Increasing Performance With Aftermarket Coil-On-Plug Ignition

There are only a few mechanical issues that could make me consider trading in a vehicle. My wife, however, has a much lower tolerance for a vehicle breaking down. It usually takes just two incidents before I start receiving a flood of marketplace ads for newer vehicles. If you’re unfamiliar with the current car market, let’s just say that both interest rates and price points are beyond what I’m willing to pay, especially considering I need room for a family of six. So, when our family hauler, a 2011 Ford Expedition, started experiencing ignition issues, and the wife noticed, I knew I had to act swiftly.

The Spark Is Gone

Several months ago, I began noticing the check engine light coming on after aggressive acceleration. This typically occurred when the tachometer needle exceeded 5,000 rpm. Although the engine didn’t feel like it was misfiring, the recurring trouble code was always P0308. To spare you the Google search, P0308 indicates that the powertrain control module (PCM) detects a misfire.

I found a way to keep it off of my honey-do list for yet another day, by simply using an SCT programmer to turn off the check engine light. My tactic worked well, until it didn’t. The ignition coils of the 5.4-liter Triton engine decided they’d had enough of my procrastination, leaving my wife to deal with a sputtering mess at partial throttle and me a well-deserved earful.

After the lecture concluded, it was time to replace the ignition coils, a task I should have tackled months earlier. But before grabbing an original equipment equivalent at the local parts store, I wanted to see if upgrading to performance coils might positively impact a non-performance-oriented engine. So, I reached out to Brian Caruth at Performance Distributors.

Understanding the Coil-On-Plug Ignition System

“A coil-on-plug ignition system is a relatively simple engineering design derived from earlier distributor-based ignition systems. It consists of a housing with primary and secondary windings inside, similar to a coil. Attached directly to the coil and spark plug is a boot,” Caruth explains. “Inside the boot, there’s a spring that establishes a connection to deliver voltage to the spark plug.”


This crude cutaway illustration displays the coil windings within the head of the coil-on-plug. A rubber boot is used to conceal the spring that establishes the ultimate connection between the coil and the spark plug.

It’s no surprise that modern vehicles rely on direct ignition technology like coil-on-plug for spark generation. Not only does it integrate the coil, distributor, and spark plug wires into a single electronic unit that the PCM can diagnose, but it’s also more efficient due to reduced energy loss from coil to plug. Moreover, instead of relying on a single coil to transmit voltage through a distributor cap’s brass pickup and a spark plug wire, the coil-on-plug system provides each spark plug with its own coil making direct contact with the plug. 

Long before the advent of the coil-on-plug setup, the distributor ignition system was considerably more prevalent. This system depended on a single coil to transmit voltage to the distributor. As the distributor's shaft rotated, it would direct voltage to the spark plug wire of each cylinder, positioned on the distributor cap, before ultimately reaching the spark plug. This process was simple and effective, but lacked efficiency and broader range of voltages that could be controlled through and diagnosed by a PCM.

Advantages Of Aftermarket Coil-On-Plug Systems

The coils equipped on the Ford Expedition in question from the factory provide roughly 25,000 volts per cylinder. A number so impressively high that I had trouble imagining the need for an upgrade, regardless of my original inquiry. However, in the case of spark generation, it turns out that more is indeed better, and with Performance Distributors’ Sultans of Spark coils offering a 60-percent increase, I was eager to learn about the advantages and how they’re achieved.

“Because of the higher voltage output, we recommend using a larger spark plug gap in a naturally aspirated setup,” Caruth says. “This exposes more of the spark kernel to the fuel mixture, leading to improved combustion, fewer hydrocarbons in the exhaust, and increased power. This translates to quicker starts, smoother idling, and overall better performance from idle to maximum RPM.”


Performance Distributors’ Sultan Of Spark coils not only boasts a remarkable 60-percent increase in voltage compared to stock, but also allows me the ability to increase the spark plug gap by ten thousandths (.010) of an inch. The outcome is a more comprehensive combustion cycle that not only conserves fuel but also amplifies power output.

While a performance boost is always welcome, the fuel-thirsty Three-Valve engine concerned me more than horsepower gains. After all, making a 6,000-plus-pound full-size SUV accelerate a bit quicker is a challenge I’m not quite ready to tackle. Fortunately, the principle of optimizing the spark plug gap to enhance the combustion process offers a more efficiently running engine, increased horsepower, and, crucially, improved fuel economy.

What Causes of Coil-On-Plug Failure (…And How Can It Be Prevented)

It would be nice to know the exact causes of coil failure, but unfortunately, that luxury eludes us. In my case, I was inclined to attribute it to time and aging, but only a post-mortem examination would provide a general idea of the underlying issues. The truth is, the answer will vary case by case, and electronic components’ lifespans are never set in stone. After all, these are electronic parts and should likely be regarded as maintenance items more than anything else.


While it might be challenging to discern in the photos, the cylinders towards the rear of the engine (on the far right) have evidently borne the brunt of the lack of airflow, leading to noticeable consequences. As you might have observed, there are only seven spark plugs present. This is due to the fact that the final one caught an early flight out of the garage when it chose to relinquish a portion of its ceramic section during removal, leading me into an impromptu fishing trip into the cylinder.

According to Caruth, “There’s no definitive answer to what causes coil failure. It could be heat-related issues over time, as the electronics inside the plug may deteriorate due to repeated heating up and cooling down. Generally, coils are quite durable and require a significant amount of stress to fail. Over time, specific components such as electronics, primary windings, or secondary windings might experience failure, but heat is typically considered the main suspect.”

So, how can you mitigate the likelihood of heat-induced coil failure? Well, according to Caruth, the best approach is to ensure that your engine bay cooling is functioning properly, fans are operational, and cool air is flowing through the engine compartment. However, even with these precautions, there’s not much that can be done for preventive maintenance, apart from using dielectric grease to prevent the boots from sticking to the plug.

Performance Through Ignition Results

With this knowledge, I decided it was worth spending the extra money to upgrade to better-performing aftermarket coils. The installation is incredibly straightforward since each coil is externally identical, making it a plug-and-play operation. As is customary, though, no act of good goes unpunished. Six out of the eight spark plugs were stuck in the spark plug threads, with two surrounded by broken ceramic pieces. To say that the job grew substantially in complexity would be an understatement.

The outcome of my fishing expedition? Well, this small culprit could have damaged the cylinder walls, kept a valve propped open, or subjected me to further troubles within my marriage. Needless to say, it’s always wise to have a borescope in your toolbox for such situations.

However, once I inspected the cylinders for any unwelcome external deposits using a borescope and received the all-clear, I was able to start the 5.4-liter engine. It purred with a smooth idle, and the throttle response was quicker. The previous sputtering mess could now be driven at partial throttle without the fear of stalling at intersections.

Performance Distributors managed to shave off 0.84 seconds from a 0-60 mph test in an F-150 equipped with the same Triton powerplant. I decided to conduct my own 0-60 mph test and achieved a time of 9.17 seconds with a gentle launch, which is respectable considering the weight and the neutered, factory-limited throttle ramp. I could claim that this was all done on a closed course, but let’s be honest here — evaluating an Expedition’s performance is akin to the Spanish Inquisition, no one expects it. Alas, instead of an element of surprise, it’s just a heavyweight gaining momentum.

With my performance assessments complete, it was time to deplete my bank account and fill up the fuel tank for some mileage testing. Generally, the brick-shaped SUV barely, if ever, manages to exceed the 14 mpg range. This might be due to a combination of poor aerodynamics and a heavy foot, but there is room for improvement. Ultimately, I averaged slightly over 15 mpg on my most recent fill-up. Coincidence or a difference in driving? Maybe, but I’ll take it as a win regardless. 

Overall Impressions

I’m all for improvements, so why not? If an original equipment or OE-equivalent replacement can restore the Expedition to its original fuel economy and performance levels, why not invest in something that can enhance these targets along the way? While the Expedition is far from being considered a performance vehicle by anyone’s standards, having performance coil-on-plugs installed on it, or any daily driver for that matter, seems like a no-brainer. Especially when the investment will ultimately pay off in fuel savings.

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About the author

James Elkins

Born into a household of motorsport lovers, James learned that wrenching takes priority over broken skin and damaged nerves. Passions include fixing previous owners’ mistakes, writing, and driving.
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