WRX Info

Frequently Asked Questions

Engine FAQ: Spark Plugs

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 nickel alloy (commonly referred to as "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.

What's some good background spark plug information?

Materials: The three main types of spark plug materials are nickel alloy, iridium, and platinum. Copper can be used in the core all plugs.

All ground electrodes are made of nickel. The use of Platinum and Iridium, which are stronger, allow for much finer CENTER electrodes (the ground electrode is still Nickel). These finer electrodes do not quench the flame core as much as a conventional style plug. This increases ignitability, therefore increasing HP. It's not a huge gain, but cylinder pressures are measurably higher.

Platinum or iridium can be used as a thin pad which is laser welded on the ground electrode (the "J" strap), this serves to increase the life of the plug.

Heat Range: Heat range deals ONLY with the temperature of the insulator of the plug (the white part). We typically only care about the temperature of insulator, as that is what gets hot enough to pre-ignite, and consequently, that’s what gets measured.

Gap: Gap has a lot to do with ignitability. The bigger the gap, the better combustion. HOWEVER, this comes at a cost. The bigger gap requires MORE voltage to spark, putting a much higher strain on your ignition system. With a turbo charged motor, the gap is usually smaller. This is due to the fact that higher cylinder pressures make it harder for the plug to fire, thereby increasing voltage requirements (this is why 2.5L NA is 0.044" gap and the 2.5L turbo is 0.030" gap). Can your ignition system handle the increased load?? Good question.

Corrugations: Ever wonder why those ribs are on a spark plug? They serve to increase the di-electric strength of the spark plug. The high voltages of modern ignition systems could cause an arc, known as flash-over, from the terminal of the plug to the metal shell of the plug. The corrugations essentially increase the surface distance from the tip to the ground of the plug.

Multi Ground strap plugs: Ever wonder what good the multiple ground strap plugs, such as bosch platinum +4, are good for? These plugs have a bit longer life and a higher fouling resistance. However, they can have a decrease in ignitability, reducing performance slightly.

Knocking and Preignition: These are SEPERATE events. Knocking occurs when the combustion flame happens much too early, causing the flame front to "slap" the piston. Preigntion occurs when the spark plug (or something else in the cylinder) gets too hot and ignites the air/fuel mixture before the spark does. This can happen only a couple of degrees before the spark plug fires, so it is possible to have pre-ignition and not know it. Pre-ignition can, and usually does, eventually lead to knocking.

Resistor (The "R" printed on the insulator) Many spark plugs have a special conductive glass seal between the center electrode and the terminal stud. This seal acts as a resistor in the plug which reduces the transmission of pulses of energy to the ignition cables. These pulses can cause Radio Frequency Interference (RFI) with electrical components in the car. For some newer cars, resistor plugs are required for effective communication between the plugs and the electronic ignition.

Radio Frequency Interference (RFI): When the spark plug creates a spark, a high frequency burst of energy is created. If this energy was to travel through the ignition wires, it could cause interference with other sensitive electronic devices, such as the radio or electronic control units. The resistor in the spark plug reduces this energy before it causes interference with other electrical components.

Fouling: Fouling occurs when a plug becomes contaminated with fuel, oil, or other contaminates that prevent the plug from generating a spark. Most plugs today are designed to reduce fouling and become self-cleaning when they reach 500 degrees Celsius. However, short trips, low speed driving, improper spark plug heat range (too cold), improper timing, or an oil leak in the combustion chamber can cause a spark plug to become fouled.

Bridging: Over time, contaminants can build on the surface of the spark plug if it does not reach self-cleaning temperature often. These buildups can grow between the electrodes until they are actually connected by a bridge of contaminants. This will often cause misfire.

Flashover: Occurs when the spark does not jump between the electrodes within the combustion chamber, but instead jumps between the metal shell and the terminal on top of the plug. This will always cause a misfire since the air/fuel mixture will not be ignited

Quenching: The purpose of a spark plug is to introduce enough heat into the combustion chamber to initiate a smooth burn of the air/fuel mixture. Quenching occurs when that heat generated by the spark is reabsorbed back into the ground electrode, the center electrode, and the ceramic insulation.

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 plugs? 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.

So really...do I need colder spark plugs? Probably not as in the end it won't make enough difference to be felt. But if you want the bling of saying you have iridium one step colder plugs and you enjoy the work, knock yourself out.

Do I need to consult with my tuner about spark plugs? Yes. Items in your car that can be tuner specific: manual boost controllers, electronic boost controllers, upgraded wastegates, restrictor pills, wastegate helper springs, external wastegates (type/size/brand), spark plug types/brands/gaps, and injector types/brands/sizes. ALWAYS take your tuner's advice on these matters no matter what the internet tells you!

So how do colder plugs actually remove heat?

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 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
IWSTI.com instructions

What is the #1 spark plug fault? Improper torque when installing them. If you under torque a plug, you could destroy your engine due to reduced heat transfer at the threads, thus causing the plug to overheat and pre-ignite. This is actually common due to the gasket making the plug feel tight when its not. Use a torque wrench each and every time! There are other methods to do this, but they should only be used in an emergency or by people who have a sense of feel and experience.

What is the torque specification on spark plugs? 15-19 ft-lbs. 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. And for you super anal retentive and smart folks, when using anti-seize compound on spark plugs, you can effectively reduce the torque needed by 1/3, but at these low torque levels it's really not going to matter.

STi + 2.5L turbo owners:

The correct spark plug for you is SOA part number 22401AA630 AKA NGK ILFR6B. A substitute might be LFR6A-11 or LFR6AIX-11 if you want iridium.

Denso Iridium IKH22 is the correct sized one step colder plug as seen on this thread.

NGK iridium LFR7AIX, is the correct sized one step colder plug as seen on this thread. Or one could use the NGK iridium ILFR7H if you change the gap.

Currently no other manufacturer makes aftermarket spark plugs designed for the STi + 2.5L turbo. 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 + 2.5L turbo plugs and the aftermarket ones.

This post contains more info on STi + 2.5L turbo spark plugs.

WRX owners (2002-2005):

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

NGK/Copper/BKR7ES/WRX/one step colder
NGK/Copper/BKR7E/WRX/one step colder
NGK/Platinum/BKR7EGP/WRX/one step colder
NGK/Platinum/PFR6G/WRX/stock <----OEM plug
NGK/Platinum/PFR7G/WRX/one step colder
NGK/Iridium/BKR7EIX/WRX/one step colder
Denso/Iridium/IK22/WRX/one step colder
Champion/Copper/RC9YC4/WRX/one step colder
Autolite/Copper/3922/WRX/one step colder
Autolite/Platinum/AP3922/WRX/one step colder
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 2002-2005 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.****