TR2/3/3A Proper distributor for 1960 TR 3A

[John_Progess]

I tried posting earlier but nothing happened so I hope this does not appear twice. My manual calls for a DM2 distributor but my project has a 25D distributor. I do not know how it ran before I got the car (disassembled). I also checked on my MGA's and they also call for a DM2 but both mine have 25D's. My question is what is the difference between the two distributors and will the TR 3 be ok with a 25D? Thanks and have a good day!

 

[Member 10617]

If you have a Haynes workshop manual, p. 94, you'll see that there is a difference between the two when it comes to max. advance, vacuum advance, and automatic advance commences. How important this is I cannot say. Someone else more experienced in the Forum can answer your question.

 

[Darrell_Walker]

And who knows if any of the advance curves are correct for current fuels. You might look into having Jeff @ Advanced Distributors rebuild yours. Not only will it look and work like new, if you can provide information about your engine he will recurve it to match (at least what he thinks is best, which is probably better than any guess I could make).

 

[TR3driver]

The first thing to understand is that 25D and DM2 are just Lucas model numbers. Lucas made a huge variety of distributors, with different advance curves, maximum advance, etc. all under the same model number. You have to look at the Lucas part number to determine what the specifications were originally. (The Lucas part number is a 5 digit number, usually with one letter after it, like 40698B.) Then on top of that, it is trivial to swap parts around, so someone might well have taken a 25D from a later car and modified it to deliver the same performance as the original distributor.

In fact, the 25D has been supplied as a replacement for the DM2 for at least the past 40 years (which probably explains why both your car and mine have them). According to an old Moss catalog I have, they supplied Lucas P/N 40783 (which is a 25D) as replacement for all TR2-TR3A (which would have used 40403, 40480 or 40698, depending on year). The 40783 doesn't have quite the same advance curve, but is fairly similar to the others.

If you really need every last fraction of a horsepower for some reason, then you'll need to spend some quality time on a dyno (aka rolling road) to find out just what your engine "likes". The absolute best ignition advance curve depends on a lot of variables, including the fuel you use (which BTW is different for different seasons), engine condition, modifications, etc. But, the curve isn't all that critical otherwise, so chances are that it will run just fine with what you have.

Or as Darrell suggested, send it off to Jeff to be recurved. Jeff also changes the recommended initial advance (the advance you set while doing a tune-up) though, so you'll need to remember what he tells you. If you forget and set it by the book, the engine will be down on power, burn more fuel and have more of a tendency to overheat.

PS, here is a little study I did some time ago, comparing some of the different distributors that have been used on TR2-4. This is only the centrifugal advance curve, the vacuum advance curves also vary somewhat.

 

[Geo Hahn]

There is an impressive amount of data here: Lucas Distributors

And I gleaned (from the above-referenced source & elsewhere) the following:

Please let me know of any errors in that summary & I'll correct it.

 

[TR3driver]

Not an error, Geo, but something that might not be clear to others. All of those distributor figures are in distributor rpm and distributor degrees; while it is much more common to use engine rpm and crankshaft degrees. The crankshaft turns twice as fast of course. So for example where your chart says a 40480 reaches maximum advance of 15 degrees at 2700 rpm; it really means 30 degrees at 5400 rpm.