How To Select An MSD Capacitive Discharge Ignition System

By: Jeff Smith | 12/01/2020 < Back to Motor Life Home

Spark ignition...every gasoline or alcohol motor engine needs one and performance engines demand a strong ignition to unlock all that power hiding in your engine. Before we get into the details of choosing among the several different MSD capacitive discharge ignition (CDI) boxes, it might be worthwhile to understand the difference between a CD and a traditional inductive ignition.

Let’s start with the inductive-discharge ignition that is the type of ignition used on every production car for the last 100 years. This spark is created using a simple device called a ignition coil, which is really a simple step-up voltage transformer. Inside the coil is a length of primary wire looped multiple times around a magnetic core that is terminated at the small terminal marked “negative” (-). A small amount of voltage is supplied to the primary windings of the coil on the positive terminal (+). In the case of a points-triggered ignition system, a wire connects between the negative side of the coil and the contact points. With the points closed, electrons flow through the windings, energizing the primary side of the coil. Wound outside of this primary loop of wire is a much longer length of wire - the secondary windings. At the point of ignition, the points open and the complete circuit to the primary windings is interrupted.

When this occurs, the magnetic field around the primary windings collapses across the greater number of secondary windings. The secondary windings are connected directly to the output pole of the coil. The secondary windings generate roughly a 100:1 ratio to the primary windings. This increased number of windings creates a tremendous increase in voltage from 12 volts to roughly 30,000 volts. This voltage is transmitted through the larger coil wire terminal on the top of the coil that directs the high voltage to the distributor and then to each individual spark plug.

Inductive systems require time to charge the primary side of the coil. This time is defined as the dwell time. Old school tuners will remember that points systems used a dwell time of approximately 30 degrees. This was a compromise dwell time that did not overheat the coil at low engine speeds but still offered suitable coil saturation at higher engine speeds. One advantage of inductive ignitions is that the spark duration is relatively long. But a significant disadvantage is that at engine speeds above 5,500, these single coils suffer a loss of output voltage and spark energy because the coil does not have sufficient time to fully saturate the primary windings. This dilemma was initially addressed with transistorized inductive ignitions that could increase the primary voltage and improve coil saturation. Eventually the OE’s solved the issue by assigning a single inductive coil for each cylinder. At 6,000 RPM a single coil must fire 375 times (3,000 / 8 = 375) while a coil for each cylinder means the coil only has to fire once for every 3,000 revolutions. Remember that each cylinder only fires once every two revolutions in a four-stroke engine cycle.

MSD LED signal light

All the digital CD boxes offer a small LED indicator light that can be useful for doing diagnostic checks. The light will indicate power and whether the box is receiving a trigger signal from the distributor. This indicator light is on the Digital 6-Plus.

In 1970, MSD created the first capacitive discharge ignition that offered some impressive ignition enhancements. The system begins with a small internal transformer in the box that steps the 12-14 volts up to around 500 to 550 volts. This voltage is stored in what is known as a capacitor. The advantage of the capacitor is that it can discharge high voltages very quickly. When 500 volts is applied to the primary side of the coil, this saturates the primary side very quickly. Again, with a turns ratio of between 70 and 100:1 on a CD style coil, this high voltage on the primary side produces as much as 45,000 volts on the secondary side. In addition, the capacitive discharge system can also produce a high current load, creating a spark that can jump a much wider spark plug gap while also delivering significant electrical power.

Of course, there are often tradeoffs with any advancement. One disadvantage to a CD ignition is a much shorter duration. The solution for MSD was to create multiple sparks at engine speeds below 3,000 RPM. The system builds voltage so quickly that this ignition system could fire the plugs up to three times at engine speeds below 3,000 RPM creating a Multiple Spark Discharge and hence the acronym.

In the beginning, there was just one MSD capacitive discharge box for all applications. But a half-century later, MSD now offers multiple ignition choices. All MSD boxes employ the same CD technology while offering different application options. We’ll take a look at six different street or street//strip MSD ignition boxes to investigate what each box offers.

MSD Street Fire

As an entry-level CD ignition, the Street Fire may be one of MSD’s least known systems. In terms of ignition power per dollar, this system delivers.

While nearly everyone knows and instantly recognizes the classic red, MSD 6A and 6AL box, there is a less well-known version that we’ll start with as an entry-level unit that MSD calls the Street Fire CDI. This piece is a full-fledged capacitive discharge, but comes in a slightly smaller package that still offers the same multiple spark discharge features of the full-size boxes but at a more affordable price. It can be connected to virtually any production engine using a distributor and offers an adjustable rev limiter.


The 6A and 6AL offer identical performance except that the 6AL also offers a rev limiter that is very easy to set. The 6AL is by far MSD’s most popular CD ignition box.

The original MSD 6A box was an analog unit that has since been replaced by digital components. The digital 6A is the first step up from the Street Fire and offers slightly higher power output and voltage and is capable of operating as high as 15,000 RPM. This box does not contain a rev limiter but is also slightly less expensive than perhaps the most popular model - the Digital 6AL.

Both the 6A 6AL offer exactly the same output voltage and power with 135 to 145 milli-joules (mJ) of current per spark event. This rating system can be used to evaluate the power behind the spark event. A joule can be defined as a measurement of the amount of heat created when one ampere of current passes through a one ohm resistor for one second. You can also think of it as the amount of heat generated across a single spark event. A milli-joule is one-thousandth (0.001) of a joule. This may not sound like much, but if you’ve ever felt the jolt of spark from an MSD unit, then you know that it has more than enough power to get your attention! The MSD 6A and 6AL boxes offer the most spark energy of all the 6 series of ignitions.

The “L” designation in the 6AL indicates the addition of MSD’s Soft Touch rev limiter. In the analog 6AL boxes, the rev limiter feature was adjusted by selecting a different plug-in modules. With the digital boxes, the rev limiter now is easily set by adjusting two rotary dials on the box cover. The Soft Touch rev limiting drops individual cylinders as the rpm hits the preset limit. This is easier on internal engine components as opposed to early rev limiters that abruptly cut the power to the entire ignition circuit, creating a harsh on-off-on application.

The rev limiter uses two dials to set the RPM. The inboard dial adjusts the 1,000 RPM point while the outboard sets the 100 RPM point. As an example, setting both dials to 6 will set the rev limiter to 6,600 RPM. Another nice feature of the 6A and 6AL boxes is the sealed multi-pin connector where all the leads for the unit exit. This makes installing the wiring a little easier and also makes switching boxes between cars very simple. If you’ve got a car with an older, analog version 6AL, the new digital box is exactly the same footprint as the original with the same mounting bolt pattern but the new box is shorter and lighter.

For supercharger or centrifugal blower engines, MSD also makes a 6BTM box. This is still the same capacitive discharge system employed on the 6 and 6AL, but the 6BTM is an analog unit – not digital. That means that the rev limiter still uses the analog push-in rpm modules.

The 6BTM’s key feature is boost retard. Integrated into the 6BTM box is an on-board manifold absolute pressure (MAP) sensor that connects via a small hose to the blower manifold. This sensor reads manifold boost pressure and then offers a range of timing retard vs boost pressure. The 6BTM dial offers a timing adjustment range from 0 up to 3 degrees of retard per psi of boost. As an example, with a maximum boost of 10 psi and a 1.5 degree retard number, this would remove 1.5 degrees at 1 psi and 15 degrees at 10 psi. A separate adjustable retard knob makes easy changes possible right from the cockpit.

Moving up another notch, MSD introduced the 6AL-2 digital unit that is offered in two configurations. The first is the standard 6AL-2. Power output is roughly similar to the 6AL box and of course offers a rev limiter but is assembled into a much smaller package. It also offers the ability to use a distributor with a Hall Effect or magnetic pickup. Most MSD distributors use a magnetic pickup as the triggering device which is a different signal than that from a Hall Effect pickup. The 6A and 6AL require the use of a magnetic pickup distributor but the Digital 6AL-2 will work with either.

Hall Effect and Magnetic Pickup Distributors

There are several different ways to trigger an ignition event. Earliest ignitions used contact points. Later distributor used magnetic pickups like this MSD distributor (left). Other systems include a Hall Effect (right) employing a shutter wheel to interrupt the signal and trigger an ignition event. A crank trigger is also considered a magnetic pickup triggering device.

The 6AL-2 is also equipped with a two-step rev limiter which allows the tuner to set a low RPM rev limiter that can be used for accurate rpm staging. The second rev limiter is most often used as the peak engine speed limiter. The low-speed rev limiter requires triggering with a 12-volt source such as a shifter-mounted switch.

Next up on the capacitive discharge chart is the programmable 6AL-2. The main difference with this unit is that it integrates MSD’s ProData+ software that allows the engine tuner to create a custom, digital timing curve. The programmable system requires the use of a locked-out MSD distributor that prevents mechanical advance. With the distributor rotor fixed, the Programmable 6AL-2 allows the tuner to create both “mechanical” and “vacuum” advance curves. Or, if the engine uses a supercharger or turbocharger, the “vacuum” side of the software can be used to create a boost retard timing curve.

The MSD ProData + software is free and is found on MSD’s website (DOWNLOAD HERE). The advantage to creating a custom digital ignition curve is that this allows asymmetrical or non-linear curves that are not possible with a centrifugal device used in traditional mechanical advance distributors. In addition, the Programmable 6AL-2 offers the step retard feature present in the standard 6AL-2 along with a total of three different rev limiters. The lowest can be set for burnout rpm, the second for a specific launch speed and the final as the ultimate high RPM limiter. The programmable ignition curve is an easy system to add to an existing engine because all MSD distributors offer a simple procedure for locking out the mechanical advance mechanism so no additional components are necessary for this system to operate.

This is a typical screen you would see using the ProData+ software for the 6530 Programmable box for a street-drive engine with vacuum advance. The lower screen illustrates the advantage of a digital advance curve. Notice that we’ve added a dip in the timing around peak torque. This cannot be accomplished with a mechanical advance in a distributor. The upper scale is a “vacuum advance” style curve. The vertical scale is the amount of timing while the horizontal scale is expressed in pressure per square inch absolute (psia). Simply multiply the top scale by 2 to equal inches of vacuum (“Hg) as expressed on a vacuum gauge. So 20“Hg would be 10 psia on the horizontal scale. That red dot that reads 10.0 22.0 means we have 22 degrees of vacuum advance at 20 inches of manifold vacuum. At the lower end of the scale, we are only adding 4 degrees at 4 inches of manifold vacuum -- expressed as 2 psia on the horizontal scale.

The ProData+ software is easy to navigate. You can add a reference point – called a dot – anywhere on the scale by left-clicking and adding. It takes about 2 minutes to create the mechanical curve you want. The “vacuum advance” curve is slightly more complex. MSD expresses the chart on the horizontal scale in terms of pressure per square inch absolute (psia).

To create a curve you would have more timing at high vacuum with low load and reduce the advance as vacuum decreases. Keep in mind that to use this feature the harness will need to be connected to a typical GM three-pin MAP sensor like those used on late ‘80s and early ‘90s GM EFI vehicles and available as a 1-bar sensor from Holley under PN 538-24.


The Digital 6AL-2 is significantly smaller than the previous boxes and will also connect to distributors using a Hall effect trigger.

This system can also be used for supercharged or turbocharged applications to operate a custom-designed boost retard curve of your own design. This will require the use of a separate 2- or 3-bar MAP sensor. A boost retard graph will substitute for vacuum advance and allows the tuner to limit timing based on boost ready by the MAP sensor. Combined with the programmable 6AL-2’s other features, this box offers a raft of features and control parameters necessary for today’s more sophisticated performance cars.

Any of these capacitive discharge ignitions will deliver more than enough spark energy to light that fire inside your street performance or street/strip engine. The only question that remains is which box fits your needs the best.

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