Welcome to our resource for MG Car Information.
MG MGA - Competition distributor
|I thought that all MGA's had the same distributor but I understand there's something called a 'competition' distibutor for the MGA. Does this mean a different vacuum advance unit or more? Can anyone explain please.|
|J H Cole|
|Competition distributor has a fixed base plate and does not have any vacuum advance|
|Yup. Vacuum advance is for quick throttle response at low engine speed and for good fuel economy at moderate steady cruising speed (partial throttle). Race cars are usually either flat out or off throttle, so neither of those conditions apply. Many racers will disconnect the vacuum unit or pin the vacuum advance plate so it doesn't move.|
When you don't need vacuum advance it is advantageous for the distributor to be simpler and more robust, leading to the "competition distributor" with no vacuum advance function. I suppose the competition distributor was a special competitions part, never installed on a production car except on special order. It does not appear in the Service Parts List.
My Mallory Dual Points distributor has no vacuum unit. I like its robust construction and long life (now over 220,00 miles), but in the long run I would prefer vacuum advance for street use.
|Vacuum advance makes a big difference in the character and tractability of the engine. My vac unit did not work, and when I put one in from British Vacuum Unit, I was amazed at the difference.|
If you use your car on the street, I would leave it as is and ensure your vacuum unit is working correctly (if it's 50+ years old, it is not...the rubber hardens up).
You can have your stock distributor rebuilt by Advanced Distributors (or another vendor) and it will be right as rain.
I love my "pointless" pertronix ignition, but that is really your personal preference.
JIM in NH
|Jim, are you saying that if you fit a Pertronix ignition |
(or any other electronic ignition?)you forsake the benifits of the vacuum advance for - say the efficacy of the electronic timing system.
|J H Cole|
|The competition distributor for the MGB is the same as the stock one ofr the 1275 Cooper S engine, and it will give decent street performance.|
I see no reason to use one on a street MGA though.
The 1 2 3 product is excellent, however.
|JH. No, Jim is not saying:" that if you fit a Pertronix ignition|
(or any other electronic ignition?)you forsake the benifits of the vacuum advance for - say the efficacy of the electronic timing system."
Any of the points replacement systems only replace the points. They have no effect on the vacuum advance system, nor do they have any effect on the mechanical advance system. If either/both of the systems were not functioning properly before the installation of the points replacement system, they will not function after the installation of the points replacement system.
Points/points replacement systems function as an on-off switch for the coil. They allow current to flow through the primary windings of the coil, thus charging it so that when the current flow is broken, a high voltage surge is induced in the secondary windings. On-off--on--off. Nothing more.
Mechanical advance moves the points cam, in relation to the points plate, causing the points to open earlier in the crankshaft's rotary movement and causing the spark plug to fire earlier as the engine revolutions increase. This allows a slightly more efficient fuel/air mixture burn cycle. It functions independently of the points but the two systems interact to give you "ignition timing" with a "mechanical advance curve".
The third distributor subsystem is the vacuum advance with is a rubber diaphragm with a wire running from the rear side to the points plate. Depending on the amount of vacuum applied to a working vacuum advance system, the points plate will be rotated in relation to the points cam, advancing the spark at higher vacuum levels and retarding the spark (but only back to some basic setting) at lower vacuum levels.
Vacuum moves the points plate in relation to the points cam. Mechanical moves the points cam in relation to the points plate. The two combine to give you a constantly variable ignition timing designed to provide best power and fuel economy.
Thus, the basis for any distributor system is that it needs to be in good working order, properly lubricated (annually for the original dizzies) and properly maintained. Straight mechanical advance is best used where the primary objective is maximum power--all the time or most of the time. A combination of mechanical and vacuum advance makes an engine that is operated at a series of different speeds (e.g. driving on the street) more responsive and provides better fuel efficiency (e.g. "miles per gallon" is one measure of this efficiency). BUT, the basis of any ignition system is good quality parts, in good condition, and well maintained.
|Thanks Les and others for the explanations. I've obviously laboured under a misapprehension about the electronic systems - I had thought that they could be tailored to give a particular advance curve to suit an engine because they seem to be sold that way. Reading the comments makes the argument to keep traditional vacuum advance system persuasive and difficult to see why one would change to electronic for normal street use.|
|J H Cole|
|The benefit of electronic points is that they are a set once and forget them system, there's nothing to wear out. they either work or they don't.|
If you want to convert to a fixed base plate, all you have to do is to undo the two small screws holding the plate to the cast body, remove the plate, do a quick weld to hold the sliding plate to the mounting plate, and reassemble.
|The 123 Distributor does have a selection of advance curves that you can select to suit your needs, there is also a programmable version so that you can create your advance curve.|
Expensive but a wonderful bit of kit, worth 1/2 a gear in my opinion.
This thread was discussed between 28/01/2013 and 31/01/2013
MG MGA index
This thread is from the archive. The Live MG MGA BBS is active now.