Holley Performance may be best known as the brand behind fuel delivery systems like the 4150 carburetor and Sniper 2, but the company’s expertise also stretches into many other aspects of automotive tuning and maintenance.

When MSD joined the Holley family back in 2015, the makers of the iconic 6AL capacitive discharge box brought decades of ignition component design experience and engineering know-how along for the ride. While this alliance has ushered in more sophisticated products that have raised the bar for ignition system capability in the years since, the company hasn’t forgotten that the foundation of an effective high-performance engine combination starts with the fundamentals. And when it comes to ignition systems, coils are a crucial piece of the puzzle.

“Whether we’re talking about a setup with a single canister-style ignition coil or one with individual units for each cylinder, the primary role of the coil is to store and deliver high-voltage energy to the spark plug,” explains Calvin Gazda of Holley Performance. And that means that if a coil isn’t up to snuff, the ignition system isn’t going to be able to provide the appropriate amount of spark that the engine needs to operate as efficiently as possible.

Ignition coils generally aren’t considered consumable parts in the same way that brake pads and spark plugs are, but they do wear out over time. And when they begin to fail, it can sometimes be difficult to hone in on the coil as the source of the issue. With that in mind, Gazda gives us a rundown of the different types of coil designs that you’ll typically encounter and how to troubleshoot them.

Single Coil Setups

The most common design that you’ll find in an old school hot rod or muscle car is the canister-style coil. Used by both foreign and domestic OEMs throughout the 1950s, ‘60s, and ‘70s, a single coil was used to provide charge to all of the engine’s spark plugs. “You have a series of internal windings sitting in oil, the latter of which provides cooling,” Gazda tells us. “It’s a very simple design.”

Toward the end of the 1970s, General Motors introduced a new High Energy Ignition (HEI) system design that quickly supplanted the traditional canister-style ignition coil setup on most GM vehicles in the years that followed.

“The concept is similar to the canister-style setup in that you have one coil providing energy to all of the spark plugs, but HEI accomplishes the same goal more efficiently,” he says. “Here, the coil is smaller and mounted on top of the distributor cap, and the winding is now in a potted gel-type substance. This improves packaging in the engine bay because now the coil doesn’t take up as much space. And you also have the added benefit of replacing the mechanical breaker points and the condenser with a maintenance-free design that utilizes an electronic internal module.”

HEI represented a significant step forward in the evolution of ignition system design, but as computer-controlled vehicle systems continued to evolve throughout the 1980s, GM and other major automakers began to turn their attention to more sophisticated solutions that would further improve engine performance and efficiency.

Multi-Coil Setups

Effectively serving as a bridge between the old-school single coil setups and the designs used in today’s internal combustion vehicles, the Coil-Near-Plug design replaced the traditional distributor with individual ignition coils that were mounted near each of the engine’s spark plugs to minimize the length of the high-voltage path and provide a hotter spark. This ECU-controlled setup is commonly found on GMT800-generation full-size trucks and other GM vehicles equipped with early iterations of the LS V8.

“This was a big technological jump,” says Gazda. “Now we can control ignition system performance on a per-cylinder basis. But unlike the designs of today, while we have individual coil packs for each cylinder, there’s still a spark plug wire running from the coil pack down to the spark plug.”

Today’s Coil-On-Plug setups improve on this concept by incorporating a “boot” into the coil pack design that allows the coil to connect directly to the spark plug, thereby eliminating the need for individual spark plug wires. “Coil-Near-Plug and Coil-On-Plug are conceptually very similar,” Gazda points out. “But with Coil-On-Plug, you’re eliminating that additional step from the process of delivering that high-voltage energy to the spark plug. These started to become really commonplace in domestic vehicles in the early 2000s, and are still used today.”

Troubleshooting Coil Issues

Since the coil’s primary job is to provide high-voltage energy to a vehicle’s engine, regardless of whether it’s a canister-style single coil setup or a modern coil-on-plug design, the symptoms of a failing ignition coil are generally similar. Engine misfires and fouled spark plugs are telltale signs of ignition coil problems, but it’s important to note that differences between old-school and modern ignition system designs affect the troubleshooting process.

“With canister-style and HEI-style setups, the symptoms tend to be more dramatic because you have a single coil providing high-voltage energy to all of the engine’s cylinders,’ Gazda says. “You’ll notice that the engine starts to run rich because it isn’t burning all of the fuel that’s being delivered, and misfires will be more common under load because the energy demand is greater in those conditions. And because you have one coil handling the distribution of high-voltage energy for the entire engine, a failing coil can also make it difficult to start the engine because you don’t have other coils in the system that can ‘pick up the slack,’ so to speak.”

These circumstances can make to tough to determine whether or not the coil is the source of the issue just by observing the engine’s behavior. “A bit of trial and error is not uncommon with these setups. You should always inspect your spark plug wires first to make sure there aren’t any melted or compromised sections and things like that, and also check the spark plugs themselves to make sure they’re in good shape. But because of the type of system we’re working with here, there are a lot of other things that can end up being the problem – it could be an issue with the distributor, the timing could be off, and so on. So it quickly becomes more of a process of going through the ignition system components one by one to find the origin of the problem. Oftentimes at the track, you’ll notice that folks with those canister-style coils will just bring an extra one with them, and they’ll try swapping that out first in order to determine whether or not the coil is the source of the problem.”

Although devices like the MSD 6AL normally do not play a role in the troubleshooting process for these types of coils, Gazda says that there are exceptions to this rule.

“Capacitive discharge boxes typically do not have a way of knowing whether or not a coil is properly responding to the signal that the box is sending. That being said, there are a few boxes that are aimed at high-end racing applications, which can give you an indication of what’s going on if you’re using a coil that supports this functionality. If you use an MSD Ultra 7AL with a current-sensing coil like the HVC-3 Series, for example, the box can actually detect the level of current that the coil is delivering, and if it’s not in the expected range, a diagnostic LED will light up on the box.”

Modern coil-near-plug and coil-on-plug systems, meanwhile, increase the number of potential culprits to consider in the troubleshooting process, and failures can be more difficult to detect because a failing coil will only affect one cylinder rather than all of them. But thanks to the wonders of modern electronics, it’s typically a much more straightforward affair.

“More often than not, a failing coil pack will trigger a Check Engine Light to come up on the gauge cluster,” says Gazda. “And when that happens, you’d just plug a diagnostic tool into the vehicle’s OBD-II port and check the code. That code will usually indicate a misfire on a specific cylinder. And from there, the easiest way to determine if the coil pack has failed is to swap that coil pack with one that’s connected to another cylinder of the engine and see if the problem follows the coil pack to that other cylinder. If it does, you know that the coil pack needs to be replaced. If it stays at the original cylinder, the coil pack most likely isn’t the problem.”

If the diagnostic code can’t be read for whatever reason, you can also use a thermal gun to check the temperatures of the primary header tubes leading away from each cylinder; the one with the bad coil pack will be significantly cooler than the rest. An inline spark tester can also be used to determine whether or not a coil pack is delivering the proper amount of spark to a given plug.

Once you’ve pinpointed which coil pack has gone bad, simply ditching it for a high-quality replacement should be enough to get you back on the road. But if your budget allows for it, Gazda recommends replacing the entire set at the same time.

“Replacing just one won’t cause any problems with the ignition system. But you also have to consider the fact that if the coils that are currently on the engine are original (or were replaced as a set a while ago) and one of them is now beginning to fail, the other ones probably aren’t far behind. You don’t hear about people replacing one spark plug at a time, and it’s wise to adopt a similar mindset here. It’s also worth noting that there’s a cost savings if you buy coil packs as a set from us. And just from an aesthetic standpoint, I know it would drive me crazy every time I popped the hood if there was one coil pack that didn’t match the rest of them.”

He also notes that anyone with other ignition system questions can reach out to MSD directly at 1-866-464-6553 to get reliable information on the subject. “You’re not going to get some scripted customer support line. These are folks who assist people with technical ignition system issues day in and day out.”

Since 1970, MSD has been leading the charge in high-performance ignition technology, delivering innovative solutions that push the boundaries of power and reliability. Trusted by top-tier racers across the most demanding forms of professional motorsports, MSD ignitions are the go-to choice when performance is non-negotiable.