This article is one in a series that have been released in conjunction with Wayne's book, 101 Projects for Your Porsche 911. The book contains 240 pages of full color projects detailing everything from performance mods to changing your brake pads. With more than 650+ full-color glossy photos accompanying extensive step-by-step procedures, this book is required reading in any Porsche 911 owner's collection. See The Official Book Website for more details.
One of the best upgrades that you can perform on your 911 is the installation of the Multiple Spark Discharge, or MSD, ignition control system. This unit replaces the capacitive discharge unit, or CD box in the 911 ignition system with better and updated technology. The CD box is the small metal box on the left side of the engine compartment that makes a high-pitched whine when the ignition is turned on. If you need to replace the CD box, I recommend that you replace it with one of the better and cheaper MSD units.
Similar to the original Bosch CD box, the MSD system uses a capacitive discharge ignition design. Other Porsches like the 356, 912 and 914 use a standard inductive ignition system that cannot match the performance of the capacitive discharge design. On these older inductive systems, the coil must step up the voltage and store the electrical charge at maximum strength in-between each firing. When the RPM of the engine increases, the coil doesn't have enough time in-between firing to ramp up to its maximum voltage. This results in a weaker spark and a loss of power at higher RPMs.
With capacitive discharge systems like the OEM Bosch CD box and the MSD unit, the ignition system contains a capacitor which can be charged much quicker (less than one millisecond) than the inductive coil. With a capacitive discharge system, the voltage sent to the coil is always at full strength, even when the engine is running at a high RPM.
In addition to the benefits of the capacitive discharge system, the MSD unit fires more than one spark per cycle. During the course of an ignition stroke, the MSD unit rapidly fires full power sparks over 20( of crankshaft rotation. This multiple spark firing aids in the combustion process and helps the fuel burn more completely, resulting in more power, and less emissions. The amount of multiple sparks fired decreases as the engine RPMs increase, and above 3000 RPM, the MSD system only has enough time in the ignition cycle to fire a single high powered spark. Within the range above 3000 RPM, the performance of the MSD system is comparable to the stock Bosch CD system.
The MSD system also incorporates a rev limiter within the ignition system that limits the maximum revolutions of the engine. Some of the early cars can use a rev-limiting rotor which performs a similar purpose, and the later 911s had a rev-limiting function built into the CD box or the tachometer itself. The MSD system allows you to adjust this RPM level by plugging in modules into the unit.
The main MSD unit recommended for the 911 is the 6AL, part number 6420. This kit contains all the wires, connectors, mounts and crimping hardware that you will need to install the unit in your 911. The kit is very well manufactured (made in the USA), and the unit is known for its high reliability. Racers under harsh conditions typically use the MSD systems in their cars.
In addition to the MSD main unit, you may also need to purchase the tachometer adapter. For early 911s with points, this tachometer adapter is required to convert the MSD signal back into one that the tachometer can recognize. Without this adapter, the tachometer will be confused by the multiple spark signal, and give a false reading. The technical gurus at MSD couldn't give me a definitive answer when asked which models needed the tachometer adapter. Apparently, changes and differences each individual car result in unpredictability in determining whether or not the system will require the adapter. Some cars may require it, and other seemingly identical cars may not. According to MSD, it may be related to manufacturing tolerances inside the VDO tachometers used on the 911. Either way, the tachometer adapter should make the system work for any 911 tachometer. I recommend that you purchase and install it to avoid any potential tachometer problems that you might experience.
Another option for the MSD system is the RPM selector. This allows you to manually dial-in the rev-limited amount that you would like to use for the unit. The unit itself allows you to use small plug-in modules to select the rev-limiting rpm, but this selector allows you more flexibility. In addition to the selector, there are also additional plug-in RPM modules available. The stock unit contains a rev-limiting plug-in for 6000 RPM, so if you want to use a value other than this, you will have to purchase another set of relatively inexpensive plug-in modules.
The final item you will have to acquire is a new coil. The MSD system will not work with the standard CD coil that is stock on the 911. The standard Bosch blue coil (0.221.119.027) can typically be used with the system, although some people have experienced difficulties with it and have replaced it with the high-power MSD coil.
Installation is relatively straightforward, and can easily be completed in about four hours. The installation instructions that come with the unit are very clear and quite concise, so the remainder of this project will focus on information specific to 911 installations that is not covered in the MSD installation documentation.
The physical mounting of the unit is important. The natural place for mounting the unit is in the same place as the old CD box- try to mount it there if possible. The MSD unit replaces the CD box, so you don't need it any more. There is a market for good, used Bosch CD boxes, so you might want to resell your old one and recoup some of the costs of purchasing the MSD system. The Bosch CD unit is mounted on an aluminum metal plate that is mounted to the left side of the car, under a plastic cover. Unplug the electrical connections from this plate and remove it so that you can drill holes in it for mounting the MSD unit. Use the supplied rubber mounts in the kit to elevate the unit off of the plate.
The thick red wire that exits the unit should be connected directly to the battery. Since this is hardly possible with the 911 battery being in the front trunk, you should connect it to the large electrical connection on the starter. This wire is connected to the battery (check it with a voltmeter) and should be live all the time. Make sure that you disconnect the battery before unscrewing this connection. The large black wire is a ground wire, and should be connected to one of the bolts that mount the aluminum plate to the left side of the car.
There are two wires (black and orange) that exit out of the unit and are covered with a black sheath. These are to be connected to the new coil that should be installed in place of the older black one. The orange wire is connected to the positive (+) terminal of the coil, whereas the black wire is connected to the negative (-) terminal.
The thinner red wire needs to be connected to a switched voltage source. The best place to do this is to tap into the electrical connector that used to go to the CD box. Peel back the rubber insulation, and underneath you should find a red wire. Tap the thin red wire from the MSD unit into the red wire that comes into the CD box connector. It's a wise idea to use some male spade connectors and plug right into the CD box connector so that you don't destroy the wiring or the connector.
In a similar manner, the white MSD wire is connected to the main green wire in the connector. This wire should come from the distributor and is connected to the magnetic pickup or points.
The tachometer wire inside the CD box connector is black with a purple stripe down the side. This wire needs to be connected to the tachometer output on the side of the MSD unit. In addition, the tachometer adapter from MSD needs to be installed inside the engine compartment. Simply wire the tachometer adapter into the system as described in the MSD installation documentation.
When you are finished installing your MSD system, it's advisable to install a new cap, rotor and spark plugs. MSD recommends that you set the gap on your plugs to a much wider distance, in the range of 1.27-1.49 mm, which is much larger than the factory specification of 0.7 +0.1 mm. MSD also recommends that you play around and adjust the spark plug gaps until you obtain the performance that you are looking for. The recommended values are simply good starting points.
When you're all done, you should have a system that gives you more lower-end power and torque, and also burns the fuel better for cleaner emissions. It's very common to simply upgrade to the MSD system if there is a problem with the standard Bosch CD box. The cost of the MSD system although expensive, is usually less than a new or rebuilt Bosch CD box.
The MSD Unit and tachometer adapter are shown here. The kit is very well put together, and contains all the small parts that you need for installation (not shown here). The tachometer adapter is recommended for all 911s. The MSD box is slightly bigger than the stock Bosch CD box, but can still fit in the prescribed mounting area.