MSD has been making these iconic red ignition boxes since the early ’80s. Since then hundreds of thousands of these units have found their way onto everything from street cars to serious racing machines. The classic 6A box is an entry-level upgrade that can be triggered by breaker points, a magnetic pickup, or an electric amplifier. It’s a vast improvement over stock, and especially helpful when the combustion chamber becomes a crazy place with the addition of compression and turbulence like from a power adder. These boxes convert your ignition system to capacitive discharge ignition (CDI). It was developed because the factory inductive discharge ignitions (IDI) have a long charge time, or dwell, that makes high rpm spark dim and inconsistent. All of MSD’s ignition control boxes take advantage of the quick spark turnaround of CDI. A step up from the 6A, is the 6AL, which has the addition of a rev limiter.
At this year’s SEMA convention MSD displayed their latest rendition of the legendary 6A/6AL boxes, the Digital 6A and Digital 6AL. True to their traditional style, the red powdercoat and machined fins keep it looking much like it always has, but it’s got a few new tricks up its sleeve. Spark control goes from analog to digital for both models. This means a microprocessor gives even more accuracy at high rpm, and a redesigned circuit board produces more power with less draw. We’re going to take a closer look at the Digital 6AL specifically since it’s one of the most substantial improvements in this unit. Toss those pills! The new Digital 6AL has two rotary dials to select the rev limit point in 100-rpm increments. As always, MSD drops one cylinder at a time, then firing that same cylinder on the next round to avoid letting the fuel collect, giving it it’s Soft Touch name. Another big change is the single-connector wiring plug. Earlier models had a set of wires that passed through a grommet on the side of the housing, but the new version has a 12-pin connector and matching pigtail harness. Wiring is business as usual using battery power, ignition key on, trigger, two coil connections, and ground. Aside from the exterior changes, the new Digital 6AL blasts a 530V charge to the coil, 50 over the previous 6AL. Subsequently, this gives us higher spark energy. If you’re looking to swap out an early 6A or 6AL, you’ll be relieved to hear the Digital 6A and 6AL continues the same mounting pattern as their originals, but in a thinner package.
CDI vs. IDI
Inductive discharge systems have the advantage of being simple and easy to build, but more notably, they have long spark duration; the downtime between firing is the only time for the coil to build and collapse a magnetic field, making it more difficult for the system to keep up when rpm is increased. Modern OE ignitions remedy that by giving each cylinder, or pair of cylinders, its own coil. Capacitive discharge ignitions use the car’s ambient voltage to charge a capacitor that, when triggered, floods the coil with 400-500 V, allowing it to create a magnetic field much faster, but this is not without its drawbacks. The high voltage is a trade for low amperage, shorting the spark’s duration. Companies like MSD combat this by taking advantage of CDI’s high-spark frequency capabilities. At low rpm, spark is triggered several times to make up for the lack of duration. As rpm increases, the number of sparks per ignition goes down. CDI is inherently more complicated, and expensive, but when you’re up against turbulence from nitrous or a turbo, for example, the extra voltage to jump the gap makes it well worth it.