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 Dwell angle
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Blue M

United Kingdom
1212 Posts

Posted - 23/09/2014 :  19:45:48  Show Profile
Does anyone know what the dwell angle should be for a DK4A distributor?

Ian

Gordon

United Kingdom
665 Posts

Posted - 24/09/2014 :  19:52:26  Show Profile
I see that no one has responded to your post on dwell angle. Here are some thoughts on dwell angle. It is written from the viewpoint of a 6 cylinder engine but the maths can easily be reworked for a 4 cylinder engine. If I have got any of it wrong I hope that anyone will comment and put me straight.

One of the considerations is that the dwell angle will need to be controlled so that the total of dwell angle (in crank angle degrees) plus the advance must not exceed the 120Ί interval between one spark and the next. So for 63Ί advance the amount of dwell possible is 120 - 63 = 57Ί crank Ί or in normal distributor convention this is 28.5Ί dwell)

A consequence of this is that the available time for charging the coil, together with the charge rate characteristic of the coil needs to be matched.

To calculate coil charge time you have first to select engine rpm, convert this to the time for one revolution and take into account the number of cylinders and then the dwell time ie time contact points are closed which is the period when the coil is being charged up.

Engine max rpm = 4500 which is 4500/60 revs per second = 75
The time for 1 revolution of crank = 1/75
For a 6 cylinder engine there are 3 firing events per crank revolution therefore 1 ignition cycle takes 120/360 of the time for 1 crank rotation = 1/75 x 120/360

We now have to consider the dwell percentage, eg if this is 70% then this is 70% of the ignition cycle time = 70% x 120 = 84 crank degrees which means that approximately 120 - 84 is available for advance = 36 crank degrees. This 36Ί crank (18Ί distributor dwell) can be changed by varying the dwell time as shown below to match the desired advance.

The following table is all at crank degrees

Dwell %….dwell……..advance……….Coil charge time…...Coil Charge……..Coil charge
………….degrees…...…degrees………..4500 rpm…………..….time 1000rpm….time 500 RPM
…………………………………………………...….(mSec)……………….....(mSec)………….(mSec)
70…………..84…………...36…………………….3.11……………………...14.0……………….28.0
66.7………..80……………40…………………….2.96………………….……13.3……………….26.7
62.5………..75……………45…………………….2.77……………………….12.5……………….25.0
58.3………..70……………50…………………….2.59……………………….11.65……………..23.3
54.2……....2.65…….….55…………………….2.41………………….……10.84……….…...21.7
50…………..60……………60…………………….2.22………………………10.0………….…….20.0
45.8………..55……………65…………………….2.03……………………….9.16 ………………8.32
41.7………..50……………70…………………….1.85……………………….8.34……………..16.68


Now consider the charging characteristics for the coil. The coil we are considering is a 12 volt coil with a primary resistance of 3 ohms and an inductance of 10 mH the current drawn is 12/3 = 4 amps and this is the max theoretical charge that the coil can take.

For an ideal coil that is fully discharged the charge time (Dwell time) for an ideal coil, starting at zero charge at zero time, is:
T = (-L/R x ln(1- (R x I/E))
where T is time in milliseconds, L is inductance in milliHenry, R is resistance in ohms, E is volts and I is current.

Dwell……..Amps…….Dwell…….Amps…….Dwell……..Amps
Time………………..……Time……………..……..Time
1.00…….…1.04…….…4.62….…..3.00………8.00….…...3.64
1.99……….1.70……….5.00…….…3.15………8.63……….3.70
2.31……….2.15……… 6.93……….3.54………9.00……….3.75
3.00……….2.47……….7.00……….3.51………9.99……….3.80
4.00……….2.80……….7.68……….3.60


Coils are characterised by their Time Constant which is calculated by L/R which in this case = 10/3 = 3.33 mSec which is when 67% of the charge has been completed. For the next Time Constant period there will be a further 67% increase of the remaining charge achieved towards the maximum possible and so on. Effectively after 3 time constants the coil is nearly fully charged and any extra time charging manifests itself in coil heating.


(formatting of tables corrected)


Gordon
ex owner of PB 0331, MG4473
Derby
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Nick Feakes

USA
2734 Posts

Posted - 24/09/2014 :  21:19:26  Show Profile
Gordon
I deleted the un-formatted version
Nick

Webmaster
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Blue M

United Kingdom
1212 Posts

Posted - 25/09/2014 :  08:48:35  Show Profile
Gordon,
Thank you for your very interesting and informative answer. It must have taken some time to do but it is much appreciated. I wanted to know because the points gap on a DK4a can be anywhere between 12thou and 20thou. Since the gap affects advance it seemed to me that measuring the dwell would give me an accurate points gap to use.

Ian
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Gordon

United Kingdom
665 Posts

Posted - 25/09/2014 :  10:34:13  Show Profile
Ian,
I should also have pointed out that over about 3 times the time constant has no benefit and only results in heating up the coil excessively and is obviously a problem at lower rpm. There is a need to balance the coil impedance with points gap to provide just enough spark energy at high temperature without overheating at cruise speed. Modern ignition systems offer control on this. Have a look at the Aldon Amethyst unit.
I think you are absolutely correct when you say that it i better to use dwell angle in setting up the points.

Gordon
ex owner of PB 0331, MG4473
Derby
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ags

United Kingdom
275 Posts

Posted - 25/09/2014 :  19:51:59  Show Profile
Hi Ian,

I have recently been looking into Lucas specifications on dwell angle, and quite honestly there seems to be no reliable documentation for the DK4A series of distributors. I did find that for older ( i.e. just pre and post war distributors) there seemed to be two families which were designed around 45 degree and 60 degree angles, but no clear indication as to which the DK4A belonged to. I have always tended to set my distributors to 45 degrees with the aid of a Lucas test gadget (very early integrated electronics) which I would not think would be obtainable nowadays, forty years later. I think my engines ran as well as most peoples so that I have always assumed that my figure was about right, even though the points needed to be closed up noticeably from the Blower figures. I have always considered that points condition was more important than ultra precise setting of the gap plus condenser and coil in good condition.

(Incidentally, a bit off topic but related, it appears that "Sports" high voltage HT coils are no longer available for points systems, though there are plenty for electronic systems. Can any one point me in the direction of a replacement for the Bosch "Sports" one on which I am running to benefit my spares boxes?).

I had started on analysing the distributor cam shape to see if I could get a mechanically based estimate of the design angle, but have not completed this. However, Gordon's electro-magnetic approach to the problem (Thanks Gordon!) has given me a fresh approach, so that when the winter approaches I shall pick up my investigation again.

More technical ramblings from

Andrew Smith MMM571
PB Abergavenny
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Gordon

United Kingdom
665 Posts

Posted - 25/09/2014 :  21:19:54  Show Profile
Hi Andrew,
You could look at the Pertronix coils. The firing voltage is going to depend on plug gap and compression ratio and of course the higher the firing voltage the greater the "electrical" stress on the distributor cap and leads. I would recommend the use of Magnecore leads from personal experience on blown and unblown engines from the 1930's. incidentally if you went the electronic advance route and developed the ideal crave for your engine you could then get the mechanical advance mechanism rebuilt to give you the equivalent curve to satisfy the VSCC!

Gordon
ex owner of PB 0331, MG4473
Derby
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Bruce Sutherland

United Kingdom
1203 Posts

Posted - 28/09/2014 :  17:13:46  Show Profile
Gordon,
Your explanation about Dwell Angle is most helpful, then you add recommendations for ignition HT leads and coils, again most helpful.

However the choice of Pertronix ignition coils is a little confusing, so are you able to advise which coil is best for the following, please?
1. For a ‘conventional’ points-type distributor?
2. For ‘123 Ignition’ electronic ignition?

My interest is for a P Type 4 cylinder application for 'enthusiastic' road use.

Bruce. (PB0564)
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Gordon

United Kingdom
665 Posts

Posted - 28/09/2014 :  17:48:23  Show Profile
Bruce,

Tnank you for your kind comments.
The inductance for the 12v Petronix coil I used was 10mH and the resistance was 3 ohms giving approximately 4 amps through the points. I used this coil because the diameter was correct for the coil holder the car was fitted with. I think that a Bosch blue coil would also be fine but don't have the details for this. The Petronix is a nominal max 40,000v. I would have thought that the unit would be suitable for a 123. Would you be using the 123 or 123Tune? Have you looked at the Aldon Amethyst unit for having the ability to develop a custom advance curve? It can also provide vacuum advance if you so wish.

I wonder how the long term reliability and service issues work with the 123. I wonder if it will be sensitive to temperature being so close to the engine. The advantage of the Amethyst is that the box of tricks is mounted remotely from the engine and is isolated from the engine vibrations and can be in a cool place. It is possible to use conventional points for triggering so the distributor looks absolutely standard as you cannot see that the advance mechanism has been locked.

I would advise that whichever system you use to check reliability and service issues carefully.

Gordon
ex owner of PB 0331, MG4473
Derby
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