Ignition Timing And Advance.

For correct ignition timing, each cylinder should receives a spark at the plug electrodes as the piston nears the top of its compression stroke (a few degrees before TDC). This is mad possible by driving the distributor shaft so that it turns are one-hall crankshaft speed. The distributor shaft may be turned by one-to-one gearing with the camshaft, which is already turning at one-half of the engine`s rotational speed. On some engines using a timing belt, the distributor is driven by the belt.

The distributor shaft gear is timed so that the spark is produced when the cylinder is ready to fire. The rotor will then point toward the cylinder is ready to fire. The cylinder`s cap plug terminal. A plug wire is attached to this terminal. The wires are attached to the cap staring at the number one cylinder and following the firing order in the direction of distributor shaft rotation. As the engine turns, the distributor shaft revolves. Each time the distributor shaft has turned enough to cause the rotor to point to a plug terminal, the ignition system produces a spark. This cycle is repeated over and over. The engine manufacturer specifies timing in regard to the number of degrees before top dead center (TDC) that the number one cylinder should fire. All other cylinders will fire at the same number of degrees before TDC. If the plug fires later than the specified setting, the timing is said to be retarded. If the plug fires earlier than specified, the timing is referred to as advanced.

Setting Basic Timing.

Most older engines, and many newer ones, have timing marks in the form of a line marked on the rim of the vibration damper. Some engines in front-wheel drive vehicles have timing marks on the flywheel. A pointer is attached to the timing cover. When the mark is exactly under the pointer, the engine is ready to fire number one cylinder. The spark will occur with the rotor pointing to the number one cap terminal. The timing is generally set by using a strobe tamp, which is a light that is operated by high voltage surges from the spark plug wire. The strobe lamp usually referred to simply as a timing light.

Typical ignition timing marks have degrees marked

before and after top dead center. This setup also

incorporates a magnetic timing probe receptacle.

To time the engine, the timing light pickup is clamped over the number one (or other cylinder as may be specified) plug wire. on most engines, special steps must be taken before setting the initial timing, such as disconnecting the vacuum line to the distributor, or grounding an electrical connector to the computer. On order engines with contact points, the point gap should be set before timing the engine. The engine is then started and operated at idle. Many modern vehicles do not have a provision for setting timing. Check the emissions sticker in the engine compartment before looking for the timing marks.

Using a strobe light to time the ignition. Every time the No.1 plug

fires, the strobe light will illuminate timing marks.

Using a Timing Light.

The timing light will illuminate the pointer over the vibration damper. The timing is checked by pointer the light at the timing marks. Every time the number one plug fires, the strobe lamp lights. Each time it fires with the damper in the same position in relation to the pointer, the damper timing mark looks through it were standing still. To adjust the timing, the distributor clamp is loosened and the distributor is turned by hand. As it is turned, the timing mark will move. When turned in the proper direction, the mark will line up with the pointer. When the two are aligned, the engine is properly timed and the distributor clamp can be tightened. Remember to reconnect any vacuum lines or electrical connectors as applicable.

Magnetic Timing Meter.

Many late model engines can be timed with a magnetic timing meter. This meter has a timing probe which is installed in a magnetic timing receptacle near the conventional timing marks. The timing meter also uses an inductive pickup which clamps over the number one spark plug. Once all connection are made, the engine is started, and timing can be read directly from the meter dial.

Timing Advance Mechanisms.

As engine speed increases, it is necessary to fire the mixture sooner. If this is not done, the piston would reach TDC and start down before the air-fuel mixture can be properly ignited. To properly fire the air-fuel mixture charge, a device is needed to advance the engine timing (firing more degrees before TDC) as the engine speed increases. It is also necessary to retard the timing to control exhaust emissions and prevent spark knock. When the engine is at idle, very little advance is necessary. At higher engine speeds, it is necessary to fire the mixture somewhat sooner. To see this concept, look at the next figure, the pressure of a burning fuel charge will end when the piston reaches 23 degree after TDC. Notice in figure A, that the combustion cycle must start at 18 degree before TDC in order to be complete by 23 degree after TDC. In figure B, engine speed has tripled. It is now necessary to ignite the charge at 40 degree before TDC in order to complete combustion by 23 degree after TDC. The three common methods of  advancing the ignition timing are centrifugal advance, vacuum advance, and electronic advance.

As engine increases, the spark must be timed sooner.

Notice in A only 41 (of crankshaft travel is required)

While in B, at 3,600 RPM. 63 degree is necessary.

Centrifugal Advance.

One method of advancing the timing is through the use of a centrifugal mechanism, which is assembled in the distributor shaft,. In effect, the distributor shaft is divided into upper and lower parts, with the upper part able to advance in relation to the lower part. When the distributor shaft turns, it turns the centrifugal unit which turns either the cam (contact point ignition) or the reluctor or shutter (electronic ignition). Centrifugal advance will advance engine timing in relation to engine speed.

Two different types of distributor centrifugal advanced mechanism.

When the engine is idling, spring pressure keeps the two weights drawn together and the shaft remains at the position for low speed timing. As the engine speeds up, the weights are drawn out by centrifugal force. As the weights move apart, they force the upper part of the shaft to move in an advance direction in relation to the lower part of the shaft. A the upper part of the shaft is advanced, it causes the triggering device to fire the coil sooner, causing the plugs to fire more degrees before TDC. The faster the engine turns, the farther apart the weights move, until they finally reach the limit of their travel.

As engine speed is decreased, the centrifugal pull on the weights is lessened and the springs pull the weights together, retarding the timing. By calculating the pull of the springs and the size of the weights, it is possible to properly advance the timing over a long RPM range. The next figure illustrates how the weights control advance by changing the weights and springs. This should be done very carefully to reduce the chance of engine damage.

Distributor centrifugal advance until in action. A--The engine is 

idling and the springs have drawn the weights in timing has no

advance. B--The engine is running at high speed. Centrifugal 

has drawn the weights outwards. As they pivot, the weight toe

ends force the cam plate to turn, advancing the timing.

Vacuum Advance.

It has been found that at the partial throttle open position, additional advance over and above that provided by a centrifugal mechanism is desirable. This is due to the fact that there is high vacuum in the intake manifold when the throttle valve is partially open. This high vacuum draws in less air and fuel. The smaller air-fuel mixture will be compressed less and will burn slower.

To maximize economy from this part of the fuel charge, it is necessary to advance the timing beyond that provided by centrifugal weights. This is provided by the vacuum advance mechanism, vacuum advance is used to advance the timing in relation to engine load. Any benefit from additional advance applies only to the partial open throttle position. During hard acceleration or wide open throttle operation, there is no manifold vacuum to operate the vacuum advance mechanism.

A vacuum advance chart. Notice the part throttle vacuum advance

is in additional to regular centrifugal advance.

Vacuum Advance Operation.

The electronic pickup or contact points are mounted on a movable plate. This plate can remove either on a center bushing, or on ball bearing on its outer edge. With either type of plate, an advance in timing may be obtained by turning the plate against distributor shaft rotation. The plate is rotated by means of a vacuum advance diaphragm. This is a stamped steel container with a neoprane-coated cloth diaphragm stretched across the center. One end is airtight and is connected to the carburetor, below or slightly above the closed position of the throttle valve. The other end is open to its center. The lever rod is connected to the movable plate.

Vacuum advance mechanism operation, A--The carburetor throttle

valve is in the part throttle position, drawing a heavy vacuum. With 

a vacuum on the left side, atmospheric pressure forces the diaphragm

to the left. The diaphragm link will pull the cam contact plate around 

and advance the timing. B--When the throttle is opened and vacuum

is lowered, the contact plate primary spring will pull the contact plate

back and retard the timing. The vacuum spring also controls the limits 

of advance.

When the throttle valve is partially open, as shown is figure A, there is high vacuum in the intake manifold. The vacuum pulls the diaphragm back toward the vacuum side. This in turn pulls the plate around and advances the timing. When the throttle is opened, the vacuum drops and the spring draws the diaphragm back toward the distributor. This rotates the movable plate in the retard direction, figure B. When the engine is idling, the throttle valve closes below the vacuum advance opening. This removes vacuum pull and the spark will be retarded for idling. The vacuum advance mechanism is constantly moving as vacuum varies with th movement of the throttle valve.

The vacuum is adjustable on some vehicles. If the vacuum advance is not operating, it can affect engine performance and fuel mileage. On some older vehicles, the vacuum may be ported, or not activated until the throttle is partially opened. 

This distributor vacuum advance until utilizes engine 

vacuum to move the distributor contact plate.

Computer-Controlled Ignition

The triggering device for electronic ignition may be inside the distributor, or it may be a camshaft or crankshaft position sensor. Since the entire spark plug production process is performed electronically, it follows that the spark timing can also be modified electronically. While many older electronic ignition use a centrifugal and vacuum advance, most modern systems use the on-board engine control computer to create electronic advance.

The latest on-board computers monitor all engine and external variables, such as engine RPM and temperature, pressure, airflow rate, air temperature, throttle opening, exhaust gas oxygen, transmission gear, vehicle speed, system voltage, and whether the engine is knocking. The computer advances or retards the timing to exactly match the needs of the engine and vehicle. There are no vacuum or centrifugal advance mechanisms on computerized ignition systems. On some systems, the computer contains the ignition module, and controls the coil directly. On other systems the ignition control module is separated, and interacts with the engine control computer. In either case, the amount of advance is set by the computer, and cannot be adjusted.