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BASICS OF PETROL ENGINE TIMING
In order for an automobile engine to function, three things are necessary: air,
fuel and a spark, to ignite the air/fuel mixture and create an explosion. The
precise instant of that explosion must be such that the maximum power is
delivered to the engine piston.
This is
”
Timing
”.
Each engine manufacturer
determines the exact timing necessary for various engines so that optimum
power is obtained from the fuel used. Due to normal engine and ignition
system wear, the accuracy of the timing can be lost, reducing both power
and fuel mileage. With the timing light, the car owner can reset the timing to
new car standards and regain lost power and fuel mileage.
Timing is given in degrees Before Top Dead Center (BTDC) or After Top Dead
Center (ATDC) in the manufacturer's specifications.
In order to completely burn the air/fuel mixture in the engine cylinders, most
timing is such that the spark occurs at a point several degrees before top
dead center (for example, 4° BTDC) to assure that full power of the explosion is
obtained. See Figure 1on page 4.
Two additional terms the engine manufacturers use when describing timing
are “Advanced” and ”Retarded”. As shown in Figure 1, when the timing is
advanced the spark will occur before the piston reaches the top of the
engine cylinder (BTDC). On some late model cars equipped with various
emission control devices, the timing is retarded so that the spark occurs after
the piston has started to move down in the cylinder (ATDC). Engine timing is
changed by adjustment of the ignition distributor.
In order to allow setting and adjustment of the engine timing, special “timing
marks” are provided on each engine during assembly. In most cases, these
marks appear on the engine vibration damper or fan pulley at the lower front
of the engine. See Figure 1 on page 4. On some early engines, this mark was
shown at the rear of the engine on the flywheel.
WHEN TO CHECK TIMING
The instant of spark plug firing is determined by the opening of the contact
breaker points and will change any time the points gap or dwell angle is
changed. In addition, normal wear on the contact breaker point contact
block will change the dwell and affect the timing. While cars equipped with
modern breakerless electronic ignition systems will not normally change
timing, since there are no contact breaker points, the timing light can still be
used to note changes in timing caused by defects in the ignition system as
well as for re-setting timing when components are changed.
