2

INSTALLATION INSTRUCTIONS

CHOOSING AN ADVANCE CURVE

The function of the advance curve is to match the ignition timing to the burning rate of the fuel and

speed (rpm) of the engine. Any factor that changes the burning rate of the fuel or the engine speed

can cause a need for an ignition timing change. Figure 1 shows some of the factors that will affect

engine timing.

FACTOR

Advance Timing

Retard Timing

For

For

Cylinder Pressure

Low

High

Vacuum

High

Low

Energy of Ignition

Low

High

Fuel Octane

High

Low

Mixture (Air/Fuel)

Rich

Lean

Temperature

Cool

Hot

Combustion Chamber Shape

Open

Compact

Spark Plug Location

Offset

Center

Combustion Turbulence

Low

High

Load

Light

Heavy

Figure 1 Ignition Timing Factors.

As you can see from the chart, most factors will change throughout the range of the engine operation.

The timing mechanism of the distributor must make timing changes based on these factors.

Example: An engine has 11:1 compression with a high energy ignition. With the specifications given,

you will have to retard the timing for the high compression and high energy ignition. By comparing

the engine’s specifications against the chart, a usable timing guideline can be found. Engines with

a combination of items from both columns will require a timing that is set in the mid range.

Obviously a full technical explanation of correct ignition timing would be very complicated. The best

way to arrive at a suitable ignition curve for your engine is to use the Ignition Timing Factors Chart

as a guide and compare it to the Advance Graphs in Figure 4 until a suitable curve is found. When

selecting your advance curve, use detonation (engine ping) as an indicator of too much advance,

and a decrease in power as an indicator of too little advance.

TIPS ON SELECTING AN ADVANCE CURVE

• Useasmuchinitialadvanceaspossiblewithoutencounteringexcessivestarterload.

• Startthecentrifugaladvancejustabovetheidlerpm.

• The starting point of the centrifugal advance curve is controlled by the installed length and

tension of the spring.

• Howquicklythecentrifugaladvance(slope)comesiniscontrolledbythespringstiffness.The

stiffer the spring, the slower the advance curve.

• The amount of advance is controlled by the advance bushing.The bigger the bushing, the

smaller the amount of advance.

CENTRIFUGAL ADVANCE CURVE

SELECTING THE ADVANCE SPRINGS

The rate, or how quick the advance comes in is determined by the type of springs which are installed

on the distributor. The MSD distributors are equipped with two Heavy Silver springs installed. These

will give you the slowest advance curve possible (Figure 2). The parts kit contains two additional sets

of springs which can be used to match the advance curve to your particular application. Refer to the

Spring Combination Chart (Figure 3) for combinations that can be achieved.