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All About Mustang Spark Plugs and Ignition Components

Written By: Josh Honeycutt

Choosing the right spark plug for you Ford Mustang requires a bit of knowledge about the plugs you're currently using, as well as the performance level of the car. When finding the best plug for your application, these are a few things you'll need to consider.

American Muscle

Facts About the Mustang Ignition System

  • Factory plugs are used for their durability and emissions qualities
  • Copper inner cores are great for conducting electricity and used in most plugs
  • Proper heat range is important to remember while choosing a spark plug. Colder plugs are recommended for force induction applications to prevent detonation
  • Heat ranges are marked by the plug’s serial number
  • Heat range dictates the plug’s best operating temperature and heat resistance
  • The heat range is based on the insulator nose in the plug
  • A larger spark plug gap is desired in a naturally aspirated Mustang, but a smaller gap is desired in a forced induction engine
  • Spark plug electrodes are made using iridium, platinum, silver, and gold. The harder the material, the more punishment they can take
  • The center electrodes or tips come grooved so the spark is more centered for more efficient combustion
  • Coil-on-plug setups provide a hotter spark for a more efficient burn
  • Upgraded coil packs can lead to better throttle response
  • Resistance in plug wires can hamper the strength of the spark
  • High performance ignition coils can stabilize idle
Foxbody Mustang Distributor with Red Spark Plug Wires

Spark Plug Overview

One of the most often overlooked items on the Mustang is the spark plug. Most Mustang owners don’t give their spark plug selection a second thought, but this can end up being a costly mistake. Ford rolled the stock 4.6 engines out with Motorcraft platinum tip spark plugs. These plugs are gapped from Ford’s factory at .055 inches. In recent years, many after market companies have researched and developed better spark plugs. These plugs allow for a more complete and efficient burn.

Motorcraft OEM Spark Plug Set - 1999-2001 Cobras and 2003-2004 Mach1s
OEM Spark Plug Set - Copper Core with Iridium Tip

Are the Factory Spark Plugs Good?

The factory plug was mainly used because of its durability and emissions qualities. Ford was required to hit 50,000 miles for the emissions warranty as dictated by the federal government. The factory plugs work quite well on stock and mildly modified cars. However, there are some gains to be made through the aftermarket. There are also situations where plugs with different heat ranges are needed.

OEM level plugs will help you regain the lost power that has slowly disappeared from your engine. They’re great for stock engines and lightly modified engines. Ford Motorcraft makes a great replacement OEM spark plug that will help to restore the lost horses.

Motorcraft 2005-2008 GT Mustang OEM Spark Plug
2005-2008 GT OEM Spark Plug

What Does Heat Range Mean?

Heat range is often a term referred to when looking at a new set of plugs, and it’s actually an important characteristic to understand. The heat range is the temperature at the tip of the spark plug electrode in a running engine. Think of it as the temperature at which the spark plug works best at, and how capable the plug is of resisting heat. The heat range does not affect the temperature of the combustion chamber directly. However, the heat from the combustion chamber can cause the temperature of the electrode to rise. Spark plugs with a lower heat range are set to be “colder” and vise-versa.

Certain plugs work better in cooler temperatures, whereas others are meant for a hotter application. It’s sort of like cooking a frozen pizza. Pizza A states it is optimal to cook it at 350C, and pizza B at 425C. Now, cooking pizza B at 350C will work too, but less effectively. The same applies to pizza A. Throwing pizza A in the oven at 425C again will work, but will probably burn if not vigilantly monitored.

Colder Set of NGK Iridium Spark Plugs
"Colder" Set of Iridium Spark Plugs

How is Heat Range Determined?

What defines a cold plug and a hot plug is the length of their insulator nose. A shorter nose offers less surface area to absorb heat and a shorter path to dissipate heat to the cylinder head. A longer nose offers the opposite – greater surface area to absorb heat and a longer path to dissipate it—resulting in the firing tip to stay hotter. Furthermore, plugs with longer insulating noses have greater exposure to the gas igniting around the tip due to their greater length (they protrude further into the ignited gas). Other factors that may affect heat range can be composition material and their thermal conductivity characteristics.

Number Four NGK Spark Plug Tip Close-Up
Number Four NGK Spark Plug Insulator Nose

The Importance of the Proper Heat Range

It is important to choose a spark plug with the proper heat range for your Mustang. For example, with forced induction a “colder” spark plug should be used. This is due to the increase in pressure inside the combustion chambers. Using a spark plug with a higher heat range will cause detonation or “pinging”. This is also the case in high combustion engines.

Spark Plug Manufactures' Heat Ranges

Most automotive spark plugs have a number indicating their heat range. This range of numbers signifies the spark plugs’ capability to resist heat. A ‘colder’ plug will not absorb heat to the same degree as a ‘hotter’ plug (meaning a ‘cold’ plug physically remains cooler than a ‘hotter’ plug). That is a pretty abstract (and obvious) way of putting it, to say the least. Thankfully, plugs are rated on a numeric scale to tell us which plug is hotter or colder than the next.

Unfortunately, each manufacturer uses their own range and description – there isn’t a standard across the board. For example, manufacturer NGK uses an inverse range from 2-11 to rate their plugs. The higher the heat range number, the more capable the spark plug is to withstanding heat, thereby it is actually a colder plug (meaning it remains cooler, it will not heat up as much). On the other hand, manufacturer Autolite uses an opposite rating system: the lower the number, the colder the plug (more resistant to heat).

Heat RangeCold

Bosch02, 0703, 08456,78
Champion55, 574, 596, 61, 637, 89, 1011, 12
Autolite (EX)254425452546

When Should I Change Heat Ranges?

Changing heat range is for heavily modified NA Mustangs or Mustangs utilizing any type of forced induction. Any supercharger/turbocharger/nitrous setup will quickly generate more heat and a colder plug is necessary. With higher pressure comes greater heat, and thus, a colder plug is necessary. Usually moving one or two ranges colder is all that is necessary to combat the greater temperatures.

For minor NA builds it is usually not a necessity in changing plugs. However, for swapping heads/camshaft(s)/building a new shortblock, changing heat ranges is not something to forget about. Major modifications require immense consideration and spark plugs are not exempt. There are many variables in order for determining a correct heat range ranging from compression ratio to engine load.

For stock Mustangs, the best plugs are stock plugs, which typically have a heat range of around 4-6, or whatever is midrange for the manufacturer in question (recall, manufacturers often use their own system).

Roush 475HP Supercharger on a 2005-2010 GT Mustang
475HP Supercharger on a 2005-2010 GT

Which Heat Range Should I Pick?

It’s important to pick the right heat range for a multitude of reasons. First and foremost, the right plug will optimize spark, and consequently air/fuel ignition (remember, igniting the air fuel mixture is what makes POWER!). Secondly, it will ensure long plug life and reliability. A plug that does not reach its optimal operating temperature (running too cold a plug) will lead to carbon build up and poor performance. Running a plug that is too hot (does not stay cool enough) can actually lead to premature detonation in the combustion chamber, which can cause severe engine damage.

  • Combustion temperature/cylinder head temperature: if you raise the combustion/cylinder head temp (via forced induction, lean fuel mixtures, advanced ignition timing, etc.) you'll want a cooler heat range plug so you don't overheat the plug
  • If you're building a high RPM engine, you'll want a cooler plug (and vice versa)
  • If the plug is too cold (too much heat resistance) carbon deposits build up on the plug resulting in incomplete combustion
  • If the plug is too hot (not enough heat resistance) you'll experience pre-ignition and in extreme cases the plug will melt
  • Test a new set of plugs by inspecting them after a half-throttle, a 3/4 throttle, and a full throttle run
Procharger Stage-2 Supercharger System Installed on a 2011-2014 GT Mustang
Centrifugal Supercharger Installed on a 2011-2014 GT

Do I Need to Retune for Hotter/Colder Plugs?

While running the correct heat range plugs for the tune it has is optimal, it is not essential to get a retune for them. Generally, if one has to change the heat range for spark plugs, there are modifications to the engine that mandate a retune.

Spark Plug Gapping Explained

The spark plug gap is the area between the ground strap and the electrode. This is where the spark jumps from the electrode to run to ground, lighting the air fuel mixture in the combustion chamber. In naturally aspirated Mustangs a larger gap is desirable as it provides a “fatter” spark. In forced induction engines the opposite is true. The gap must be reduced because of the increased turbulence and pressure in the combustion chamber. If not, the spark could be blown out causing a misfire.

Checking spark plug gaps are quite simple. The only thing needed is a gap tool which helps measure the gap itself. The easiest way to confirm the gap is to close the gap with a little bit of pressure. Then regap it with the tool itself. This not only checks the gap and reapplies it, but confirms what the gap is.

Autolite Mustang Spark Plug with a Flat Tip

What Spark Plug Cores Exist?

All automotive spark plugs have a copper core. Or to be on the safe side, 99.9% of all automotive spark plugs utilize a copper core. Whether you’re looking at a set of OEM cheapies or the top-dollar Super Power Performance Plugs, both of them have the same element at their respective cores, and that element is copper. Now why would a set of $20 plugs versus a set of $60 plugs use the same stuff down under? Well, quite simply, copper is exceptional stuff to conduct electricity, promote jumping the gap and dissipate heat. There isn’t anything better that is readily available or cost-efficient. However, the key word at play here is ‘electrode core’. Copper makes up the electrode core, but NOT the entire electrode itself. The electrode itself is composed of a core material (copper), and an outer electrode material. Spark plug manufacturers use varying outer electrode materials for different benefits, which we will look at next. So just to sum things up, when you see the term ‘copper’ thrown around, it is always referring to the inner electrode core, and the inner electrode core only.

Autolite Mustang Spark Plug Set

Spark Plug Electrodes - What Materials are Used?

The primary change in spark plug material in recent years has been in the electrode of the plug. In the past the electrodes were made of copper. So where do the crazy terms like ‘nickel-alloy’, ‘platinum’ and ‘iridium’ come from? Well, these materials are what encompass the core, and are what make up the outer electrode. The first and foremost, nickel-alloy, is the usual material of choice for OEM spark plugs. It is the cheapest, and handles its duties without issue in stock motors. The subsequent two, platinum and iridium, are harder, more durable materials, able to withstand higher heat, with the latter being the hardest of them all (supposedly iridium is 6X as hard as platinum). 

When engines are modified out of stock parameters (i.e; more power) is when these harder materials will outlast basic nickel-alloy, ensuring a strong spark over a greater period of time, and under hotter conditions. The primary advantage being they last longer and produce a more consistent spark kernel. This results in better fuel economy and better throttle response.

2011 V6 Mustang on a Dyno

What About Spark Plug Tips?

At the tip of the spark plug is the center electrode, where the spark is generated, and the ground electrode, where the spark jumps to. Conventionally, spark plugs have always had a concentric, flat tip, which work absolutely fine. However, several manufactures out there have started grooving their center electrodes such that they are no longer flat (for example, NGK V-Groove means the center electrode has a v-shaped cut/groove, in the centre of it). The concept behind this is simple. With a traditional flat tip, the spark is generated in the middle of the tip. With these grooved tips, the groove forces the spark to occur at the edge of the electrode, where manufacturers claim it is easier to ignite the fuel/air mixture, resulting in a better burn and requiring less voltage to produce the initial spark. MPG conscious vehicles often employ these types of plugs, where perfect ignition is another trick to boost efficiency.

NGK V-Power Spark Plug for 1993-2001 Cobra, 1996-2004 GT, Mach 1, and 1998-2004 V6 Mustangs
V-Groove Spark Plug

What Brands are Available?

NGK is one of the most popular aftermarket spark plug producers. They offer a great iridium spark plug for the mustang with noticeable gains in off idle throttle response. These can be picked up for around $70. The stock Ford Motorcraft plugs can be had for around $30. The E3 spark plugs resale for roughly $50 and use a nickel alloy electrode.

Brisk spark plugs rival those of NGK and offer great gains in off idle throttle response as well and come in at right around $80. Taking things to the extreme are Pulstar's Plasmacore spark plugs that use plasma-assited combustion and a nickel-chromium center for improved throttle response and and fuel economy, coming in at around $120.

Brisk Silver Racing Spark Plug for 2015-2017 EcoBoost Mustangs
Silver Racing Spark Plug for 2015-2017 EcoBoosts

Coil-on Plugs (1999-2014)

On 2005-2014 GT Mustangs, 2007-2014 GT500s, and 2011-2014 V6 Mustangs, Ford used coil-on plugs rather than the coil pack that was traditionally used. Coil-on plugs are also found in the 1999-2004 Mustang GT, Cobra, 01 Bullitt and 2003-2004 Mach 1 Mustangs. When upgrading your coil-on plug with an Accel Super Coil-On Plug Kit, you can expect 10-15% more energy than the factory OEM coils. The coil-on plugs work simply by giving you a hotter spark. The hotter spark helps to burn the fuel better, creating more power and better fuel economy.

GMS 4V Coil on Plug Pack for 1999-2004 Cobra and Mach 1 Mustangs
Coil on Plug Set for the 1999-2004 Cobra and Mach 1

Coil Packs (1996-1998 4.6L & 1994-2009 V6)

1996-1998 4.6L Mustangs and 1994-2009 V6 Mustangs all utilize coil packs in their ignition systems. We upgraded the coil pack on our 2000 V6 Mustang bolt-on build-up as well as our 2005 V6 bolt-on build-up using the Accel Super EDIS Coil Pack. Upgrading the coil pack gave us more power than the factory OEM coils could offer, which complemented our other mods and gave us peak performance and better throttle response.

1994-2009 Mustang Coil Pack

Ignition Coils (1983-1995)

1994-1995 Mustang Ignition Coils: The 1994-1995 5.0L Mustangs use an ignition coil just like the Foxbody Mustangs, rather than the coil packs listed above, to help increase throttle response, idle quality and starting power.

1983-1993 Mustang Ignition Coils: The ignition coil is found in the 1983-1995 5.0L Mustangs. Aftermarket ignition coils make more power than OEM factory coils to increase throttle response, idle quality, and starting power. It is a perfect match for stock engines and engines with minor performance upgrades.

1983-1995 Foxbody Mustang Ignition Coil
1983-1995 Ignition Coil

Mustang Spark Plug Wires - Does Thickness Matter?

Wire size matters when a vehicle is heavily modded. The purpose of a larger wire is to deliver a better spark to the engine. On stock engines, upgrading the size is truly minimal. However, on a boosted or built engine, they can make a solid difference in spark plug performance.

Stock wires can become worn and cracked, which can cause them to slip off or even let water into the engine. When putting on new spark plug wires, you’ll be upgrading the actual wire itself, which features a low resistance conductor for minimum spark loss and more power. When installing new spark plugs, it’s the best time to upgrade your other components as well. You’ll be removing your coil or coil pack and spark plug wires to access the spark plugs anyway. With a fresh system, you’ll be able to notice and feel your engine running better.

MSD 8.5mm Spark Plug Wire Set on a 1999-2000 V6 Mustang
8.5mm Spark Plug Wire Set on a 1999-2000 V6
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