Technical information

AUTOTRONIC CONTROLS CORPORATION • 1490 HENRY BRENNAN, EL PASO, TEXAS 79936 • [phone] • FAX [phone]
www.msdignition.com email: [e-mail]
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Most MSD Distributors with a magnetic pickup fea-
tures an adjustable mechanical advance assembly. The
distributors with this feature are supplied with three
sets of advance springs and four stop bushings.
The advance assembly is made up of weights, springs,
an advance cam and an advance stop bushing. The
distributor can be used in a wide selection of applica-
tions by changing the springs and stop bushing only.
There is no need to change the weights or advance
cam.
Timing Functions
There are several different timing specifications that
are used when referring to the ignition timing.
Initial Timing: This is the base or idle timing. It is
the amount of timing set in the engine before
any advance begins.
Mechanical Advance (Centrifugal): This is a mechanical
feature on most distributors that advances the timing
as engine rpm increases. The amount of advance is
determined by the stop bushing and the rate of
advance is determined by the tension of the springs.
Vacuum Advance: Some of the MSD Distributors are
equipped with a vacuum advance canister. Under
partial throttle (high vacuum) conditions this vacuum
advance increases the timing to increase the
economy.
Total Timing: This is the total of the Initial Timing, Cen-
trifugal advance and if equipped, the vacuum ad-
vance. For example: 8° Initial + 21° Centrifugal =
29° Total Timing.
Tips on 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 changing the burning
rate of the fuel or the engine speed can cause a need
for an ignition timing change. These factors range from
fuel octane to the shape of the combustion chamber.
Check with your engine builder to determine what
ignition curve will best suit your engine and driving
habits.
Selecting the Advance Springs
The advance springs determine how quick the ad-
vance comes in. The starting point of the advance curve
is controlled by the length and tension of the spring.
The slope, or how fast the curve comes in is deter-
mined by spring tension.
MECHANICAL ADVANCE SETUP TIPS
To change the advance springs, simply use a set of
needle nose pliers and pull the spring off.
SPRING COMBINATION RATE OF ADVANCE
2-Heavy Silver SLOWEST
1-Heavy Silver
1-Light Blue
1-Heavy Silver
1-Light Silver
2-Light Blue
1-Light Silver
1-Light Blue
2-Light Silver FASTEST
Selecting the
Advance Stop Bushings
The advance stop bushing determines the amount
of advance available to the centrifugal assembly. There
are four different bushings with their amounts shown
below.
Red - 28° Blue - 21°
Silver - 25° Black - 18°
To change the advance stop bushing, remove the
locknut and washer on the bottom of the advance as-
sembly. Remove the bushing and install the new one.