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morisani
Italy
109 Posts |
 Posted - 25/03/2016 : 14:56:06
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Does the PA distributor Lucas DK4A have a mechanical advance? Is it fixed or it changes according to the engine running speed? In this case how is it possible to check if it works or not?
Thanks
Francesco |
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george
United Kingdom
862 Posts |
Posted - 25/03/2016 : 16:25:18
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| Francesco, yes the dk4a has auto advance. To tell if the advance works in the correct sense gently try to turn the rotor arm clockwise [viewed from above] It should turn a small amount against a spring. The only way to check that the advance works when the engine is running is with a strobe timing light. Geoff |
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morisani
Italy
109 Posts |
Posted - 25/03/2016 : 20:37:11
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Thank You very much. Considering that the Pa ignition advance data is 20° how much it will change increasing the speed? Is there a fixed relation between advance and speed? Thanks again.
Francesco |
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ags
United Kingdom
275 Posts |
Posted - 25/03/2016 : 21:13:40
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Hi Francesco,
As you have been told the PA/PB DK4A distributor does have a centrifugal advance mechanism in the housing under the baseplate which carries the points. Blower quotes the advance specification beyond the static setting as a total of 20 degrees (crankshaft figures) commencing at 2000 rpm and ending at 4000 rpm. The advance pattern is a straight line so that at 2500 rpm there will be 5 degrees extra advance over static, at 3000 rpm there will be 10 degrees extra advance beyond static and so on.
However these specification figures were set for Thirties' fuels, and so whilst they are a reasonable starting point now they may not necessarily be ideal for the very much improved petrols available today.
More technical ramblings from
Andrew Smith MMM571
PB Abergavenny |
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Andrew Fock
Australia
374 Posts |
Posted - 25/03/2016 : 22:16:03
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So... Andrew, What is considered a better advance curve for modern fuels?
Andrew
PA 1294
NA 0279 |
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Gordon
United Kingdom
692 Posts |
Posted - 25/03/2016 : 22:24:41
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I think that the optimum advance curve will be dependent on the build spec of your engine and the only way to determine this is to lock the automatic advance and determine the optimum advance curve on a rolling road. Armed with this information you can then get the advance mechanism rebuilt to give the desired curve.
Gordon
ex owner of PB 0331, MG4473
Derby |
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Simon Johnston
United Kingdom
6123 Posts |
Posted - 26/03/2016 : 08:15:04
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Two questions:
First, if one uses a fixed or locked distributor to determine the optimum advance curve how can this be translated into a mechanical distributor with, as Andrew says, a straight line advance pattern? One can use two springs, the second of which cuts in at higher revs than the first, to give a two stage linear advance pattern, but a curve? (Bosch uses flexural springs in some of its distributors which results in a curved as opposed to a linear advance pattern but this is probably not an option for Triple-M distributors, whether Lucas or Bosch.)
Second, it seems that we should set our distributors to give maximum advance around 3000-3500 revs so any adjustment of the 'advance curve' will be at lower revs than that. But when pressing on we need to keep our engines well above 3000 revs, so what is the benefit of fiddling with the advance setting below that other than ensuring that it idles properly and doesn't pink at low revs, both of which are easily accomplished with the standard distributor or the Bosch 009?
Simon J
J3437 |
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Simon Johnston
United Kingdom
6123 Posts |
Posted - 26/03/2016 : 13:17:54
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Sam,
There's nothing very sophisticated about the 009 distributor. It has a linear advance from about 800 rpm to 1300 rpm (1600 to 2600 at the crank) with about 17 degrees of advance at the crank at full advance. The advice I was given was to set the distributor so that there is about 35 degrees of advance at 3000-3500 revs.
Simon J
J3437 |
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sam christie
United Kingdom
3101 Posts |
Posted - 26/03/2016 : 13:31:40
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Simon,
Whatever your 009 distributor does it seems to do it very well.
Sam |
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morisani
Italy
109 Posts |
Posted - 30/03/2016 : 16:42:24
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I asked about Pa distributor Lucas DK4A advance because recently I noticed poor engine power when going uphill. I checked the distributor advance bought as rebuild in 2014. It doesn't seem blocked but i doesn't go more than 20° or 21°. Starting from 15° it goes easily to 20° at 2000 but doesn't go further at 3000 or 4000. I don't Know if didn't work from the beginning. Every suggestion will be very much appreciated ?
Thank
Francesco
Mg Pa 1121 1934
Distributor Lucas DK4A rebuild - rotor arm 400052 |
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ags
United Kingdom
275 Posts |
Posted - 31/03/2016 : 14:36:24
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Hi Francesco,
I am not yet ready with all the test results which I think that I might need for recommending better ignition advance patterns. However Bruce is undoubtedly right that the best way to do it is for each individual engine. It is generally easier and quicker on a rolling road but with care, a stopwatch and a suitable selection of slight hills which are relatively traffic free the job can be done on the road if you allow enough time for the job (that is days, weeks or in my case months).
I disagree with Simon over the need to set the distributor to have advance beyond 3500 rpm. I am quite sure that my engine shows responses to changes in ignition at 4000 and even 4500 rpm. (With standard cam and on OM carburettors but with a lot of attention paid to details under the surface.) The starting rule of thumb is that advance should continue at least until the point of maximum torque (that is the point of maximum volumetric efficiency of the engine, carburettor, camshaft, manifold and cylinder head combination). Different engines may or may not show improvements or losses higher than the max. torque rpm.
Having said all that it does sound as if your distributor is not giving enough advance to suit your engine. Any good rebuilder of distributors should also supply you with a chart of the advance figures at various rpm levels in the range, after he has done his rebuild. Remember of course that you do need to be careful to find out whether the rebuilder is quoting crankshaft or distributor degrees and speeds. (As I am sure that you know the distributor turns at half engine speed so that the degree figures are also halved.)
I do agree with Simon that about 35 degrees is a reasonable figure for maximum advance but once again different engines may need slight variations of about 1 to 3 degrees more than this figure.
More technical ramblings from
Andrew Smith MMM571
PB Abergavenny |
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Simon Johnston
United Kingdom
6123 Posts |
Posted - 31/03/2016 : 19:48:55
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quote:
Originally posted by ags
I disagree with Simon over the need to set the distributor to have advance beyond 3500 rpm.
Hi Andrew,
I'm not at all certain what you are saying here but to be clear, I was not offering any personal advice, rather I was conveying the advice given to me, i.e. beyond about 3500 rpm the timing advance should be pretty much set at around 35 degrees. Can a Triple-M Lucas distributor in fact be modified to continue to advance the ignition up to 4000 or 5000 rpm?
I'm also still at a loss to understand how one can create an advance 'curve' with a mechanical distributor. What sort of variations in timing can one achieve other than a linear 'curve' or at most a two stage linear one?
Simon J
J3437 |
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ags
United Kingdom
275 Posts |
Posted - 31/03/2016 : 23:53:11
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Hi Simon,
I agree that I, and most other people, do tend to use "curve" rather loosely when describing centrifugal advance patterns. As you point out you can only achieve a one slope or at the most two slope set of lines between the fixed horizontal portions at static and maximum advance. Further I believe that the slope of the later part of the pattern, if any, must for mechanical reasons be less than or equal to that for the first part. That is you can get a set of lines that is concave downwards but not upwards, when conventionally plotted with the advance increasing vertically and the revs. horizontally.
I think probably that "curve" tends to be used because the patterns for distributors with vacuum advance are indeed truly curved, but such things could not be done with Lucas DK4A machinery. (Though I am aware that people have adapted BMC A-series distributors with vacuum advance to fit XPAG engines with good results claimed.)
The answer to your question as to whether you can get a Lucas distributor to advance beyond 4000 rpm I am sure is yes, and my old one does do this, with the correct springs fitted. My detailed experiments recently have been with a box of "electrickery" for ease and range of adjustments, but I have been careful to stick to settings that I think can be achieved mechanically. I will shortly find out for real when my original Lucas one comes back with a respecified pattern.
More technical ramblings from
Andrew Smith MMM571
PB Abergavenny |
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Gordon
United Kingdom
692 Posts |
Posted - 01/04/2016 : 12:05:37
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Andrew,
Can you reveal what your "electrickery" is?
Gordon
ex owner of PB 0331, MG4473
Derby |
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george
United Kingdom
862 Posts |
Posted - 01/04/2016 : 12:18:57
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| Francesco, What static timing figure are you setting [degrees on the flywheel?].Blower gives I think 20 but I use a bit less say 17. With the engine running at idle that goes up to about 22/25 on the strobe , increasing revs brings an increase to about 35/37 at about 3000/3500 revs. The car goes well . Geoff |
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Simon Johnston
United Kingdom
6123 Posts |
Posted - 01/04/2016 : 12:32:55
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Geoff,
Your experience mirrors mine, albeit I use a Bosch distributor, and I think it's not far out. My car certainly goes well on that setting
Andrew,
Re vacuum advance, I can't see any benefit to this for our cars as, like vacuum wiper motors, it doesn't work when accelerating hard. Its purpose is to give more advance on idle and light throttle to improve fuel economy and emissions. I can't see how it is at all relevant to maximising the performance of our engines. But if one really want to try it there is a vacuum advance version of the Bosch 009 called the SVDA.
Edited to correct spelling of 'Bosch'!
Simon J
J3437 |
Edited by - Simon Johnston on 03/04/2016 08:00:44 |
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