Distributor Timing Essentials
by Don Caithness
photos by Don Caithness
Distributor timing is one area that scares many people but what if you could gain an extra 10-30 horsepower by understanding and applying a few of the simple principles? Distributor timing is comprised of 3 key factors that all need to work in unison for maximum performance and mileage: initial timing, mechanical advance and vacuum advance. So let’s begin by covering some of the basics first.
The first factor is initial timing. The reason that timing is normally advanced before Top Dead Center (TDC) and is required to change with load and RPM is fairly simple. The piston is traveling towards TDC and at the same time, the explosion of the fuel mixture (Combustion) takes a period of time to occur. If you had the spark occur at TDC, then the force of the explosion would occur too late after the piston had reached TDC and was on the down stroke resulting in less power. So you advance the spark a few degrees, which give the explosion time to develop as the piston is still on its way up, with the full force of the explosion occurring just as the piston has passed TDC.
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| 1-GM HEI distributors have been around since the mid 70’s and have proven to be very reliable. | 2-Before you can remove the distributor, unplug the 3 connectors from the cap. The yellow wire is the tachometer lead and the pinkish wire is the 12-volt power lead. |
The next factor is called mechanical or centrifugal advance. As the engine RPM increases you are also changing the speed that the piston is moving at, but the time for combustion remains the same. So you need to advance the spark even further if you still want the explosion to happen just past TDC. This is referred to as the rate of advance.
The last factor is the vacuum advance. Vacuum advance plays no role under full throttle operation since manifold vacuum drops to zero. However, vacuum advance will greatly increase fuel economy under steady load conditions, which is its primary purpose. In addition vacuum advance can have a positive affect on idle quality and engine temperature.
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| 3- Place a chalk mark to reference where the rotor tip is pointing before you pull out the distributor. | 4-After 25 years of service, the internal parts are rusted and in our case the mechanical advance was seized. |
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