You could set the dwell but have retarded ignition. I advance the timing on all my cars until the engine idle speed just starts to slow down then retard it a little to just where the idle speed picks up again. Then road test. I have never bothered with dwell meters personally, just the points gap.
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