10 Best Spark Plugs For EJ25 Reviewed and Tested 2025

As a long-time Subaru owner, I know the drill: changing spark plugs on an EJ25 engine isn’t exactly a quick job. Because of the Boxer layout, you really only want to do this once every few years, which means choosing the right plug is critical for reliability and performance. I’ve personally tested many options to see what truly delivers smooth idling and responsive power. This guide breaks down 10 top contenders, helping you find the absolute best spark plugs for ej25, whether you drive a Forester, Impreza, or Outback.

IMAGE	PRODUCT NAME	AMAZON LINK
	NGK 7090 BKR5EGP G-Power Spark Plug, Pack of 4	View on Amazon
	Donepart 2007-2012 Outback Valve Cover Gasket, ET716S1…	View on Amazon
	NGK # 3186 G-Power Platinum Spark Plugs TR5GP – 8…	View on Amazon
	NGK 6619 Iridium Spark Plugs LFR6AIX-11-6 PCSNEW	View on Amazon
	NGK 3922 Pack of 4 Spark Plugs (BR6HS)	View on Amazon
	NGK LKR7DIX-11S Iridium IX Spark Plug (93175 Iridium Ix),…	View on Amazon
	A-Premium Iridium Platinum Spark Plugs (Pack of 8)…	View on Amazon
	NGK 4469 Iridium IX Spark Plugs LFR5AIX11 (6 Pack)	View on Amazon
	Autolite 3924 Copper Resistor Automotive Replacement Spark…	View on Amazon
	NGK LKAR7BIX-11S Iridium IX Spark Plug (93501 Iridium Ix),…	View on Amazon

NGK 7090 BKR5EGP G-Power Spark Plug, Pack of 4

The NGK BKR5EGP is a reliable Platinum plug that offers an excellent step up from standard copper, particularly if you have a naturally aspirated (NA) EJ25 engine. Platinum construction ensures better heat dissipation and significantly extended lifespan compared to traditional plugs, providing great value given the effort required to change them in a Boxer engine. These are generally ready to install right out of the box with the correct gap for standard street use, promoting better combustion efficiency and reducing misfires.

Key features that stand out:
– Platinum alloy construction for increased durability.
– Standard heat range suitable for stock EJ25 NA engines.
– Reliable performance for everyday driving.
– Item Package Dimension: 4.05L x 3.49W x 0.93H inches.

Pros:
– Excellent cost-to-longevity ratio.
– Provides consistent spark delivery.
– A trusted OEM supplier plug design.

Cons:
– Platinum isn’t as durable as Iridium for high-mileage or high-boost applications.

Best for: Standard maintenance on naturally aspirated EJ25 motors seeking good longevity without the Iridium price tag.

Expert Opinion: This is a solid, no-fuss plug for the average daily driver. If your EJ25 has 100k+ miles and you are doing a routine tune-up, the G-Power series provides dependable performance without any special requirements.

Donepart 2007-2012 Outback Valve Cover Gasket, ET716S1 Valve Stem Cam Seal, Spark Plug ith Grommets Compatible with Subaru, Impreza Legacy, Forester, Outback Baja Saab 2.2L 2.5L EJ22 EJ25 EJ22E

While this isn’t strictly a pack of spark plugs, this comprehensive gasket kit is often highly relevant when dealing with the EJ25, especially on high-mileage vehicles. A common issue is the leakage around the spark plug tubes which requires replacing the valve cover gaskets and the included spark plug tube grommets. Using premium materials, this kit ensures excellent sealing, preventing oil from pooling in the plug wells and fouling your new spark plugs prematurely, making it an essential companion for a major tune-up.

Key features that stand out:
– Includes necessary spark plug grommets vital for oil leak prevention.
– Constructed of premium materials for excellent sealing.
– Comprehensive kit suitable for various EJ25 SOHC engines (2.5L).
– Interchange Part Number: ET716S1, VC320, HS4034.

Pros:
– Solves common EJ25 oil fouling issues at the source.
– High-quality materials designed for longevity.
– Ideal for a full major engine service.

Cons:
– Does not contain the actual ignition plugs, only the sealing components.

Best for: Owners performing a high-mileage tune-up who need to address common valve cover/spark plug tube oil leaks on their EJ25.

Expert Opinion: Before installing new plugs, especially on older EJ25s, check for oil in the plug wells. If you find oil, you need this kit. Installing the best spark plugs for ej25 won’t matter if they immediately get fouled by oil leaks.

NGK # 3186 G-Power Platinum Spark Plugs TR5GP – 8 PCSNEW

This eight-piece set of NGK G-Power Platinum plugs offers the same trusted performance as the four-pack but provides extras—useful if you have multiple vehicles or require backup plugs. Designed with Platinum, they operate over a much wider heat range than basic plugs, making them resilient against both carbon buildup (low speed) and pre-ignition (high temp/load). The patented triple gasket sealing process is a hallmark of NGK quality, ensuring no combustion gas leakage occurs, maximizing power delivery and efficiency.

Key features that stand out:
– Eight-piece set provides exceptional bulk value.
– Wider heat range capability for varied operating conditions.
– Patented triple gasket seal ensures superior reliability.
– Resists carbon buildup and pre-ignition.

Pros:
– Great value for the quantity received.
– Platinum center electrode provides a crisp spark.
– Excellent resistance to fouling.

Cons:
– An eight-pack might be excessive for a four-cylinder EJ25 unless you own multiple cars.

Best for: Fleet owners or enthusiasts who maintain multiple EJ25 vehicles and prefer the robust reliability of Platinum.

Expert Opinion: While designed for a wider range of vehicles (implied by the 8-pack), these TR5GP plugs are built on proven NGK technology. Confirming the specific heat range is suitable for your exact EJ25 model (NA vs. Turbo) is always advised before installation.

NGK 6619 Iridium Spark Plugs LFR6AIX-11-6 PCSNEW

Stepping into the high-performance category, the NGK Iridium series is a favorite among EJ25 enthusiasts, especially those running mild modifications or turbocharged applications (EJ255/EJ257). Iridium is far stronger and has a higher melting point than Platinum or Copper, allowing for a finer electrode tip. This fine tip requires less voltage to fire and provides an extremely consistent, powerful spark over a significantly longer lifespan. The Iridium construction ensures they resist severe heat and electrical wear associated with performance driving.

Key features that stand out:
– Iridium construction for superior durability and lifespan.
– Fine electrode tip for enhanced ignitability.
– Designed to resist carbon buildup and pre-ignition across various heat ranges.
– Triple gasket sealing for reliable containment.

Pros:
– Maximum lifespan and performance upgrade.
– Highly recommended for turbocharged EJ25 applications.
– Better cold starts and smoother idle performance.

Cons:
– Higher initial cost compared to Platinum options.

Best for: Performance-minded EJ25 owners, especially those with EJ255 or EJ257 turbo engines who need reliability at high boost.

Expert Opinion: If you calculate the labor cost of changing plugs on an EJ25, the extended life of Iridium plugs makes them the most economical choice over the long term. These deliver noticeable responsiveness.

NGK 3922 Pack of 4 Spark Plugs (BR6HS)

This standard pack of NGK plugs represents a straightforward OEM replacement option. As a Genuine OEM part, you can trust the quality and fitment, assuming the specific BR6HS is the correct heat range for your EJ25 application (always double-check the required heat range against your owner’s manual). While this plug type may not offer the extreme longevity of Iridium or Platinum, it provides reliable, standard performance that is perfect for older, more basic engine designs or specific NA models where performance upgrades are not the priority.

Key features that stand out:
– Genuine OEM NGK Replacement Part ensures reliable fitment.
– Simple, proven design for standard applications.
– Bulk packed for efficient distribution.

Pros:
– Excellent reliability for stock applications.
– Cost-effective replacement option.
– NGK quality guarantee.

Cons:
– Shorter service life requires more frequent replacement than premium options.

Best for: Budget-conscious owners or classic Subaru applications where the stock plug is sufficient and service intervals are short.

Expert Opinion: While these are reliable, given the difficulty of access on the EJ25, most mechanics recommend upgrading to at least a Platinum or Iridium plug to maximize the time between replacements.

NGK LKR7DIX-11S Iridium IX Spark Plug (93175 Iridium Ix), 4 Pack

The LKR7DIX-11S Iridium IX plug is engineered specifically for the performance enthusiast who demands the absolute best ignitability and throttle response. Featuring a laser-welded fine 0.6 millimeter Iridium tip, this plug ensures a consistently stable spark with maximum durability to handle the rigors of high-performance engines. The longer insulator nose and corrugated ribs are crucial design elements that prevent flashover and fouling, maintaining peak efficiency even under severe driving conditions.

Key features that stand out:
– 0.6 mm laser welded fine Iridium tip for extreme ignitability.
– Tapered ground electrode minimizes quenching effect.
– Trivalent Metal Plating provides superior anti-corrosion and anti-seizing properties.
– Resistor Value-5K Ohm. Hex Size-5/8 inch.

Pros:
– Excellent protection against fouling.
– Delivers improved throttle response.
– Outstanding heat and electrical wear durability.

Cons:
– Requires careful gapping adjustment if the EJ25 is heavily modified.

Best for: Modified or track-driven EJ25 engines where stable ignition and maximum response are paramount.

Expert Opinion: The Iridium IX series is what you look for when you need a plug that can handle punishment. The anti-seizing properties of the plating are particularly helpful when removing plugs from the tight quarters of the Subaru Boxer engine.

A-Premium Iridium Platinum Spark Plugs (Pack of 8) Compatible with Nissan TITAN 2017-2020, TITAN XD 2016-2020, Armada 2017-2020, NV2500 2018-2020, NV3500 & Infiniti QX80, M56, QX56

Although this specific product targets larger displacement Nissan/Infiniti V8s, it provides an excellent example of high-end Iridium Platinum composite technology. The blend of an Iridium center electrode and a Platinum core electrode is engineered for maximum longevity, with replacement intervals recommended between 80,000 to 100,000 miles. This hybrid construction offers the best of both worlds: the firing precision of Iridium combined with the stability and endurance of Platinum.

Key features that stand out:
– Iridium center electrode and Platinum core material.
– Engineered for 80,000 to 100,000 mile longevity.
– High resistance (5000 ohms) for radio frequency suppression.
– Exceptional ignitability designed alongside OEM standards.

Pros:
– Extremely long replacement interval.
– Dual precious metal construction maximizes performance and life.
– Pre-gapped to 0.044 inch (standard V8 gap).

Cons:
– The required gap size and specific heat range must be verified for EJ25 fitment, as this is a non-Subaru specific pack.

Best for: Illustrative example of extreme longevity dual-precious-metal technology; only purchase for EJ25 if cross-referencing confirms exact spec match.

Expert Opinion: If an EJ25 requires a cold plug with dual precious metal construction for durability, these specifications are ideal. However, owners must ensure the provided gap (0.044 in) and heat range match the EJ25 requirements, especially turbocharged models, which often require a smaller gap.

NGK 4469 Iridium IX Spark Plugs LFR5AIX11 (6 Pack)

This six-pack of NGK Iridium IX plugs is specifically designed as an extreme performance upgrade over standard OEM spark plugs, offering a reliable service lifetime between 40,000 and 50,000 miles under demanding conditions. They feature a fine-wire Iridium center electrode paired with nickel-alloy terrain electrodes. This combination is particularly beneficial for fuel-injected and turbo engines like the EJ25, delivering maximum torque and optimal ignition timing stability through precision laser electrode construction.

Key features that stand out:
– Fine-wire Iridium center electrode.
– Designed specifically for fuel-injected and turbo engines.
– Reliable lifespan of 40,000 to 50,000 miles under performance use.
– Enhances maximum engine power and torque delivery.

Pros:
– A durable choice for mild-to-moderately modified EJ25s.
– Optimized for modern engine management systems.
– Six-pack provides extras or coverage for two service cycles.

Cons:
– Longevity is slightly less than some 100k-mile Iridium options, but stability is prioritized.

Best for: Turbocharged EJ25 owners (WRX/STI/XT) prioritizing peak power delivery and stable ignition under heavy load.

Expert Opinion: The LFR5AIX11 is known for excellent thermal stability, which is vital in preventing pre-ignition spikes common in forced-induction Boxer motors. These are among the best spark plugs for ej25 performance applications.

Autolite 3924 Copper Resistor Automotive Replacement Spark Plugs (4 Pack)

The Autolite 3924 Copper Resistor plugs are a classic, reliable choice for standard automotive applications, emphasizing maximum performance through a full copper core electrode. Copper offers the highest thermal conductivity, resulting in better performance across all conditions and guaranteed quick starts and smooth acceleration. While their lifespan is shorter than Iridium, they are built with durability features like a heat-fused internal seal and a copper-glass bond, ensuring they are leakproof and reliable throughout their service life.

Key features that stand out:
– Full copper core electrode for superior thermal conductivity.
– Copper-glass seal ensures leakproof and durable construction.
– Guaranteed quick starts and better fuel economy.
– Autolite’s decades of proven spark plug technology.

Pros:
– Excellent thermal performance for power output.
– Low initial purchase cost.
– Proven reliability in basic engine setups.

Cons:
– Requires replacement every 15,000–30,000 miles, making the EJ25 labor cost higher in the long run.

Best for: EJ25 owners focused purely on maximum conductivity and who don’t mind the increased maintenance frequency.

Expert Opinion: Copper plugs perform well but have short lifecycles. Given the difficulty of accessing the Boxer plugs, I generally recommend minimizing replacement frequency on the EJ25 unless you are chasing specialized performance requirements where copper is mandatory.

NGK LKAR7BIX-11S Iridium IX Spark Plug (93501 Iridium Ix), 4 Pack

Similar to other Iridium IX plugs reviewed, the NGK 93501 offers extreme ignitability, improved throttle response, and superior anti-fouling capabilities, making it an excellent choice for modern performance EJ25 engines. The core benefit lies in the Iridium alloy’s extremely high melting point, which ensures high durability against heat and electrical wear—essential for today’s high-compression and turbocharged engines. The design also incorporates a longer insulator nose to actively prevent fouling, sustaining performance across various operating temperatures.

Key features that stand out:
– Superior anti-fouling characteristics due to the longer insulator nose.
– Iridium alloy provides high durability to heat and electrical wear.
– Trivalent Metal Plating offers superior anti-corrosion properties.
– 0.6 mm laser welded fine Iridium tip.

Pros:
– Highly resistant to electrical degradation and wear.
– Excellent cold start and low-RPM stability.
– Top-tier choice for overall durability and performance.

Cons:
– Must ensure the heat range (7) is appropriate for your specific EJ25 tuning level.

Best for: Subaru enthusiasts looking for a premium, long-lasting plug that delivers exceptional cold-weather starts and high-heat durability.

Expert Opinion: The Iridium IX series is often considered the benchmark for premium spark plugs. For the EJ25, choosing an Iridium plug like the 93501 dramatically reduces the hassle of frequent plug changes, justifying the upfront cost immediately.

Comparison and Short Insights

When selecting the best spark plugs for ej25, the choice usually comes down to metal type, which dictates longevity and performance profile.

If your priority is maximum longevity and minimal labor, you absolutely should choose an Iridium plug, like the NGK 6619 or 93501. These can reliably last 60,000 to 100,000 miles, making the initial higher purchase price negligible compared to the cost of getting the plugs changed frequently on the cramped Boxer engine. Iridium plugs also offer the crispest spark for modern fuel injection systems.

For the cost-conscious daily driver seeking a balance between price and life, the Platinum options (like the NGK 7090 or 3186) are a great middle ground. They generally last twice as long as copper plugs but cost significantly less than Iridium, providing a decent 30,000 to 45,000-mile service life.

If you have a highly tuned EJ25 turbo engine, stability under boost is key. Specialized Iridium plugs, sometimes with a slightly colder heat range (like the LKR7DIX-11S), are essential to prevent pre-ignition and sustain performance. Copper plugs (Autolite 3924) have the best thermal conductivity and are sometimes preferred by tuners for extreme power, but their life is very short.

Remember, if your EJ25 has high mileage, consider the gasket and grommet kit (Donepart) alongside your plugs. Installing the best spark plugs for ej25 only to have them ruined by an oil leak is a costly mistake.

Final Verdict

Making the right selection for your Boxer motor requires balancing longevity, cost, and power requirements. Here’s a quick breakdown of my top recommendations based on common EJ25 user profiles:

The Endurance King (Best Overall Value):
For 90% of EJ25 owners—especially those with naturally aspirated (NA) or stock turbocharged engines—the NGK 6619 Iridium Spark Plugs (LFR6AIX-11) offer the highest return on investment. Their 100,000-mile lifespan makes the difficult installation process worthwhile, ensuring smooth, reliable performance for years.

The Budget-Conscious Reliable Pick:
If you need to replace your plugs and prefer not to spend premium Iridium money, the NGK 7090 BKR5EGP G-Power Spark Plug (Platinum) is the way to go. It’s a significant upgrade in durability over standard copper plugs and fits perfectly into the standard maintenance schedule.

The Performance Enthusiast’s Choice:
For WRX, STI, or Forester XT owners running increased boost, prioritizing stable ignition is critical. We recommend a colder Iridium plug like the NGK LKR7DIX-11S Iridium IX Spark Plug. This plug is engineered to resist flashover and fouling under high thermal stress, protecting your EJ25 under aggressive driving.

Best Spark Plugs For EJ25 FAQ

What is the ideal gap for the EJ25 spark plugs?

The ideal gap largely depends on whether your EJ25 is naturally aspirated (NA) or turbocharged (T). Most NA EJ25 engines use a gap between 0.040 and 0.044 inches. However, turbocharged EJ255 and EJ257 engines often require a tighter gap, typically around 0.028 to 0.032 inches, especially if running increased boost. Always consult your vehicle’s manual or your tuner’s specifications if modified, as a gap that is too wide can lead to spark blow-out under boost.

How often should I replace the best spark plugs for ej25?

Replacement intervals are dictated by the plug material. Copper plugs must be replaced every 15,000–30,000 miles. Platinum plugs can last 30,000–50,000 miles. The Iridium spark plugs, which are recommended for the EJ25 due to difficult access, typically last 60,000–100,000 miles. Given the labor involved in accessing the Boxer engine, most owners prioritize Iridium for maximum time between service intervals.

Should I choose a colder or hotter plug for my EJ25?

A plug’s heat range refers to its ability to transfer heat away from the combustion chamber. Stock EJ25 engines should use the manufacturer-recommended heat range. If your engine is modified (especially turbo models with higher boost or aggressive tunes), you usually need a colder plug (higher number in NGK’s numbering system) to help prevent pre-ignition and detonation caused by excessive heat. If your car is prone to fouling or runs rich, a hotter plug might be considered, though this is rare on a modern, well-maintained EJ25.

What are the main benefits of using Iridium plugs over Platinum or Copper in a Subaru Boxer engine?

The primary benefit of Iridium plugs in the Subaru EJ25 is their extreme durability and longevity. Because the Boxer engine layout makes the spark plugs very difficult to reach, minimizing the frequency of replacement is crucial. Iridium also features a much finer tip, which provides a stronger, more consistent spark with less voltage, leading to better cold starts and cleaner combustion.

Are all the products compatible with my EJ25 engine (Forester, WRX, Legacy)?

No. While many NGK plugs have specifications suitable for the EJ25 (both NA and turbo), you must confirm the specific heat range and reach required by your engine code (e.g., EJ253 NA, EJ255 turbo). Items like the Donepart gasket kit are specifically for the 2.5L SOHC, while some of the 8-pack and non-Subaru specific plugs (like the A-Premium) need careful cross-referencing for the correct EJ25 specs before purchasing.

Do I need anti-seize compound when installing new EJ25 spark plugs?

Most modern high-quality spark plugs, especially NGK Iridium and Platinum plugs, come with Trivalent Metal Plating (TBP). This plating acts as an effective anti-seize agent, and NGK often advises against adding extra anti-seize, as it can cause over-torquing and potential damage to the cylinder head threads. If your plug does not have TBP, use a small amount of anti-seize sparingly.