Wayne Cook
December 15, 2009

By using either mechanical centrifugal advance or a vacuum-actuated mechanism, we can change the engine timing adjustment and advance the timing as the engine runs to suit the requirements according to rpm or load. The reason for the variation in timing requirements is because although the engine runs at different speeds and under different loads, the air/fuel mixture burns at only one speed. The spark merely ignites the mixture and it will only burn at that one rate regardless of how fast the engine is turning. Thus if the piston is going to return sooner, as happens at higher rpm, the air/fuel mixture must be fired off sooner in order to allow optimum combustion time for the rapidly returning piston. Hence, as engine speed increases, the engine timing must automatically advance to occur sooner in the cycle. In other words, the faster the engine is spinning, the earlier we have to fire the plug to produce optimum power. Once computer controls were able to directly control the engine's timing, vacuum and centrifugal advance mechanisms were no longer necessary and were eliminated.

If you'd like to enjoy the advantages of an oversized distributor cap but you don't want to replace the whole distributor you don't have to. You can have the larger diameter using production Ford equipment pirated from later-model Mustangs and other Fords of the late '70s.

Ignition Upgrades:Capacitive Discharge
A stock ignition system uses an inductive discharge. In this arrangement the coil must store and then step up the voltage between each firing. At normal rpm ranges this system works fine. However, as engine rpms climb the process begins to happen too quickly for the coil to have time to store up a complete charge between each event. The result is that the firing occurs faster than the coil's ability to store up a complete charge, meaning that the spark is fired at less than peak efficiency. The use of an MSD capacitive discharge ignition box, such as the 6AL, will eliminate this problem. The 6AL unit also incorporates a built-in rev limiter that can be set to the desired rpm with a simple adjustment.

Distributor Gear Gotchas!
One thing to be aware of when changing distributors in your engine is the material that the distributor drive gear is made out of. It must be made of a material compatible with your camshaft. If your car has the original flat tappet cam, be sure that your new replacement distributor is also equipped with an iron drive gear. If your car has a more modern roller cam, then the camshaft will be made of steel. In this case the new distributor must have a steel or bronze distributor drive gear. Any mismatch between the material the cam is made from and the distributor drive gear will result in premature wear for both gears. Besides destruction of these gears, metal shavings will end up in the engine oil and permanent damage to the engine could result.

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