Ignition Upgrade: MSD 6AL
One area we had overlooked
on our project '67 was the ignition. Sure we had done the typical enhancements such as re-curving the stock distributor, and replacing the points with a solid state module. That was fine for the street and occasional mid 13 second runs at the strip. However now that the car has become predominantly a quarter mile machine, and the engine is run repeatedly beyond 6000 rpm, we figured it was time to upgrade the ignition system.

Ignition systems can be broken down into two categories, inductive-disharge (IDI) and capactive-discharge (CDI). The differences are in how the coil fires the spark. A coil is simply a step-up transformer; taking 12 volts in from the battery and putting out around 30,000 volts to fire the spark. If you remember from high-school physics, the flow of electricity generates a magnetic field, and a magnetic field can be used to generate electricity. In an IDI system, the breaker points in the distributor contact to pass 12 volts through to the primary side of the coil, causing a magnetic field to build up. As the breaker points open up again, the field collapses and generates over 150 volts in the primary windings. Multiply that by the 200:1 ratio in the secondary windings, and you have around 30,000 volts firing the plug, typical for a stock system. All of this takes place in less than 0.015 seconds to fire one spark. A V8 engine spinning at over 6000 rpm, requires over 400 sparks per second!

Although it may seem impossible for this system to accurately and precisely produce the required spark energy to each cylinder, it does do it reliably to a point. As engine rpms increase, and build up time decreases, inductive systems do show weakness, in that the coil simply cannot build-up the fill voltage. This is where capacitive discharge comes in.

In a capacitive-discharge system the coil is not used to build-up the primary voltage source. Instead a transformer steps up the battery voltage to over 450 volts and stores it in a large capacitor. When triggered by the distributor, the voltage is dumped into the primary side of the coil. The high voltage input to the primary windings results in as much as 50,000 volts output. The benefit with CDI is that the capacitor charge itself in less than one millisecond (0.0001 seconds). This means that at very high rpms there is full energy to the plugs. The ability to charge the capacitor rapidly also allows for firing multiple-sparks at lower rpms, and increasing spark duration to 20 degrees of crank rotation.

Images courtesy Autotronic Controls Corp.

MSD pioneered the capacitive discharge and multispark ignitions many years ago. Judging by the majority of racers using MSD ignitions, we felt it was a safe bet to go with a tried and true MSD 6AL system and Blaster coil. The MSD 6AL comes with a rev limiter, an important safety measure in case something fails.

We were very impressed with the completeness of the MSD kit, and the level of detail in the instruction manual. The kit came with all the wiring, connectors, and miscellaneous hardware needed to complete the installation.


Installing the MSD is surprisingly very straightforward. It took us less than an hour to complete the installation. The MSD 6AL will run with all distributor trigger mechanisms on the market. The instruction manual comes with detailed schematics for most systems. Although our Petronoix Ignitor wasn't in there, we had no problem figuring it out, since the wiring is very basic. These connections are the same regardless of your distributor or pick up:

Heavy Red: Full time 12V (Battery)
Heavy Black: Ground
Red: 12V "Key on"
Orange: (+) coil
Black: (-) coil

All that remains to connect is a white lead, or green/violet leads, depending on what triggering mechanism you are running. If you previously had points, early Duraspark, Mallory Unilite, or any sort of amplifier from the distributor to the negative side of the coil, you'll connect to the white lead. If you are running a magnetic pick-up like a crank trigger, or later Durasparks, you'll connect to the green/violet harness.

With the Petronix, being a Hall-Effect Switch, we weren't sure if it qualified as a magnetic pickup, or standard "white wire." We tried it with the magnetic pick up, and the engine would crank but not fire. The MSD manual shows how to test for spark, and we followed the directions to isolate the problem to how we'd wired the Petronix. Turned out that the Petronix takes its own 12V, ignition-on source, and the other lead connects to the white wire. Once we got that straight, the engine fired right up. In fact we barely heard the starter crank and the engine immediately roared. We noticed the idle was smoother and an intermittent miss we previously had around 6400 rpm had disappeared.

At the track we made several passes with the MSD connected and then disconnected, running off the Petronix alone. We noticed a consistent gain of 1 to 1.5 mph and dropped 0.05 to 0.10 seconds through the traps with the MSD firing the plugs.