Spark Plug FAQ

What type of spark plugs should I run in my car? Refer to your owners manual for recommendations. Alternately, you can visit an auto parts store or online retailer for recommendations on suitable spark plugs designed for your vehicle. Major manufacturers are:
a. NGK
b. Bosch
c. Denso
d. Autolite
e. Champion

Who are the specialty spark plug manufacturers? These manufacturers make specialty plugs that have unique compositions or designs that claim increases over traditional plugs. They are listed for advanced users or those with interest.
a. Torquemaster
b. Beru (specifically the Silverstones found here)
c. SplitFire
d. PREP spark plugs
e. E3 spark plugs
f. Pulstar plugs

What types are there? There are really three main types:
a. conventional (copper)
b. platinum
c. iridium

Which type should I use? That depends on how often you are interested in changing the spark plugs. Conventional spark plugs generally last one year. Platinum or iridium can last, depending on manufacturer specifications, up to seven years.

How often should I change my spark plugs? Refer to and use the maintenance schedule in your owner’s manual when using stock plugs. For other plugs, refer to the plug manufacturer’s recommendation.

Are there any special considerations for nitrous users? Zex has a line of spark plugs specifically designed for nitrous applications. Some claim that platinum spark plugs should not be used in vehicles equipped with nitrous.

Which type produces the most power? This is a hotly debated topic with few consistencies. Conventional wisdom says that copper spark plugs produce more power than the longer life platinum or the newer iridium. In the end, there really is no concrete evidence one way or the other as any dyno testing of spark plugs results in differences that are far less than the dyno tolerances from run to run.

What is the gap specification for my plugs?
Refer to your owners manual. In the case of some iridium plugs and certain plugs by other manufacturers, they will come pre-gapped. These certain type of plugs are not designed to by gapped by the end user. This post is very informative on proper spark plug gapping theory.

How do I gap my plugs? To decrease the gap, tap the ground electrode onto a hard surface. This should be a slow process as you are removing thousandths of an inch. Too hard a tap to start off with could damage the center electrode. To increase gap, use gapping wire or feeler gauges. Many parts stores also sell cheap 99¢ circular spark plug gap key chains. These should be avoided as they leave a slight angle to your gap. For the ultimate gapping tool, Jacobs Electronics makes a professional gapping tool. Some manufacturers’ spark plugs are not designed or intended to be gapped. If you purchase one of these types, do not re-gap these plugs to factory specifications!

What is “indexing” and how much more power will I get? Indexing (AKA “degreeing”) refers to how the spark plugs are installed so that the ground electrode is oriented to face the intake valve in an effort to “open up” the spark to the incoming air/fuel charge. This is accomplished by placing a washer underneath the spark plug’s shoulder area (of a specified thickness) so that after properly torquing the spark plug, the electrode would be pointed in the desired direction, usually towards the incoming air/fuel charge from the intake valve. It is important to note that any power increase will be small and the results can only be determined after extensive dyno testing. It is generally a waste of time unless you have the training or the dyno facilities to document the results.

Do I need a colder spark plug? A rule of thumb is one heat range colder for every 75–100hp you increase. In identical spark plug types, the difference from one full heat range to the next is the ability to remove 70°C to 100°C from the combustion chamber. The heat range numbers used by spark plug manufacturers are not universal, so a 10 heat range in Champion is not the same as a 10 heat range in NGK or in Autolite. When making the decision to change the heat range of your plugs, it is a wise idea to make changes in 1 heat range increments, checking all your plugs for suitability during your test. Vehicle modifications during the test phase of your plugs is not recommended as this may skew your results.

How do I troubleshoot my spark plugs upon removal to diagnose problems?
a. .pdf document by Autolite
b. .pdf document by Bosch
c. html page by SplitFire
d. html page by Denso

How do I replace my spark plugs?
scoobymods.com instructions (with photos)
scoobymods.com instructions (with photos)
scoobymods.com supplemental information about WRX plug coils

What is the torque specification on spark plugs? 15-19 ft-lbs. Whenever possible, it is recommended that the spark plug should be installed by using a torque wrench. If a torque wrench is not available, tighten hand tight. Then tighten with a standard socket wrench and additional 1/4 to 1/2 of a turn.

STi owners:

The correct spark plug for you is SOA part number 22401AA630.
Denso Iridium IKH22 is the correct sized one step colder plug as seen on this thread.
NGK LFR7AIX (#2309) is the correct sized one step colder plug as seen on this thread.

Currently no other manufacturer makes aftermarket spark plugs designed for the STi. Many use aftermarkets anyway, but if you look at the pictures on this thread, you can see there is a pretty sizeable difference between the STi plugs and the aftermarket ones.

This post contains more info on STi spark plugs.

WRX owners:

Plug Maker/Type/Part Number/Car Type Used/Heat Range

NGK/Copper/BKR6ES/WRX/stock
NGK/Copper/BKR7ES/WRX/one step colder
NGK/Copper/BKR6E/WRX/stock
NGK/Copper/BKR7E/WRX/one step colder
NGK/Platinum/BKR6EGP/WRX/stock
NGK/Platinum/BKR7EGP/WRX/one step colder
NGK/Platinum/PFR6G/WRX/stock <—-OEM plug
NGK/Platinum/PFR7G/WRX/one step colder
NGK/Iridium/BKR6EIX/WRX/stock
NGK/Iridium/BKR7EIX/WRX/one step colder
Denso/Iridium/IK20/WRX/stock
Denso/Iridium/IK22/WRX/one step colder
Champion/Copper/RC10YC4/WRX/stock
Champion/Copper/RC9YC4/WRX/one step colder
Autolite/Copper/3923/WRX/stock
Autolite/Copper/3922/WRX/one step colder
Autolite/Platinum/AP3923/WRX/stock
Autolite/Platinum/AP3922/WRX/one step colder
Autolite/Platinum/APP3923/WRX/stock
Autolite/Platinum/APP3922/WRX/one step colder

Denso Spark Plug Locator
NGK Spark Plug Locator
Autolite Spark Plug Locator

****PFR7B (NGK Part Number 4853) is the correct one step colder plat plug for the WRX no matter what their website says. The out of the box gap should be at or close to .030. I called the NGK USA toll free # today to find this out and they said they are aware that their site sucks butt and will update it “soon”. Info as of April 2005.****

Editors Note

This post was created because I wasn’t able to find a good spark plug FAQ. I came up with the text based on LOTS of searching here and the internet. Upon reading this you should have an idea of what type of spark plug best suit your needs.

If you find an error in this FAQ, please PM me with factual details and I will update this post. If you feel this post is missing an important or common spark plug information item, let me know and I will research it and update this post.

One Response to “Spark Plug FAQ”

  1. NGK/Copper/BKR7E-S are highly recomended for those who are running an untuned JDM ej20 motor, with limited availibility to 100 octane pump.

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