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All About Mustang Spark Plugs and Ignition Components

Written By: Josh Honeycutt

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To perform at its peak, your Mustang needs to have spark plugs that are in good condition and up to the task. Make sure you select a set that is going to last and can handle the power your Mustang is putting out.

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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.

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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 & Gapping

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 .053 inches. The stock range can be from .049-.053, but anything past that will need adjustment. In recent years, many aftermarket companies have researched and developed better spark plugs. These plugs allow for a more complete and efficient burn. 

Consult the chart below for appropriate gaps for your model year. Gaps for boost are for psi between 10-15. For those of you running 17psi or more, you will want to have a smaller gap. The main goal is to have the largest gap possible without having spark blow out. Always start with the largest gap setting and work down.​

1979-1993 (Foxbodies)0.049-0.0540.028-0.034
1994-2004 (New Edge and SN95)0.052-0.0550.032-0.035
2005-early 2006 (S197)0.040-0.0500.032-0.035
Late 2008-2010 (S197)0.052-0.0560.032-0.035
2011-2018 (Coyote engine)0.049-0.0530.028-0.032

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.

NOTE: 2005-2010 GT Spark Plug Removal

The Ford 3 valve modular motors utilized a unique 2 piece style spark plug which has caused issues when removing the plugs from a high mileage vehicle as they are prone to cracking. The plugs look like this:

Used 2005-2010 GT Mustang Spark Plug Set
115K miles on 2006 GT Spark Plugs

Tools required: Ratchet with various sockets, spark plug socket, torque wrench set to 33 lb-ft, 10 inch extension, PB Blaster, air compressor with blow gun attachment

Step 1: Make sure the engine is COLD. Let it sit overnight if need be. Some people argue the engine should be warm, but when it's warm the aluminum of the head expands and makes the hole the plug sits in tighter. In a cold engine situation the aluminum has not made the hole tighter and the metal threats on the plug have not expanded either.

Step 2: Remove all plugs and ignition coils to access the spark plugs. Once removed take the blow gun and blow the debris out of the holes.

Step 3: Use your torque wrench with the extension and spark plug socket and try the plugs. DO NOT TORQUE THE PLUGS OVER 33 FT LB AND GO VERY SLOWLY. DO NOT USE AN IMPACT. If it will move, move the plug about 1/8-1/4 turn and stop. With luck, one or two of the plugs will come loose completely and you can remove it. 

Step 4: In the holes that still have the plugs in them, whether they were able to be partially turned or not, pour some PB Blaster, Seafoam deep creep, or some other penetrating fluid and let the plugs sit for 15-30 minutes.

Step 5: After waiting tighten the plugs back while not torquing over 33 ft lbs and then try loosening again. Repeat this process with stubborn plugs until they come out. DO NOT TRY AND FORCE A PLUG, IT WILL CRACK!

Step 6: Apply dielectric grease to top of plug and anti-seize to the threads if you want and reinstall plugs, again not torquing over 33 ft-lbs. Reinstall ignition coils and plug everything back in.

Check all connections of the ignition coils and injectors and start engine. Some smoke may come out from burning carbon buildup from the plugs and from the penetrating agent. If one of the plugs cracked, refer to Ford TSB 08-7-6 which illustrates how to use the motorcraft broken spark plug removal tool.

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