Bolt-Ons for a Small Block Chevy: Choosing the Right CD Ignition


Drag Race 101: More fire in the hole—choosing the right CD ignition for your small block.

Making any combustion engine run requires three variables; air, fuel, and spark, but the ignition system of your car is sometimes an afterthought. Let’s face it, if the engine runs, it must be working. But there is more to it than that, and the ignition system must be up to the task of doing what is needed. Since nobody reading this probably runs a stock engine, why would you run a stock ignition? Without a proper ignition, you might be sacrificing power.

Most factory ignition systems are more than capable of handling basic upgrades such as an intake, exhaust, and/or header system with no problems. In reality, a mildly modified engine might show little or no power increase. But, introduce power-producing components like a high-lift cam, increased-compression, nitrous oxide, forced induction, or whatever, and these increases demand a hotter ignition spark.

It is impossible to throw a blanket-statement out there and try to cover all ignitions systems and come up with a cut-and-dry set of rules to live by. It is imperative that you know your vehicle and have a plan of attack when it comes to modifications, including ignition upgrades.

Installing an MSD ignition box in your car almost guarantees a complete burning of all the air and fuel you can supply your engine. Leave it to two missile engineers to create the MSD ignition, which stands for multiple spark discharge. An MSD ignition system is effective because it allows for multiple sparks per combustion cycle. This makes sure that the air and fuel mixture is completely burned. An MSD ignition is also known as a capacitive discharge (CD) ignition.

In hopes of keeping it simple, a capacitor discharge ignition is a type of ignition system that was developed to overcome the long charging times associated with high inductance coils that are used in inductive discharge ignition systems. This makes the ignition system more suitable for high engine speeds. The capacitive-discharge ignition uses capacitor discharge current output to fire the spark plugs.

SO which MSD box is right for your car? Selecting the right MSD ignition for your application can be a daunting task. MSD offers ignitions for everything from 300 horsepower street cruisers to 5,000 horsepower dragsters. Choosing the right box is as simple as knowing what your engine’s needs are. Below are a few of the offerings, and what they are designed to do.


Digital 6 Plus

The Digital 6 Plus uses a high speed RISC microcontroller to control the ignition’s output, while constantly analyzing the various inputs such as supply voltage, trigger signals and rpm. The high speed controller can make extremely quick compensations to the output voltage, multiple spark series, timing and rpm limits while maintaining accurate timing signals to within 1 degree, and 1 percent of the rpm limits.

The Digital 6 plus offers a built in Soft Touch Rev Control with two different rpm limits. The Soft Touch circuitry provides a smooth and accurate rpm limit, by dropping the spark to individual cylinders. It produces a load-free rpm limit that is within 1 percent of the selected rpm. You can select two rpm limits—one for a low limit, that can be used for staging the car, and the other for top end over-rev protection. Both rpm limits are externally adjustable in 100 rpm increments and adjusted by easy to use rotary switches so there is no need to carry extra chips.

It also has a single-stage retard that can be activated when 12 volts are applied. This is great for nitrous applications. The retard will remain in effect until the 12 volts are removed. This retard step can be connected to a nitrous solenoid activation wire or micro switch on the shifter. The retard is adjustable from 0 to 9.9 degrees in 1/10 degree increments.

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Programmable Digital 6AL-2

The programmable Digital 6AL-2 is based on the original 6AL-2, but rather than having rotary dials for rpm adjustments, it uses a serial port that connects to a PC. Once you load MSD’s Windows-based software on your PC, you’re ready to start mapping and programming. The digital technology also allows you to get rid of the weights and springs that control the mechanical advance of your distributor. Simply lock-out the advance mechanism, and create a timing curve with your computer that allows you to manipulate the every 100 rpm. The advantage is precise timing control, with the ability to ramp-in the timing or retard it at the rpm that you want it to move.

The ignition also features a timing curve that is based on boost, so if you’re running a supercharger, you can map out a timing curve based on boost pressure. With this feature, you have the ability to pull timing out during the hole-shot, and ramp it back up during the run. Since many cars have more power than traction capabilities, being able to soften the power at the first 60-feet of the run can help reduce the tire shake off the line.

There are also three different rpm limits that can be set in 100-rpm increments; one for the burnout, one for the hole-shot, and a top end limit for over-rev protection. If you have a nitrous system, the ignition also has an activation wire that ties into the nitrous wiring. When the nitrous is activated, the timing can be retarded instantly.


A great function of several of the MSD boxes is the ability to adjust your ignition timing with a computer. This is a great asset for those looking to extract every possible horsepower from their engine. When using the MSD software and your PC, adjusting any part of the ignition timing is as easy as entering the desired degree of timing for a given rpm.


Digital-7 Plus

For racers that don’t need a lot of programming options, the Digital-7 Plus is a great choice. Below 3,300 rpm, there is MSD’s series of multiple sparks. This spark series burns in the cylinder for over 20-degrees of crankshaft rotation, resulting in improved combustion and increased power. The Digital-7 Plus provides racers with two smooth rpm limits, one for top end protection, and another to activate with a clutch or transbrake switch for use at the starting line. Both are adjustable, from 2,000 to a maximum of 12,500 rpm. For nitrous applications or for racers looking for a little more mph, there is a single-stage retard circuit that is adjustable up to 9.9 degrees. MSD’s digital 7 box also has compatibility for certain applications including; Digital-7 Programmable w/Data Acquisition, Programmable Digital-7 with Boost Retard.

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The 7AL-2 has been a staple in the drag race community for years. From Super Stock to Comp Class racing, it’s the ignition of choice for racers. The familiar gold box has been redesigned to deliver a 40-percent increase in spark energy. It also features a popular two-step rpm control for staging and over-rev protection.



The MSD 7AL-3 features a host of the most popular drag racing bells and whistles. A built-in Three-Step Rev Control provides three different rpm limits that can be used for the burnout, staging, and top end over-rev protection. There is also a Multi-Step Retard with four different modules that can be activated at different times. Many race engines with crank trigger systems or locked-out distributors will benefit from the 7AL-3’s built-in Start Retard option.

When cranking your engine, the Start Retard will retard the timing 25-degrees, allowing the engine to turn over easier. When the engine starts, the timing will automatically go back to its mechanical setting. One last option is an rpm-activated switch that allows you to activate a solenoid or shift light at an adjustable rpm. When the engine hits the desired rpm the circuit provides a ground to the component to activate it.



The entry level MSD box is the analog MSD 6A. The MSD 6A has been around forever, and is the base-model of the capacitive multiple-spark discharge 6 Series design. This ignition works on modified street machines or street cruisers. The powerful multiple sparks of the MSD 6A ensures complete combustion. Easier starting, improved throttle response, reduced plug fouling, more power and increased fuel economy are all benefits from the 6A’s high energy multiple spark strikes. The 6A ignition produces a series of full-power sparks through mid-range rpm to ensure complete combustion of the air and fuel mixture.

Like all of the MSD 6 Series Ignitions, the 6A will work with any vehicle as long as it has a 12-18 volt electrical system. It can be triggered using breaker points, a magnetic pick-up or the output of an electronic amplifier.

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The MSD 6AL ignition is a good starter box to deliver high-horsepower spark, and for saving that engine. The 6AL shares the same circuitry of the 6A with the addition of a built-in adjustable rev limiter. The combination of powerful sparks with the safety of a Soft Touch Rev Control is what makes the 6AL a popular ignition box for entry level drag racers.

MSD’s Soft Touch rev control prevents the engine from overreving in the event of a missed shift or parts failure. The rpm limit is adjustable in 100 rpm increments with MSD’s plug-in modules. When the engine reaches the selected RPM limit, the Soft Touch circuitry begins dropping the spark to various cylinders. On the next cycle, these plugs are fired again to prevent fuel from loading up in the cylinder. The result is a smooth and accurate rpm limiting action without backfires or blowbacks through the carburetor.



The 6 BTM is the perfect choice for engines with a turbo or supercharger. Not only will the engine benefit from MSD’s full power spark, but there is also an adjustable boost/timing retard circuit to prevent detonation. When your turbo or blower forces the air and fuel mixture into the engine, the cylinder pressure inside the combustion chamber increases, making more power. While this increase in power is great, it can also lead to detonation. The 6 BTM lets you eliminate detonation by retarding the timing in relation to boost pressure. A dash mounted control knob lets you adjust the amount of timing retard. It can be adjusted from 0 degrees per pound of boost to 3 degrees per pound (up to 15 degrees).The 6 BTM also shares the Soft Touch Rev Control of the 6AL Ignition for overrev protection. The BTM is supplied with rubber shock mounts and rpm modules for 3,000, 6,000, 7,000 and 8,000 rpm.


Since boost is monitored by pressure, the 6 BTM is connected to the intake so it can sense vacuum or pressure, and adjust the timing accordingly.


Source: Autotronic Controls



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