MSD 6421 6AL-2 Ignition Control Installation - Manual (Page 2)

2 INSTALLATION INSTRUCTIONS
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REV LIMITER
This Ignition features a built-in Soft Touch Rev Control with two different rpm limits. The Soft Touch
circuitry provides a smooth and accurate rev limit by dropping the spark to individual cylinders. The
Soft Touch produces a load-free rev limit that is within 1% of the selected rpm.
You can select two rev limits; one for a low limit that can be used when staging the car, and another
limit for top end overrev protection. Both rpm limits are adjusted in 100 rpm increments with the
sealed rotary switches on the side of the ignition. Using and programming the Two Step is explained
in detail on page 5.
CYLINDER SELECT
The MSD is programmed at the factory for use on 8-cylinder engines. If you are installing it to a different
engine you will have to program the Ignition. This is easily achieved through the cylinder select switch
on the side of the ignition. Page 5 outlines setting the cylinder select.
GENERAL INFORMATION
BATTERY
An MSD 6AL-2 Ignition Control will operate on any negative ground, 12 volt electrical system with a
distributor. The MSD can be used with 16 volt batteries and can withstand a momentary 24 volts in
case of jump starts. The Ignition will deliver full voltage with a supply of 9 - 18 volts and will operate
with a supply voltage as low as seven volts.
If your application does not use an alternator, allow at least 15 amp/hour for every half hour of
operation. The MSD uses about .9 Amps for every 1,000 rpm. If the engine is cranked with the same
battery or other accessories such as an electric fuel or water pump are used, the amp/hour rating
should be higher.
COILS
The MSD 6AL-2 Ignition can be used with most stock coils and aftermarket coils designed to replace
the stock coils. The line of MSD Blaster Coils, PN 8202, PN 8207 and PN 8222, are great for street
and mild racing. For extended high rpm operation the Blaster HVC, PN 8252 or HVC II, PN 8253,
are recommended. For more information on recommended coils, check with our Customer Service
Department at (915) 855-7123.
TACHOMETERS
The MSD Ignition features a Gray Tach Output wire that provides a trigger signal for tachometers, a
shift light or other add-on rpm activated devices. The Tach Output wire produces a 12 volt square
wave signal with a 25° duty cycle.
Some vehicles with factory tachometers may require a Tach Adapter to operate with the MSD. For
more information on Tachometers and MSD Tach Adapters, see the Tachometer Section on page 7.
If your GM vehicle has an in-line filter it may cause the tach to drop to zero on acceleration. If this
occurs, bypass the filter.
SPARK PLUGS AND WIRES
Spark plug wires are very important to the operation of your ignition system. A good quality, helically
wound wire and proper routing are required to get the best performance from your ignition, such as the
MSD or Street Fire 8.5mm Super Conductor Wire. Helically wound wires provide a good path for the
spark to follow while keeping Electro Magnetic Interference (EMI) to a minimum. Excessive EMI, such
as the amount that solid core wires produce, will interfere with the operation of the MSD. Solid Core
spark plug wires cannot be used with an MSD Ignition.