Washington, D.C. Centre
the Spark


by Jim Lunson


In viewing several recent car shows, I noticed a wide variety of arrangements used to regulate the ignition timing on our MGs. Some are exactly stock, some are rather neat upgrades, and some appear to be really hurting power and efficiency. This variation lead me to a discussion on ignition timing and the options available.

Ignition timing is the instant in the cycle of a running engine when the spark plug fires. For complicated reasons, this timing of the spark plug firing must vary in conjunction with the speed of the engine (RPMs) and load imposed as it propels the car. Otherwise the engine would run very poorly and without nearly its potential power. In cars with conventional set-ups (non-computer controlled) such as our MGs, this ignition timing is varied by small rotations of the distributor body. This rotation changes the instant of firing in its internal spin in relation to the position of the pistons, valves and the rest of the engine function and provides the timing variation needed to make an engine run properly.

There are two main methods for creating this distributor rotation, vacuum and mechanical. Vacuum timing control operates off the vacuum created in the intake manifold as the engine runs. It works through a round diaphragm disc connected to the manifold by a long tube. When the manifold vacuum increases, this pulls on a diaphragm inside the disc which moves in or out, turning the distributor slightly as it moves. Mechanical timing control works off of a set of small weights and springs inside the distributor that spin with the distributor shafts rotation, and due to centrifugal force, move outward as the spin increases, turning the distributor body slightly as it moves. Each system has advantages and disadvantages. Most cars should have both systems for proper operation at all speeds and conditions found on the road.

Vacuum vs. Mechanical Control - Each system as its advantages and disadvantages. Vacuum advance is a proactive system while mechanical advance is more reactive. Vacuum advance dies out at higher speeds. Here's how they work. Vacuum (negative pressure) is greatest inside the intake manifold when the engine is at idle (such as at a stop sign). The pistons are sucking in the air/fuel mixture as they go up and down in the cylinder, yet the throttle valves on the carburetors are almost closed so very little gas gets to the engine. These valves seal off the ability to intake air to offset the withdrawal caused by the pistons; hence the negative pressure. This pressure is transmitted by the hose noted above to the distributor where it pulls a disc its farthest, increasing the timing. As the engine accelerates from an idle, the vacuum decreases as the throttle valves open (more air/fuel mixture can enter the manifold). As the engine speed increases further, the suction continues to drop (the throttle valves are open pretty wide at this point), yet there the timing stays advanced because, as the RPMs increase, the mechanical advance takes over, maintaining the advance and then increasing it further as RPMs increase further. This is why most cars utilize both systems to provide the advance. Vacuum advance gets going and then mechanical advance works after the engine attains the higher RPMs moving the weights and springs. This means that vacuum advance gives the engine additional pep when first accelerating from idle, while mechanical advance give a steadier and more accurate timing advance at higher sustained speeds.

Some system of carburetion and distributors do not use vacuum advance at all. These manifold and carburetors often have no port to connect the hose required to run to the distributor. And certain distributors do not have the round diaphragm system to make it rotate. This is often true with Weber carburetor set-ups, especially side draft types, as well as with certain distributors. This elimination of the vacuum advance is generally best used in racing operations only. Remember, mechanical timing advance is reactive, only increasing the timing after the engine RPMs have increased enough for the weights and springs to take action. Only after more RPMs are added, more timing advance follows. This is perfect for racing type arrangements that do not need great acceleration from idle and the primary function is to strictly maintain higher speed for long durations. One way to tell if a carburetor or distributor is designed for racing only is that they will advertise that they have only mechanical advance systems and no provision for vacuum advance. Think about what application you need.

Distributor Advance Curves - Another factor in the ignition timing set-up is the called the distributor advance curve. This refers to the mechanical advance system inside the distributor and concerns with how far the timing is advanced mechanically in relation to the RPMs of the engine. This is controlled by the size of the weights and springs responsible for the internal rotation. Heavier weights and/or weaker springs will move the advance forward more at less RPMs, affecting the performance of the engine. It is important to match the advance to the engine application. Some distributors come with several sets of weights and springs so they can be adjusted to change performance depending on usage of the car. This advance curve also ties to the camshaft installed in the engine, as the shape of the cam lobes affects how fast and how far the valves open. All these factors affect the performance of the engine, depending on what use it is going to get (normal street driving, interstate cruising, or race applications. Most distributors originally sold for our MGs come with a set advance curve. Therefore, it is important to watch which distributor is installed in which engine, as the curve interacts with the vacuum advance, the carburetor, and the cam shaft, all affecting performance. It all ties together.

During the original design and production, the MG factory spent a lot of effort analyzing the interrelationships between vacuum and mechanical advance, and the curves required, and the intended application. They ended up basically designing the best combination they could at the time for the car they produced. We have MGs produced over a period of almost 40 years, a period when huge technological advances were made in engine design. With these changes constantly occurring, plus all the modifications available today due to further advances in technology, we now have a lot of options to improve performance of our cars. We need to be very careful, however, on the combination of carburetor, distributor, cam shaft as they relate to the timing advance mechanisms installed in our cars. The systems have to work together as one engine. So take a minute to look at what you have under your hood, study all the equipment in the engine and see if what you are using is the best.

I'll go into this subject further next month with some more details on ignition timing. This will include how timing is set, what settings to use, effects of modern ethanol in gas, ported vacuum vs. manifold vacuum, and some of the pitfalls to avoid.


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