Lucas distributor

[BLOWNMM]

Hi guys
I have a Lucas 23D4 dissy in my car. The guy I bought it off told me it came from a Mk, 1 Cooper S. The dissy is marked
23D4 40943E and 4675. However I have found a Lucas distributor chart from which the only match is for a 40943 without the E. In in the vacuum code column there is no info. In the models column it says 'BMC special tuning C-BHA 44? HC'. Can anyone shed some light on this. Have attached pics which show the bob weight stamped 12 which agrees with the chart, and a pic of the markings on the dissy. Also link to the dissy chart refered to.
Bob


http://www.sterlingbritishmotoringsocie ... _Chart.pdf

[RobThomas]

Looks like it is a BMC Special Tuning item. I've found a couple of references but am not sure if you've already got this info. The fact that it says 25D4 and then lists no vac figures might suggest 23D4 and a typo?

http://osneyconsulting.com/maps/Lucas%2 ... 040943.dat

// Aldon Amethyst ignition map
// Provided for convenience without any warranty
//
// BMC Special Tune
//
// Lucas 25D4 distributor model 40943
// Static timing: 12 degrees BTDC
//
// Kevin Jones 11/4/13

CYLINDERS 4
REVLIMIT 6400
ADVLIMIT 24
OFFSET 0
CENTRIFUGAL 0,0,8,16,17,18,19,20,21,22,23,23,24,24 (Every 500 rpm? Rob)
VACUUM 0,0,0,0,0,0,0,0
____________________________________________________________________________________________________________________
http://www.mgb-stuff.org.uk/curvestext.htm

Special Tuning
Stage 3/4 1967 on

25D4 40943

Degrees

RPM

Chart

Dwell

60 +-3

Static

6 (suggested)


Strobe

not given

Centrifugal

0
400
4
550
8
700
12
850
16
1000
18
1550
20
2100
22
2700
24
3250

Vacuum - none


Quick cut-n-paste.

[BLOWNMM]

Thanks Rob I was not aware of the extra info you provided. The only thing missing is the suffix 'E' which is still a mystery.
Bob

[bmcecosse]

For some reason the S dizzy had no vacuum unit -no-one has ever been able to explain why to me.....we know better now, and a vacuum advance unit should always be fitted on a N/A engine. However a 'blown' engine is a special case and maybe it is better with out the vac advance - since there is little chance of any vacuum in the manifold!

[RobThomas]

The general belief is that the suffix relates to changes in product/component design that didn't affect the curve of the item. Different materials, different connections on vacuum ports (screw v plastic sleeve), different length pins for the points (32oz v quickfit, if you know what I mean).

For the MM, 40056 was a leftover from the pre-war Morris 8, then it went to 40251 with the same cam shape until 40251E which had the high-lift cam profile along with the 40333. I have a 40251D which has the old cam shape but otherwise appears to be the same as the later ones.

Perhaps this explains why the 40943 is listed as a 25D4 with no vac and then the 40943A is listed as a 23D4? That might be the only change.

None of the above info I listed is confirmed so you might want to treat it as a starting point for further investigation?

Rob

[RobThomas]

Just checked the curves for the 5 listed versions of the 40056 and the only 40251 that I could find. All very close when plotted apart from the 40251 having 2 degrees less at 500rpm than the 40056s. Another manual lists the baseplate for the MM as being 18 degrees and 22 seconds.
Doesn't answer your original question but it might help to clear the fog?

[bmcecosse]

I think with a blown engine you will want much less advance than a N/A engine. Otherwise detonation = holes in pistons will soon follow!