Ignition timing refers to the precise time spark occurs. Ignition is specified by referring to the position of the #1 piston relation to crankshaft rotation. Ignition timing reference timing marks can be located on engine parts and on a pulley or flywheel to indicate the position of the #1 piston. Vehicle manufacturers specify initial or basic ignition timing.

When the marks are aligned at TDC, or 0, the piston in cylinder #1 is at TDC of its compression stroke. Additional numbers on a scale indicate the number of degrees of crankshaft rotation before TDC (BTDC) or after TDC (ATDC). In a majority of engines, the initial timing is specified at a point between TDC and 20 degrees BTDC. A few manufacturers specified initial timing from 1 to 5 degrees ATDC for vehicles built during the 1970s.

If optimum engine performance is to be maintained, the ignition timing of the engine must change as the operating conditions of the engine change. Ignition systems allow for these necessary changes in many ways; these are covered in greater detail later in this chapter. All the different operating conditions affect the speed of the engine and the load on the engine. All ignition timing changes are made in response to these primary factors.

Engine RPM

At higher rpms, the crankshaft turns through more degrees in a given period of time. If combustion is to be completed by 10 degrees ATDC, ignition timing must occur sooner or be advanced.

However, air/fuel mixture turbulence increases with rpm. This causes the mixture inside the cylinder to turn faster. Increased turbulence requires that ignition must occur slightly later or slightly retarded.

These two factors must be balanced for best engine performance. Therefore, while the ignition timing must be advanced as engine speed increases, the amount of advance must be decreased some to compensate for the increased turbulence.

Engine Load

The load on a engine is related to the work it must do. Driving up hills or pulling extra weight increases engine load. Under load, the pistons move slower and the engine runs less efficiently. A good indication of engine load is the amount of vacuum formed during the intake stroke.

Under light loads and with the throttle plate partially opened, a high vacuum exists in the intake manifold. The amount of air/fuel mixture drawn into the manifold and cylinders is small. On compression, this thin mixture produces less combustion pressure and combustion time is slow. To complete combustion by 10 degrees ATDC, ignition timing must be advanced.

Under heavy loads, when the throttle is opened fully, a larger mass of air/fuel mixture can be drawn in, and the vacuum in the manifold is low. High combustion pressure and rapid burning results. In such a case, the ignition timing must be retarded to prevent complete burning from occurring before 10 degrees ATDC.

Firing Order

Up to this point, the primary focus of discussion has been ignition timing as it relates to any one cylinder. However, the function of the ignition system extends beyond timing the arrival of a spark to a single cylinder. It must perform this task for each cylinder of the engine in a specific sequence.

Each cylinder of an engine must produce power once in every 720 degrees of crankshaft rotation. Each cylinder must have a power stroke at its own appropriate time during the rotation. To make this possible, the pistons and rods are arranged in a precise fashion. This is called the engine¡¯s firing order. The firing order is arranged to reduce rocking and imbalance problems. Because the potential for this rocking is determined by the design and construction of the engine, the firing order caries from engine to engine. Vehicle manufacturers simplify cylinder identification by numbering each cylinder. Regardless of the particular firing order used, the number 1 cylinder always starts the firing order, with the rest of the cylinders following in a fixed sequence.

The ignition system must be able to monitor the rotation of the crankshaft and the relative position of each piston to determine which piston is on its compression stroke. It must also be able to deliver a high-voltage surge to each cylinder at the proper time during its compression stroke. How the ignition system does these things depends on the design of the system.