View Full Version : TR4/4A 3A and 4A distributors?

12-02-2014, 03:27 PM
There is a spec page in the Moss catalogue that shows the 3A distributor as LucasDM2P4 and the 4A one as 25D. Is there much difference between the two? Could you pull out 4A dist and drop in a 3A dist without any adverse affect?

Geo Hahn
12-02-2014, 06:11 PM
different advance curves...


also different vacuum advance numbers.

my guess is that you could get the tr4a distributor to work reasonably well in a tr3 engine but perhaps with some compromise between good idle and peak performance.

12-02-2014, 06:37 PM
Man, that plot sure gets around :)
The application information isn't quite correct (or complete); for example 40795 is also listed for TR3B and later TR4.

Geo Hahn
12-02-2014, 09:24 PM
if you want a thought to live forever... just post it on the internet.

google tr4 advance curve and guess what is the first image that comes up?

12-02-2014, 10:37 PM
google tr4 advance curve and guess what is the first image that comes up?
Yeah, but that's your post, not mine.

Not that I'm complaining, I'm glad it's useful. Makes it a lot easier to visualize the curves than just reading the numbers in a table.

02-08-2015, 08:14 PM
We were discussing the total amount of advance and how it hits max at a fairly low rpm. The previous chart shows that the TR2/3and 4 curves have more advance over a longer range. Why is that and what would be the positives and negatives of having more advance in a 4A.

02-08-2015, 08:45 PM
I don't know.

Optimum ignition timing is one of those things that borders on black magic. It's affected by all sorts of things, including combustion chamber shape, exhaust and intake, camshaft and so on (all of which changed at least once during TR2-4A production).
I've been told that, even after all these years, engine makers still run a new engine design on the dyno to find the "sweet spot" for timing and then work from there to allow for changes in fuel quality, air temperature and so on. Here's a write-up that goes over a few of the concepts (although it's written from the standpoint of needing less timing when adding supercharging, duh). https://www.innovatemotorsports.com/resources/myths.php

Adding more timing may get you more power/economy, or less, or result in disaster.
That damage was caused by a failed valve stem seal (combined with badly worn guides), but the effect would be the same if you get preignition (aka knock) from too much timing. Since it only happened at higher speeds, I had no idea anything was wrong until the piston broke (laying a cloud of blue smoke behind me on the freeway).

Marvin Gruber
02-08-2015, 09:10 PM
Bottom line, if you had choice of distriubutors which one would be best for normal driving


02-08-2015, 09:28 PM
I'm a real novice when it comes to distributers and the best decision I've ever made is to send mine to advanced Distributers for a rebuild to my engines specs...night and day.

02-08-2015, 09:40 PM
My opinion, stick with the stock setup unless you have the time, inclination and tools to validate any changes. The factory was certainly conservative in their choices, but at the same time interested in the best performance they could deliver consistent with long engine life under a wide variety of conditions. You could probably squeeze out a few percent more power and/or fuel economy; but is that really worth risking the kind of damage shown above? If you are going racing, then the answer is certainly "Yes", but not (I think) for normal driving.

In other words, even though I don't know the specific whys and wherefores, I'm certain they had valid reasons for making the timing curve more conservative when they put so much time and energy into making the engine more useful on the street (by optimizing mid-range power rather than peak power). It wasn't just laziness or stupidity.

02-08-2015, 11:57 PM
Ok, I have learned something. I also suffered a broken piston fortunately mine held together by the pin and I was able to make the 200 mile drive home. Albeit with limited power. Good thing was that by pulling the engine it helped make the decision to do a resto. Finished it last summer after 18 mo.

Marvin Gruber
02-09-2015, 11:19 AM
I had a couple of 4 cyl distributors apart last week. While cleaning up I did notice one was stamped 10 degrees and the other 16 degrees. Only differences I could see was the lenght of the little arm where the stamping was done. I thought then a person could probably modify advancement on the 10 degree distributor and what effect that would do so this thread was timely for me.


02-09-2015, 02:48 PM
The only other differences in the centrifugal advance are the spring rate and length of the advance springs, which are difficult to see with the naked eye. There's a good article at https://www.starchak.ca/tech/pdfs/lucastuning.pdf
which talks about the various relationships.

FWIW, Marcel has also posted a good article on converting a later "electronic" distributor as a replacement for the Lucas. Not written for Triumphs, but the same mods should work for the TR2-4 motor.