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MG MGB Technical - DISTRIBUTOR

I HAVE A 77 MGB WITH ORIGINAL 45DE EID DISTRIBUTOR. SHOULD I REPLACE IT WITH MALLORY DUAL POINT DIST. OR UNILITE ELECTRONIC DIST.? ANY PROS OR CON'S ?
MANNY

The Mallory is a great unit, but there may have been some rumbles about the quality of the points.

Others will know more about this.
Jack

Manny-
You may wish to develop a customized spark advance curve to meet their individual needs. If you seriously want to leave this option open, a distributor that has an adjustable advance curve is desirable, such as the one made by Mallory. It is available in both vacuum advance and centrifugal advance versions (Victoria British Part#'s 17-501 and 17-500, respectively). Victoria British has a website at http://www.victoriabritish.com/ . In both versions, the centrifugal advance mechanism is adjustable from 16 to 28 degrees by means of a simple Allen wrench, the vacuum advance curve of the vacuum advance version of the distributor is adjustable by using a 3/32" Allen wrench and inserting it into the hose connection nipple and altering the tension value on the diaphragm. An advance curve kit consisting of both an assortment of centrifugal advance weight springs and the Allen wrench is readily available (Moss Motors Part # 143-236). It also has the advantage of having a dual point spark triggering system. In this type of system both sets of points are joined by a wire so that when the first set of points open, nothing happens until the second set of points open. The second set of points open just as the first set are closing. This quick closing of the circuit (approximately 5 degrees) gives the coil a maximum amount of dwell time (72 degrees) to charge, thus increasing the voltage of any given coil. This makes the system highly appropriate for engines equipped with a camshaft designed for high engine speed applications. However, other than the ability to have the spark advance curve custom tailored to work with almost any camshaft, there is no practical advantage to the increased coil charging time of the Mallory distributor when used on a four cylinder engine. Converting the distributor to electromagnetic or electro-optical triggering will entail no tangible sacrifice. For the MGB with a special camshaft, however, a custom spark curve can help exploit that last bit of potential power as well as deliver better response to changes of the throttle while avoiding the dangers of preignition. A six cylinder engine fires 50% more often and an eight cylinder engine fires twice as often. In such engines equipped with radical camshafts, the increased coil charge time can become critical at high engine speeds. Both versions are also available as Unilite distributors with solid state triggering (Victoria British Part #'s 17-503 and 17-502, respectively). Moss Motors has a website at http://www.mossmotors.com/ .
However, you don't necessarily have to go such a big bucks route, and you don't have to place your trust in the appropriateness of Aldon's spark advance curve, either. If your Lucas 45D distributor is in good shape and you don't mind doing it the labor-intensive way, Cambridge Motorsport in the UK offers a package of five advance springs that will enable you to tailor the rates of your spark advance curve. Unless you've got an assortment of spare advance plates to work with, however, your total advance will remain the same. Don't bother trying to refine the spark advance curve by experimenting with different bobweights as they are all the same weight. Only after the centrifugal advance curve is successfully established can different vacuum advance capsules be tried.
In developing a custom spark curve, the object is to achieve peak combustion pressure. Although engine speed can vary, the fuel/air mixture combusts at a fixed rate. Therefore, the fuel/air mixture has to be ignited progressively earlier as engine speed increases. However, if ignition occurs prematurely the pressure wave inside the combustion chamber will reach the piston crown while the thrust axis of the connecting rod is aligned with throw of the crankshaft, overcoming the pressure of the oil in the bearing and thus causing engine knock and resulting in damaged bearings, journals, and even a collapsed or broken piston crown. However, should ignition occur later than the optimum moment, the pressure wave will reach the piston crown too late for maximum power to be achieved. As a reasonable starting point, the static setting should be 14 degrees BTDC and the maximum mechanical advance setting should be 20 degrees BTDC for a total of 34 degrees of advance. If a very hot camshaft is used, more advance may be necessary to obtain the best idle. With these initial settings in place as a starting point, you should be able to develop the optimum ignition advance curve for your engine. Be sure to use no more advance than is necessary to obtain optimum power or you?ll risk burning the valves.
Steve S.

Manny. If you truly have an operating Opus unit (Lucas 45DE4) you should sell it to a car show type. I have not seen an operational one in many years. The one on my 79 (purchased new) went out in the first six months of ownership and was replaced by the dealer with the 45DM4 unit which worked well for the next five years. What I have seen, commonly, is the body of the 45DE4 with switched out internals. My daughter's 77 had a Lumenition system installed into the 45DE4 when we purchased the car. Hence, you might want to examine your current distributor and see exactly what you have inside it. That would be your basis for any changes.

As Steve notes, setting up the adjustable type distributors is a fairly complex job, requiring either a great deal of road testing or access to a chassis dynamometer (rolling road) if it is to be done properly. This is worth the trouble if you have significantly modifed your engine, but might not be if you are still running a relatively stock engine. Peter Burgess does not mention any need for such a special distributor in his book on power tuning the MGB engine. You might simply consider having John Twist rebuild your current distributor back to its original specifications. Another alternative is the "Euro Spec" distributor. The ones I have used have been the UK 45D4 (points type) distributors used on the UK cars from about 74 onwards. This distributor will also take a little testing to determine the best timing as it was designed for use with the high compression engines and the US engines of that era, unless they have been rebuilt, are all low compression engines. The LC engines used slightly more ignition advance than the HC engines, so a distributor designed for the HC engines may have to be advance slightly more, initially, when used with the LC engines. Les
Les Bengtson

LES,
I PICKED UP THIS CAR ABOUT 1 YEAR AGO. HAS ONLY 18,000 MILES. CAR HAS SAT FOR A LONG TIME. ALL ORIGINAL INCLUDING 8-TRACK. I THINK DIST. IS GIVING PROBLEMS, I WAS THINKING ABOUT CHANGING.
MANNY

Manny. If you really have an Opus system installed, changing would be a good idea. I am currently running the "Euro Spec" distributors in both my 79 and my daughter's 77 with no problems. Beware of the "rebuilt" distributors available from Auto Zone. They are inexpensive, but seem to be put together from whatever parts seem to be in good condition and on hand. The one I picked up to use in my 68 had 16 degrees too much mechanical advance. Fortunately, I examined the thing before installing it. Les
Les Bengtson

LES,
SHOULD I REPLACE W/ POINT DIST. OR ELECTRONIC DIST.?
MANNY

Manny. That is entirely up to you. You can install and aftermarket electronic trigger into a points type distributor if you wish. However, if an electronic distributor breaks, you cannot install a set of points to get you home. If you want my complete thoughts on the subject, go to my website, www.custompistols.com/ then click on the MG section, followed by the articles section. There are some articles which may help you to make your decision. Les
Les Bengtson

A dual-point is a waste of time, the low-revving engine of the MGB just doesn't need it, after all a single point is good enough for the V8. Any replacement distributor is going to be a poke in the dark advance curve wise, but a replacement that allows you to fiddle with the curves may be a better bet than something else. So-called 'Eurospec' distributors are marketed in the USA, but most of these are for the high-compression European engines and not the low-compression American which had significantly different curves originally. Whatever distributor you fit can be made to work - after a fashion - but unless you spend hours on a rolling road tuning the curve your actual will likely only touch the ideal (for your engine, they are all different) in one place. You can find that point simply by running with the greatest advance that does not induce any pinking under any combination of load, throttle opening or engine revs for your grade of fuel. It takes long enough on a rolling road to tune a curve to an engine's requirements with a non-vacuum distributor, with vacuum it will take much longer. And if you use a non-vacuum distributor you will suffer from reduced cruising mileage.

If you really do have an Opus unit then congratulations, it must be one of the very few left working. In your case I think I would investigate replacing the points plate and upper shaft with parts from any other 45D4 and using conventional points and condenser (or electronic points like pertronix if you really must) to retain your existing centrifugal and vacuum advance curves.
Paul Hunt

This thread was discussed on 02/02/2004

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