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MG MGB Technical - Distributor 25D and 45D4 differences?

Hi,

I am building up my Sunbeam and I have Lucas 25D and 45D4 distributors. Both are unmodified. Which has better curve for performance or has some other kind of advantage over the other? It is a 4 cylinder engine making around 130 horsepower fed by Dellorto sidedraught carburators.

My mentor used to race a MGB and said both of these distributors were available on these cars so I am hoping this club will know :)

Thank you very much,
Andrew Zizzo
Andrew Zizzo

Andrew. No one can tell you for sure. The basic designations of the 25 series and the 45 series are modified by a "specification number" which should be stamped on the body of the distributor. On the 25D, there will be a raised pad which will have a five digit number series and may, or may not, an an alphabetic suffix. An example would be the 41121 B specification. Below the specification code would be a four digit number like 4266. This would indicate that the distributor was built in the 42nd week of 1966.

All of the specifications are slightly different and there are dozens, if not hundreds of specifications. Each one is set up for a specific engine application and may not be ideal for any other engine application. So, what you would have to do is to locate the specification numbers on the distributors, then cross reference these numbers to a specification chart to obtain the original advance curve of the distributors. Then, you would need to compare the advance curve of the distributors to the requirements of your engine.

Each engine will have an ideal advance curve. One that, if we could obtain it, would provide the maximum power and economy that the engine was capable of. They are coming close to doing this with the computer controlled engine management systems and knock sensors in the engines. When we go back to the old distributors, however, the situation is significantly different. We must find out what the ideal advance curve for the engine is, then custom build a distributor that will come as close to that required curve as possible without any possibility of going too far and causing engine damage.

Hence, the modern engines can go far closer to that "ragged edge" of engine damage due to their computer controls and engine feedback devices. The older distributor cannot operate to that level of precision and must be set up as close as possible, realizing that it lacks the precision of modern systems.

Most of the factory distributors performed well with factory engines--they provide adequate performance and are safe to use. With custom engines, especially where maximum performance is required, plotting the actual advance requirements and building a custom curved distributor are a must if one wants maximum performance.

If maximum performance is not a must, say as simply an upgraded driver, you still need to plot the required curve, then find a distributor which approximates the required curve without showing too much advance, or too little advance, but within a much broader range than the performance distributor.

So, you see, that no one can tell you which of these distributors might be better until the engine's requirements are determined and then compared to the advance curves of the two distributors. Les
Les Bengtson

You can find distributor curves on Paul Hunt's web site. You will need the five digit part number of the distributor to determine the curve of your distributors. Hope this helps, Clifton
http://www.mgb-stuff.org.uk/wn_suspensionframe.htm
Clifton Gordon

Sorry, I didnt know Les had already posted. Clifton
Clifton Gordon

Not a problem Clifton. I did not include a hot link to Paul's site which is a good thing for anyone to have. But, the fellow also need to know that there are some significant differences between what an MGB dizzy might be programed to provide and what his engine might require. Thus, our posts tend to complement each other. There is another internet posting of distributor advance curves, which may be available from Paul's site at the bottom of the tables on advance curves. (Just noticed it and have not had a chance to chase the links yet.) Just because the dizzies came from an MG source does not guarantee that they are MGB dizzies. Like SU carbs, the basic units are all the same, physically. But, the internals can be changed to set the two up in very different configurations. Les
Les Bengtson

Hey guys,

Thanks for the detailed responses. Sorry if I am a bit uneducated on the subject, I've never had to do this before. How would I go about figuring out what my engine needs? And then what companies do this service in the States?

Thank you for your help,
Andrew Zizzo
Andrew Zizzo

Also,

for the numbers on the side of the 45D it goes:

41554 E 45D4 >>---> 0477

(yes, it looks like an arrow on the side of the distributor ??? )

Would this be a "45DE4" like I see mentioned on this site or does the E mean something else? The 25d reads:

41077 A 1166


This computer doesn't work with adobe acrobat for some reason, I will go downstairs to see if using the file linked from the link you guys gave me I can find some info on these units.

I have no idea where this 45D came from but I suspect it is from a car with stromberg of unknown CD with white (or yellow I should say) plastic automatic "choke" (fuel enrichment)
Andrew Zizzo

Andrew. The only way to choose the ideal advance curve is to have the car tested on a chassis dynomometer (rolling road). This type of place is normally found under the automotive section of the yellow pages in the larger cities. Race and performance shops may have a dynomometer or may be able to tell you where they take their cars for testing. Actual, controlled testing is the only way to determine the exact curve any engine needs. Thus, as a general thing, because they are dealing with a wide variety of engines, the manufactures produce a distributor which is close to the average requirements, but somewhat conservative. Thus, engine damage is almost impossible, but performance is reduced.

As to the distributors you have. The first one is a points type Lucas 45D4 to specificaion 41554E, produced in the fourth week of 1977. It is not, according to my Bentley manual, a specification used on the MGB.

The second distributor is a Lucas 25D4 points type distributor to specification 41077A which was produced in the eleventh week of 1966. Again, my copy of the Bentley manual does not show this specification as having been used with the MGB.

Since these are points type distributors, it is relatively easy to determine what the current advance curve is if you can find access to a distributor test machine. Such machines are sometimes found in speed shops and race preparation shops. Telephoning around to find out what is available in your area would be a good place to begin your researches. Les
Les Bengtson

OK so I looked them up,

The 45D is from a later model Rootes vehicle, the information suggested a Talbot GT model. This makes some sence, maybe a Rootes dealership sold this distributor to the previous owner. No information was available for this unit.

For the 25D it lists:

3* advance at 500 rpm

12.5* advance at 1050rpm

13* advance at 1800rpm

and then for the vacuum it reads 4/8/5

The 25D also has a vernier adjustment, what does this adjust? Could I use this for tuning it in for better performance?

It's a 1767cc aluminum head motor revving just below 7000
Andrew Zizzo

The vernier adjuster allows you to fine tune the position of the advance plate which affects the initial advance. This in turn affects the total advance but it does not change the value or the curve of the mechanical or vacuum advances. I suppose in a way it adjusts performance but it's a set and forget kind of thing.

I leave the adjuster in the middle of it's rotation travel when I set the timing. After I clamp the dizzy back down I recheck the timing. If I bumped it off a degree or two I can easily spin the wheel and get the timing right where I want it without the hassle of trying to rotate the dizzy just the perfect amount.

I have also heard of it being used to back off the timing to prevent pinging should you happen to get a bad batch of gas or have to run a lower octane than usual.

-Jared
Jared Snider

Andrew. The vernier adjustment is used for fine tuning as you suspect. You set the initial timing by rotating the distributor, then fine tune using the vernier adjustment. Some sort of zero mark would be of use so that you can return to the original adjustment if desired.

The distributor specifications you quote need some analysis. I suspect they are in distributor rpms and degrees rather than engine rpms and degrees.

The distributor turns at one half engine speed (because it takes two complete revolutions of the engine to complete the four cycle process). Thus engine rpms are twice the distributor rpms and engine degrees of advance are twice the distributor degrees of advance (once again because the engine is turning twice as fast as the distributor). This is something that most people do not realize needs to be stated when giving out information relating to the distributor. Some of the information, such as that contained in most of the workshop manuals, is listed at the crankshaft pulley where you can cross check what is happening with a dynamic timing light. This is the information most of us need to set and check the timing of the engine.

Some of the manufacturer's data, however, is listed as it was taken from the distributor test machine--in distributor rpms and degrees of advance. To find out what this represents at the crankshaft, multiply by two. Thus, your figures would show, at the crankshaft pulley,

1,000 rpm 6 deg advance
2,100 rpm 25 deg of advance
3,600 rpm 26 deg of advance

This is further complicated by the "programed in" or "static" advance. This is advance which is programed into the system when it is being set up. Say, for instance, we set the crankshaft pulley at 10 deg BTDC and statically time the distributor to this. Then, at 1,000 rpm our total advance is 10 deg static, plus 6 deg mechanical for a total advance of 16 deg at 1,000 rpm when timed dynamically. That programed in advance holds true across the rpm range and would result in a final, total, advance of 36 deg BTDC at 3,600 rpm. Normally, with a high compression engine, you are looking for about 32 degrees of total advance, at least with the MGB engine. With the Sunbeam engine, that might be a worthwhile place to start, but it will be an approximation only. You really need to find a source of information familiar with that engine and the specific modifications you have made to get an accurate idea of what an acceptable advance curve and maximum advance would be. Les
Les Bengtson

Andrew,
The 25D4 41077 that you have is off a Sunbeam Rapier 1.7 and is listed for the years 1967 to 1976 for that car.
Regards
Graham
Graham Cherry

The distributors you have should both give satisfactory performance if they are set up as your manual suggests (meaning static timing).
You won't likely get "maximum" performance without implementing the suggestions given by the other VERY knowledable posters.
I happen to prefer the 25D myself as I go from 4800' elevation to near sea level on a frequent enough basis to need the handy adjuster on the side to retard at low elevations (they sell lower octane fuel in the mountains).
I use a Mallory Unilite for racing and set it up for each venue.
A nice upgrade to any stock distibutor is a pertronix breakerless system as it greatly reduces the maintenence involved (don't forget to oil the shaft under the rotor periodically, however!).

mike!

Thanks for all the help guys.

My 25D shaft has a little play in it so I'm swapping for another one with a sunbeam guy. I already have the pertronics setup for the 25D, the main reason I started this post was to see if I should send it back and get one for the 45D instead. I will have to run race gas for optimum performance at my compression but would like to be able to turn timing back should it not be available.

The standard for the sunbeam is 8* BTDC, so 34 degrees total advance. The car is a 1767cc Alpine built in December 66, so I suspect the distributor I found in the trunk is original
Andrew Zizzo

This thread was discussed on 23/06/2004

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