MSD 8572 Corvette V8 Tach DriveDistributor w_Vacuum Advance Installation - Manual (Page 2)

 INSTALLATION INSTRUCTIONS
M S D I G N I T I O N
• w w w . m s d i g n i t i o n . c o m • ( 9 1 5 ) 8 5 7 - 5 2 0 0 • FA X ( 9 1 5 ) 8 5 7 - 3 3 4 4
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.