I did a little research on spark plugs recently and after visiting a number of different reputable websites including:
...and reading through many of the contained articles and numerous forum posts, I've gathered enough information to make what I feel is an informed decision about which plugs to buy for my own car. By posting this, my intent is not to take credit for the information (much of it was copied from the above sites and hopefully I didn't forget to mention any), but simply to gather as much up-to-date information about spark plugs as I can that is relevant to WRX/STis and put it in one place. And as always, if I've omitted any significant information or made any horrible mistakes, please let me know and I'll be happy to fix them. Otherwise, enjoy!
Just about every spark plug currently available can be found here: NGK Spark Plugs-Denso Spark Plugs-Champion Spark Plugs-Bosch Spark Plugs-Autolite Spark Plugs
Following is a list of popular spark plugs that I found are currently being used:
2002-2005 WRX (2.0 L Turbo Engine)
NGK/Copper/BKR7E/BKR7ES/one step colder
NGK/Iridium/BKR7EIX/one step colder
Denso/Iridium/IK22/one step colder
2006+ WRX and 2004+ STi (2.5 L Turbo Engine)
(including 3rd gen. 2008+ WRX/STi)
NGK/Copper/LFR7A/one step colder (some claim this plug exists, but it is hard to find)
NGK/Iridium/LFR7AIX/one step colder
Denso/Iridium/IKH22/one step colder
NGK vs. Denso
- Denso (1949) is a newer company than NGK (1936)
- “24” on Denso’s heat range scale is the equivalent of “8” on NGK’s scale
- Construction of NGK and Denso plugs is similar
- Denso plugs have a smaller center electrode (0.4 mm – claimed to be the smallest in the world) than NGK plugs (0.6 mm)
- The narrower the center electrode, the lower the potential difference (voltage) that is required to jump the gap between center and ground electrodes
- Denso plugs require a lower ‘spark jump’ voltage than NGK plugs
- The smaller diameter of the Denso center electrode means that less wear can take place before the plug’s optimum service life is exceeded
- NGK plugs are more popular than Denso plugs
- Both NGK and Denso are more popular than any other brand
- Denso plugs are more fragile than NGK plugs and don’t last as long
- The Iridium electro-tip on Denso plugs is easy to knock off which can lead to detonation
- If the engine is Japanese, use NGK plugs
- Buy the cheapest plugs (any brand) you can find
- Denso plugs utilize a ‘u-grooved’ ground electrode which is claimed to aid in the production of a ‘flame core’ which further improves the combustion process
- In theory, all the benefits offered by NGK Iridium plugs are further improved by the Denso Iridium plugs, except service life
Stock Heat Range or One Step Colder?
- Selecting a spark plug with the proper heat range will ensure that the tip will maintain a temperature high enough to prevent fouling, yet be cool enough to prevent pre-ignition.
- The spark plug heat range has no relationship to the electrical energy transferred through the spark plug
- The heat range of a spark plug is the range in which the plug works well thermally
- Spark plugs don’t create heat
- A heat range refers to how much heat a spark plug is capable of removing from the combustion chamber
- The major structural difference affecting the heat rating is the length of the insulator nose. A hot type spark plug has a longer insulator nose. The insulator nose of a hotter spark plug has a longer distance between the firing tip of the insulator, and the point where insulator meets the metal shell. Therefore, the path for the dissipation of heat from the insulator nose to the cylinder head is longer and firing end stays hotter. The insulator nose of a hotter spark plug also has a greater surface area that is exposed to more of the ignited gases and is easily heated to higher temperatures. A colder spark plug functions in an opposite manner.
- The optimal firing end temperature is approximately between 500 deg. C (932 deg. F) and 800 deg. C (1472 deg. F). The two most common causes of spark plug problems are carbon fouling (<450 deg. C) and overheating (>800 deg. C)
- Causes of Carbon fouling = continuous low speed driving and/or short trips, spark plug heat range too cold, air-fuel mixture too rich, reduced compression and oil usage due to worn piston rings/cylinder walls, over-retarded ignition timing, and ignition system deterioration.
- Carbon fouling occurs when the spark plug firing end does not reach the self-cleaning temperature of approximately 450 deg. C (842 deg. F). Carbon deposits will begin to burn off from the insulator nose when the self-cleaning temperature is reached. When the heat range is too cold for the engine speed, the firing end temperature will stay below 450 deg. C and carbon deposits will accumulate on the insulator nose. This is called carbon fouling. When enough carbon accumulates, the spark will travel the path of least resistance over the insulator nose to the metal shell instead of jumping across the gap. This usually results in a misfire and further fouling.
- If the selected spark plug range is too cold, the spark plug may begin to foul when the engine speed is low or when operating in cold conditions with rich air-fuel mixtures. In some cases, the insulator nose can usually be cleaned by operating the engine at higher speeds in order to reach the self-cleaning temperature.
- Causes of overheating = spark plug heat range too hot, insufficient tightening torque and/or no gasket, over-advanced ignition timing, fuel octane rating too low (knock is present), excessively lean air-fuel mixture, excessive combustion chamber deposits, continuous driving under excessively heavy load, and insufficient engine cooling or lubrication.
- The most serious result of selecting a heat range that is too hot is overheating. Overheating will cause the electrodes to wear quickly and can lead to pre-ignition. Pre-ignition occurs when the air-fuel mixture is ignited by a hot object/area in the combustion chamber before the timed spark event occurs. When the spark plug firing end (tip) temperature exceeds 800 deg. C, pre-ignition originating from the overheated insulator ceramic can occur. Pre-ignition will dramatically raise the cylinder temperature and pressure and can cause serious and expensive damage. When inspecting a spark plug that has experienced overheating or pre-ignition, blistering on the ceramic insulator and/or melted electrodes can sometimes be found.
- As a general guideline, among identical spark plug types, the difference in tip temperature from one heat range to the next is 70 deg. C to 100 deg. C.
- If the engine is to be operated at high RPM, under a heavy load, or at high temperatures for long periods, a colder heat range may be needed.
- Conversely, if the engine is to be operated at low speeds or at low temperatures for long periods, a hotter heat range might be needed to prevent fouling
- When using an ethanol blend fuel with high ethanol content in high performance applications, a colder heat range may be necessary.
- Advance ignition timing by 10 degrees will cause the spark plug tip temperature to increase by approximately 70 deg C to 100 deg. C. In such a case, a colder heat range spark plug may be necessary.
- Significantly increasing the static/dynamic compression ration will increase cylinder pressures and the octane requirement of the engine. Knock may occur more easily. If the engine is operated near the knock level, a colder heat range spark plug may be necessary due to the resulting increased cylinder temperatures.
- A colder heat range spark plug may be necessary due to the increased cylinder temperature as boost pressure (manifold pressure) and subsequent cylinder pressure and temperature increase.
- The WRX/STi stock temperature range is 6 on NGK’s scale and 20 on Denso’s scale
- Too much fuel or a bunch of oil in the combustion chamber fouls plugs
- The plug electrode material isn’t what gets the heat out, it’s the ceramic insulator
- Colder heat range plugs have more porcelain on the plug. The purpose is not to insulate, but to conduct. More porcelain allows more area to get hot so the plug can transfer heat directly into the head (cooling system) and lowering the cylinder temperature.
- Stock heat range plugs should always be used in a stock to slightly modified daily driven WRX/STi
- Stock heat range plugs in high horsepower engines do not have fouling problems
- One step colder plugs shouldn’t cause problems in a stock to slightly modified WRX/STi
- Plugs too cold shouldn’t foul, but may not fire correctly
- Colder plugs should be reserved for higher boost applications (say, 18-19 psi or more)
- For every 100 hp over stock, you can go one range colder
- More boost = more heat = colder plug
Spark Plug Gap
- Subaru manual recommends a 0.028-0.031” gap
- The larger the gap, the more voltage is needed to bridge the gap
- Opening gaps up to present a larger spark to the air/fuel mixture maximizes the burn efficiency
- The smaller the gap, the more likely the plug will fire
- The smaller the gap, the faster the plug will wear
- Plugs wear the fastest when there’s no spark
- Ideally, you want to run the widest gap you possibly can without misfiring
- “-11” at the end of NGK spark plug part numbers usually refers to a 1.1 mm (0.044”) pre-set gap
- Even if plugs come pre-gapped, they need to be double checked before being installed
- The more power you make, the smaller gap you will need
- Stock turbo WRXs are fine gapped to factory specifications
- 0.030” is a safe gap for every stock to semi-modded (up to Stage 2) WRX/STi
- 0.028” is a good compromise between power and longevity
- Any turbo larger than stock should have plugs gapped between 0.028-0.033”
- WRX/STis with turbos over 650 CFM should be gapped between 0.025-0.030”
Copper vs. Iridium (vs. Platinum)
- Conventional Nickel Alloy, a.k.a. “Copper” = inexpensive, best conductor, good performance
- Platinum = spark not as good as Copper, costs more than Copper
- Iridium = performance similar to Copper, longevity similar to Platinum, more resistant to fouling and oxidation than Copper, costs more than both
- The difference between “Iridium” and “Iridium IX” is the ground electrode material. “Iridium” (OEM) plugs have a Platinum tip on their ground electrode, so they cost more. “Iridium IX” (aftermarket) plugs lack the Platinum tip – they’re steel, so they cost less.
- Copper plugs require slightly more energy than Iridium plugs to fire
- Coil-on-plug systems (like on the WRX/STi) provide sufficient energy to easily fire Copper plugs, so Iridium plugs are essentially the same as Copper plugs as far as performance is concerned
- Copper plugs last 10-15k miles
- Platinum plugs last 30-60k miles
- Iridium plugs last 60k miles
- Copper plugs seem to be used by tuners as they tweak different parameters of the car and sometimes end up damaging plugs during the process since they’re cheap to replace. They’re also common in cars with high power levels since they’re likely to wear out plugs quicker than stock and slightly modded cars. They’re also for people who don’t mind changing their plugs (or who can afford to pay someone else to do it) every 10-15k miles. Changing plugs often and examining them can also be used to monitor an engine’s health.
- Iridium plugs seem to be the choice for owners who don’t want to worry about changing their own plugs (or paying someone else to change them) very often and still maintain a high level of performance
- Copper plugs are the only ones that should be used in cars with Nitrous
- Iridium plugs are better than Copper plugs in a forced induction engine because they require less spark energy to fire. As cylinder pressure increases (due to forced induction), more energy will be required make any plug fire, so Iridium plugs will be less likely to misfire than Copper plugs.
- Iridium plugs aren’t responsible for horsepower gains on dynos. They just happened to be firing correctly in cases when Copper/Platinum plugs were misfiring.
- Copper plugs are better for more aggressively tuned and boosted engines
- If you are pushing 22+ psi, consider Copper plugs because the electrode on an Iridium or Platinum plug is so small that it will continue to glow after the combustion event, setting the cylinder up for a pre-ignition issue on the next stroke.
- Copper spark plugs are your best bet
- The only real difference in plug material is service life (Iridium and Platinum plugs last longer than Copper plugs)
- If you are going to spend the money on Platinum plugs, you should opt for Iridium plugs instead because they perform better for about the same price
- Unless you're going to change your plugs yourself (read: free labor, buy Copper plugs because they offer the best performance), Iridium plugs are the best bang for your buck
Anti-seize on plugs: yes or no?
- Anti-seize is a lubricant
- Applying anti-seize to threads changes their torque values
- Torque directly affects the spark plugs’ ability to transfer heat out of the combustion chamber. A spark plug that is under-torqued will not be fully seated on the cylinder head; hence heat transfer will be slowed. This will tend to elevate combustion chamber temperatures to unsafe levels, and pre-ignition and detonation will usually follow. An over-torqued spark plug can suffer from severe stress to the Metal Shell which can distort the spark plug’s inner gas seals or even cause a hairline fracture to the spark plug’s insulator. In either case, heat transfer can be slowed.
- Most NGK plugs come from the factory with a shiny silver metal coating (Trivalent coating, a form of “anti-seize”)
- “Anti-seize” or “Anti-corrosion” coatings are applied to threads to prevent them from seizing to the head and reduce the risk of pulling out the threads
- If you reinstall a set of NGK plugs, it is recommended to use anti-seize because when you remove the plugs from the head, some of the Trivalent coating will be lost
- Many anti-seize compounds contain graphite
- Graphite corrodes Aluminum almost instantly (even more with added heat)
- WRX/STi spark plugs should be torqued to approximately 15-19 ft/lbs
- The best method is to apply a small amount of copper-based anti-seize and using the gasket-touches-plus-1/4-turn tightening method
- It is unnecessary to apply anti-seize to the threads of NGK plugs because of the Trivalent coating, but it certainly shouldn’t hurt anything unless you over torque them
- NGK does not recommend the use of anti-seize for spark plugs with special metal plating
- If you have a tuner, consult with him before buying plugs. Spark plugs are a tuner-specific item and you should take your tuner’s advice before you believe what you read on the internet.
- When installing spark plugs, compare them to your stock/current plugs. Make sure they are the same size.
- Indexing (a.k.a. “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. It is supposed to make your engine a little more efficient, but generally it is a practice reserved for racing teams. It is unnecessary on daily-driven cars.
- Pictures are:
1. Comparison of a stock heat range plug vs. a colder heat range plug
2. Comparison of the right length plug vs. the wrong length plug
3. Part number decoder page for NGK plugs