Why A CD Ignition Like PerTronix’s Digital HP Is Perfect For You

In order for the “controlled mini-explosion” to occur in each cylinder of an engine, you need three things: fuel, air, and spark. Regardless of what performance enhancements you have made to your engine, those three items are a constant requirement. Remove any one of them, and the internally contained firestorms go away.

When enthusiasts hurl most upgrades at an engine, they will typically be targeted at improving airflow, compression, and/or fuel delivery. However, the folks at PerTronix want to make sure you realize when looking for that ever-elusive power increase, one should not forget about upgrading the ignition.

When our classics were first sent to the dealerships, many electronic ignitions were barely adequate when the engine was stock. But who leaves anything stock? Performance upgrades typically require modifications to the ignition system, which are paramount to delivering a reliable spark to the engine. Why is that you ask? Modifications like an increased compression ratio, camshaft duration and overlap, and even the type of fuel used, play a huge role in how those small, controlled burns happen within a cylinder. If the ignition is not sufficient to light the fires, you can guess what happens.

CD ignition

The small-ish size of the Digital HP box means it can be mounted virtually anywhere. Even if mounted out in the open, like on the firewall, it doesn’t make the underhood area appear cluttered.

We have heard time and time again that having the multiple-spark capability of a CD ignition box can be a huge ignition improvement. We decided to speak to Russell Stephens of PerTronix so he could elaborate more on the subject.

Faithful Fire

“A CD ignition is a great upgrade, and the PerTronix Digital HP is the perfect ignition-box choice for performance enthusiasts,” he affirms. “Most performance vehicles have larger carburetors, higher lift camshafts, increased compression ratios, or even power adders such as a turbo or supercharger. All of these factors increase the demands on the ignition system. The Digital HP will ensure the air/fuel mixture in the cylinder will light no matter what the conditions. It does this by providing powerful, multiple sparks for 20 degrees of crankshaft rotation and up to 7,000 rpm.”

According to PerTronix, most CD boxes actually stop delivering multiple sparks around 3,000 rpm. This is due to the rate at which other CD ignitions can multispark (about 1 time per millisecond). At 3,000 rpm, their second spark occurs about 20 crank degrees later. The Digital HP sparks 50-percent faster. This means that you have two to three times as many sparks at 3,000 rpm and multiple sparks to 7,000-plus rpm. More importantly, the following sparks are closer to the desired ignition timing for better combustion.

The Digital HP ignition system does need a coil that is capable of handling the increased power without overheating. For normal street driving, the Digital HP will work with most coils that have a primary resistance of 3 ohms or less. A low-resistance coil, such as the Flame-Thrower III canister coil, or Flame-Thrower HC or HP, are highly recommended. For extended high-RPM use, such as circle track or road racing, only use the Flame-Thrower HP coil (PN 60100) or equivalent. The Flame-Thrower HP coil is an ultra-low resistance coil designed specifically for CD ignitions.

Completely Burned

There are some enthusiasts who feel that a multiple spark ignition is not really necessary. Some think this is because once the fire within a cylinder is lit, it will burn completely — it shouldn’t need to be relit. However, according to Russell, there is a flaw in this thought process.

CD ignition

Upgrading to a Digital HP ignition control does not mean eliminating what you are already using. This CD ignition can be used with your existing HEI, PerTronix’s Flame-Thrower distributor, or a traditional point-style distributor that has been upgraded with a PerTronix Ignitor. That means, depending on how/where you mount the ignition box, you can keep a stock appearance under your hood.

“The big variable is ‘if’ the combustion mixture is lit and ‘if’ it burns completely,” Russell states. “The combination of multiple sparks, higher spark energy, and the 20-degree firing duration offered by the Digital HP box guarantees the mixture will ignite and burn completely. Factors such as an overly rich mixture (or even extremely lean) and higher cylinder pressures all reduce the chance the mixture will ignite and completely burn. The Digital HP provides all the ignition spark an engine needs to ensure the fuel charge is burned in the cylinder and produces the best power.”

Features For All

In addition to a complete combustion burn, there are other features that make this CD ignition the perfect choice for performance enthusiasts. First, there is an integrated three-stage, digital rev limiter. If an enthusiast wants to do some weekend drag racing, this rev limiter can be activated for burnout, launch, and high-rpm over-rev protection. The Digital HP also has an RPM-activated output that can turn on a shift light or trigger solenoids for nitrous or Co2 shifting. There is also a start retard to help start high-compression engines. From mild performance upgrades to high horsepower, big-inch engines, the Digital HP is a great ignition choice.

However, is a CD ignition box something the average non-racing enthusiast should consider? Is this a performance-only ignition box, and if so, why should the average “driver” car enthusiast consider it for their classic? “Not only is this a true high-performance box that includes some serious high-performance features as described above, but this ignition box is also very well suited for the average driver,” confirms Russell. “Its high energy, multiple sparks will help “rich” engines start, allow the engine to reach maximum RPM, and will ensure complete combustion of the air/fuel mixture. Even if the engine is just a 325-horsepower small block, this ignition has the power and features to get the best power and performance out of it.”

Before we go any further, many of you are probably thinking, “I have an HEI distributor, can I add the CD box to that”? The answer is yes. While the HEI is a great distributor, adding the Digital HP has some real benefits. In fact, Russell had this to add, “The HEI uses a single spark, inductive ignition module. This design is very limited when engine RPM is increased. By adding the Digital HP to an HEI distributor, you now have an ignition with capacitive discharge technology, multiple sparks, long-duration firing, and a much higher RPM range. An HEI is greatly enhanced when paired with the Digital HP.”

When it comes to ignition systems, many hot rodders don’t think twice about it as long as it is working. However, If you ever mash the throttle, even in your cruiser, as engine RPM increases, the ignition system has to work harder. This is when an OE system can wreak havoc on a perfectly running engine when at lower RPM.

CD ignition

Functions of the Digital HP are controlled via the switches. The first switch controls how many cylinders, the mode switch controls features like rev limiting, shift light triggering, and type of distributor used. The RPM switches allow you to select at what RPM those functions occur.

As Russell stated, as engine RPM increases, there is less time for the ignition to light the air/fuel mixture. Consequently, the spark must occur earlier in the power stroke in order for it to have sufficient time to ignite the air/fuel mixture properly. Again, this where a CD ignition shines, as it delivers multiple sparks for that time span of 20 degrees of crankshaft movement. This definitely covers the combustion process. When the burning mixture ignites, it expands. As it expands, it is creating the energy needed to force the piston down to make power. The more complete the burn, the more potential power is realized.

On a properly running engine, peak cylinder pressure occurs between 10- to 20-degrees after top dead center (ATDC). The actual time is dependent on piston and combustion chamber design, as well as camshaft timing. Since the amount of burn time is relatively stable, we can see that the spark must actually start at an earlier “time” in the process for the burn to take place in that same window of 10- to 20-degrees ATDC as RPM increases.

When an engine is spinning at roughly 6,500 rpm, the spark plug ignites the air/fuel mixture nearly 60 times-per-second. If any one of those 60 sparks “misses,” It can, at best, be mildly annoying. Multiply that scenario by eight cylinders, and it’s easy to see why a properly functioning ignition system is paramount to performance and reliability.

ignition upgrade

The Digital HP is designed to improve the performance of an engine by ensuring the air/fuel mixture in the cylinder is burned completely. How does just the addition of an ignition box do this? The box amplifies the energy of the ignition system, lengthens the duration of how long it fires, and provides multiple sparks during the firing duration. The PerTronix Digital HP Ignition is considered a capacitive discharge ignition because it stores energy in a large capacitor inside the unit.

While an ignition system might seem extremely complex, it really isn’t that hard to see why a CD ignition — like the Digital HP — could be a great upgrade for your hot rod. However, if you’re worried about the appearance of a big ignition box cluttering your engine bay, the Digital HP is available in either black or silver. That means you can let it blend in or stand out, depending on your engine bay. What’s more, the Digital HP is actually a small unit (about one-third of the size of a traditional ignition box) that can be tucked out of sight.

“The Digital HP is made in the USA and uses modern digital design and circuitry,” Russell concludes. “This is how the Digital HP can produce more power and include more features than the popular red boxes on the market. More importantly, it does this in a package that is 70-percent smaller, so it is easier to find space under the hood to mount it. Smaller, more powerful, and lots of bonus features make Pertronix’s Digital HP is the perfect ignition for your hot rod.”

Lastly, this little wonder is 50-state legal too!

Article Sources

About the author

Randy Bolig

Randy Bolig has been working on cars and has been involved in the hobby ever since he bought his first car when he was only 14 years old. His passion for performance got him noticed by many locals, and he began helping them modify their vehicles.
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