MSD 84891 Installation

MSD Pro-Billet Flat-Top

Chevrolet V8 Distributor

PN 84891

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Important: Read these instructions before attempting the installation.

Parts Included:

1 - Pro-Billet Distributor 1 - Rotor, PN 84673 1 - Distributor Cap, PN 8531

WARNING: Before installing the MSD Distributor, disconnect the battery cables. When discon-

necting the battery cables, always remove the Negative (-) cable ﬁrst and install it last.

Note: The terminals of this Distributor require spark plug style terminals. You may need to change the

terminals and boots of your wires. MSD offers two kits, PN 8849 or PN 8848 that are supplied with nine boots and terminals.

Note: If the gear is ever replaced, MSD Gear (PN 8531) is required for replacement due to the .500"

diameter shaft.

Note: For a replacement rotor, contact MSD Customer Support at 915-855-7123.

1 - Advance Kit 1 - Gasket 1 - Tube of Gear Lubricant 2 - O-Rings

TIMING FUNCTIONS

Before continuing with the installation, here are a few deﬁnitions you should be aware of:

Initial Timing: This is the base timing (also referred to as idle timing) of the engine before the centrifugal advance begins.

Centrifugal Advance: The centrifugal (or mechanical) advance mechanism is made up of weights, springs, advance cams, and an advance stop bushing. The amount and rate of advance that your distributor is capable of is determined by the centrifugal timing. If you ever wish to lock out the centrifugal advance, refer to the centrifugal advance section.

Total Timing: This is the total of the initial timing plus the centrifugal advance added together. Example: 10° Initial + 25° centrifugal = 35° Total Timing. (When checking Total timing, disconnect the vacuum canister and plug the vacuum source.)

Note: MSD Distributors are supplied with the heavy (slow) advance springs installed. This is to prevent

detonation in certain applications. Review the information on pages 2-4 to determine the best advance curve for your application.

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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

• Use as much initial advance as possible without encountering excessive starter load.

• Start the centrifugal advance just above the idle rpm.

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

• How quickly the centrifugal advance (slope) comes in is controlled by the spring stiffness. 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.

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INSTALLATION INSTRUCTIONS 3

CENTRIFUGAL ADVANCE CURVE

SELECTING THE ADVANCE SPRINGS

The rate, or how quick the advance comes in is determined by the type of sp rings which are installed on the distributor. The MSD distributors are equipped with two Heavy Silver springs installed. These will give you the s lowest 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 particula r ap plication. Refer to the Spring Combination Chart (Figure 3) for combinations that can be achieved.

To change the springs, remove the cap and rotor and use needlenose pliers to remove the springs. Be sure the new springs seat

in the groove on the pin.

Timing Curve From Factory

Figure 2 The Factory Equipped Curve.

SPRING COMBINATION RATE OF ADVANCE FIGURE 4

2- Heavy Silver SLOWEST A 1- Heavy Silver B 1- Light Blue 1-Heavy Silver C 1-Light Silver 2- Light Blue D 1- Light Silver E 1- Light Blue 2- Light Silver FASTEST F

Figure 3 Spring Combination Chart.

Figure 4 Advance Curves.

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