By Sam Logan, July 7, 2014
MSD 6-series multi-spark capacitive discharge ignition systems feature prominently on Muscle cars everywhere and their installation is easily performed.
Though the ignition pioneer makes competition systems for applications with distributor-less ignitions, Muscle car owners are drawn to the 6-series systems that are designed to be used with a distributor, whether equipped with contact points or of electronics style.
Because capacitive discharge (CD) ignitions fire very quickly, the system has the ability to actually fire multiple times within the same firing sequence. While a standard ignition fires the spark plug only once, the MSD (which stands for Multiple Spark Discharge) actually fires three times in the same session at engine speeds below 3,000rpm.
The objective is to help ignite any part of the charge that wasn’t already ignited by the initial spark. Hence complete combustion is accomplished which improves drivability and heightens performance.
The capacitive discharge concept means that a capacitor in the ignition box is charged such that the electrical force traveling to the coil is around 500 volts, instead of the customary12 to 14 volts from the battery. It also carries the advantage that the MSD capacitor discharges very fast, so you achieve a very steep curve on the voltage traveling through the coil and to the spark plugs. With the old school points-and-coil system, the output voltage resembled a sine wave with the voltage building up to full power slowly. In sharp contrast the capacitor’s energy is almost instantaneous.
Six ignition boxes in MSD’s 6 series for Muscle cars: How to select the right one
For the average street-strip enthusiasts with compression ratios under 12.5:1 or boost pressures below 8lbs, the 6A box is the most suitable.
The most basic of the 6-series and in common with all others in the series, the 6A offers multiple sparks up to 3,000rpm and capacitive discharge. It’s also provided with a convenient wiring diagram.
Importantly, all 6-series ignition systems are of modular design, which means you can plug-in external boxes that control engine speed or ignition timing. However the 6A box can only manage the timing controls, not the rev controls. For off-road and marine applications the 6A system is available in water-proof form.
Happily, for those seeking rev control the 6AL is available, which has an integral soft-touch rev limiter. Using a single-stage rev limiter for high rpm only, the 6AL is a practical concept: the chips provided offer multiple rpm stages and you simply fit the appropriate one in the side of the box.
Alternatively, the 6BTM model was conceived for Street turbocharged and supercharged applications. This design has the same features of multi-spark, capacitor discharge and rev-limiting as the 6AL. But its distinguishing feature is its ability to retard the ignition timing in proportion to boost. To plumb you simply connect your vacuum line to the nipple which relays the boost level in the manifold. To adjust the amount of ignition timing per lb of boost a control knob is supplied which is installed on the dash. Set to zero the timing remains unaltered. Set to 1 degree the system retards the ignition timing by 1 degree at 1lb of boost. Or perhaps 2 or 3 degrees per lb; you can deduct ignition timing from the engine up to a total of 15 degrees.
As we ascend the 6-series range we encounter the new digital 6AL-2. Compared with the other members of the 6 series, this box is a little different in size and shape and has more power than the older 6AL system. It also features an integral two-step soft-touch rev control. This offers both a staging rev control for the drag strip and a high rpm limit. In addition, instead of having to slide chips into the box it is adjusted with dials. Also, it features an LED that tells if the input voltage is correct. A 6AL-2 Plus is also available, which includes single-stage timing retardation.
Finally the Programmable 6AL-2 operates with a laptop computer and adjusts the timing curve in any way desired. This is a significant advantage because you no longer have to experiment with a distributor’s mechanical weights and springs. Using MSD’s Pro-Data+ software (which can be downloaded for free from MSD’s website) allows you to very quickly and easily create your own custom advance curve. The Programmable 6AL-2 contains 3 rev limits allowing step retards.
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