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Ignition Advance

* Initial ~ Mechanical ~ Vacuum *

Theory of Operation

There are two things an engine must have: spark and fuel.
The proper amount of fuel and air, compressed and ignited at the ideal moment, produces power. This Nostic, will deal with ignition, the spark that ignites the air/fuel mixture. Ignition timing is typically split into three types.

The first is initial timing�that is, the amount of spark lead that�s set using a timing light (aimed at the harmonic balancer), with the engine at idle.

The second type is mechanical advance, which is determined by the distributor�s mechanical advance system.

The final, and third, type is vacuum advance, which is determined by the vacuum canister attached to the distributor. The three types of ignition timing combine to produce the total amount of ignition timing in the engine at any given load or rpm.

The mechanical distributor is still the most popular form of ignition, it' s important to understand how these events interact. Initial advance is set by moving the distributor in the engine. Once initial timing is set, it�s complemented by the mechanical and vacuum-advance curves.

Mechanical advance adds more timing to the engine and is based strictly on rpm. The mechanical advance weights and springs inside the distributor combine to produce a curve that begins with initial timing (with the engine at idle) and increases to a certain rpm but generally peaks below 3,000 rpm. This is often referred to as the mechanical ignition curve. Vacuum advance also adds timing to the engine, but it�s based on engine load and is controlled by intake-manifold vacuum. When the throttle is partially open (highway cruising), manifold vacuum is high. This vacuum signal is routed to the distributor and �pulls� on the vacuum advance canister/diaphragm to create more ignition timing. As the throttle opening increases, load increases and manifold vacuum decreases. At wide-open throttle (WOT), manifold vacuum is at or near zero, and the vacuum advance produces no additional ignition timing.

With the throttle valve part way opened, there is high vacuum in the intake manifold. This high vacuum draws in a lesser amount of fuel mix. The smaller amount will be compressed less and will burn slower. To get the max performance and economy from this reduced fuel/air mixture, it is necessary to advance the timing beyond what is provided by the centrifugal/mechanical weights. Vacuum advance will add a total advance of 30 degrees....centrifugal total: approx. 15 degrees.

Under hood temps deteriorates the vacuum diaphragm. When it ruptures, it is no longer operational. The only advance timing providied is by the initial & centrifugal advance.

Trouble Shooting: connect a piece of vacuum hose { 1/4 in x 12in. } to the port on the vacuum advance canister....place hose into mouth....start sucking, if you can continue 'sucking', the diaphragm is ruptured. Replace vacuum advance. Inquiries: BobRobert@MotorNostics 129.1

BobRobert @ MotorNostics Ignition Advance/Theory {Click '97 HOME link}