Everything you need to know about your Subaru Engine
Everything you need to know about your Subaru Engine.
Make Your Subaru Engine Last.
You’ll notice in the title of this blog, we say everything you need to know about your Subaru engine—not just what we want you to hear, or what our opinion is. This is the most up-to-date information available, so you can make educated decisions about what you do under the hood.
Here at Crawford, you’ll only ever hear proven facts, and the information that allows you to choose what’s best for your Subaru.
Engine Part Sources
If your car came new from the dealership, then it’s 100% stock or OEM (Original Equipment Manufacturer). These are the parts Subaru installed from the factory.
If something is aftermarket, then it’s been manufactured or modified by another company. Genuine Subaru OEM parts remain the benchmark: decades of R&D, investment, and real-world testing have gone into them. They’re expensive, but they’re designed for durability, reliability, and safety.
Aftermarket has improved since 2019. Some brands now produce excellent, near-OEM quality components. However, bargain parts—especially knock-offs manufactured with lower-grade materials—are still risky. You get what you pay for. For critical systems like turbos, sensors, and internal engine components, OEM is often worth it. For cosmetic or simple bolt-on parts, quality aftermarket options can be a smart choice.
New factor: With Subaru introducing hybrid boxer engines, parts compatibility and ECU/software integration matter more than ever. Using the wrong part may not just reduce performance—it could interfere with hybrid systems or emissions compliance.
Types of Subaru Engines
Subaru’s boxer engine heritage remains, but the lineup has shifted significantly since 2019.
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EJ Series
Long-time workhorse of Subaru performance cars. Today, the EJ is almost entirely phased out in new models, surviving mostly in enthusiast builds and motorsport. -
FB Series
Designed for efficiency, smoother torque delivery, and lower emissions. Found in modern Impreza, Crosstrek, Forester, Outback, and Legacy models. -
FA Series
Lightweight, efficient, with both naturally aspirated and turbocharged versions. Subaru’s newer performance and midsize vehicles (like the Ascent, Outback XT, Legacy XT, and BRZ) rely on FA24 engines. -
CB Series
Subaru’s most recent boxer family, more compact and efficient. The CB18 is found in models like the Levorg and select Forester trims overseas. -
Hybrid Boxer (new frontier)
In 2024, Subaru unveiled its next-generation hybrid boxer engine. Production begins in Japan in late 2024, with rollout to Crosstrek and other models soon after. This marks a turning point in Subaru’s engine history.
Subaru Engines by Model (Current Highlights)
Here’s a breakdown of which engines power today’s Subaru lineup:
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FA24D (NA) — 2022-present Subaru BRZ & Toyota GR86.
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FA24F (Turbo) — Subaru Ascent (2019-present), Outback XT (2020-present), Legacy XT (2020-present).
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FB25D (NA) — 2019-present Forester, 2020-present Legacy & Outback (non-turbo trims).
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FB20D / FB20 — Current Impreza and Crosstrek (non-turbo).
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CB18 (Turbo) — Levorg (2020-present, outside US) and select Forester models in global markets.
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Hybrid Boxer — Crosstrek Hybrid and upcoming next-generation models (production begins 2024–2025).
If you’d like to see the full history and lineup of Subaru engines, this Wikipedia article has it all: https://en.wikipedia.org/wiki/Subaru_EJ_engine
Most of the aftermarket power and engine components you will find are for turbocharged Subaru models. This is due to the fact that until 2013, no NA (Naturally Aspirated / Non-Turbo) models were tuneable. That means you couldn’t adjust the car’s computer, or ECU, to accommodate for the changes made under the hood past an intake or headers.
There is also no need to build the engine if you’re not making big power. There are ways around this, but they usually come at a heavy cost and require completely re-wiring your engine and making it no longer road legal. Swaps are popular with NA models for this reason, but take a phenomenal amount time and money.
Parts of your Engine
The complete engine refers to everything that comes out of the engine bay as one unit—minus the intercooler, piping, hoses, and fluid reservoirs. This is often called a Turn-Key engine.
Inside, major systems include the block, pistons, crankshaft, cylinder heads, valvetrain, and (for turbo engines) turbochargers and intercoolers. Each plays a role in reliability and performance.
A complete long block strips away bolt-on components such as your turbo, manifold, and headers.
A bare long block is the same thing, minus the valve covers, timing components, cams, and pulleys.
The short block is the center of the engine, with the cylinder heads removed. It includes its internals: the pistons, rods, crank, bearings, and rings.
Cylinder heads are bolted up to each end of the short block. 
Inside the short block, you will find the moving parts that make your engine purr. These are known as internals.
At the very heart of the short block is your crankshaft (crank), which turns the crank pulley on the back of your engine. This rotating motion is translated to your drivetrain which turns your wheels. 
Connected to your crank are your rods, with the appropriate official name of Connecting Rod. It connects the crank to the piston, which is forced to move in and out of the cylinders by internal combustion (explosions). RPM’s are how many times per minute an explosion happens between all cylinders in your engine. When an explosion happens, the top of the piston gets the direct impact, and the rod has to be even stronger due to its length and direct connection to the piston via a wrist pin.
The piston rings are placed into grooves on the top edge of the pistons, and they close the small gap between the piston and the cylinder wall in order to hold back exhaust from entering your crankcase. Some still slips by through the ring gap, and this is know as blow-by.
Engine Myths: Ringlands, Bearing Failures, and Reliability
You’ve probably heard the myth: “Subaru engines break because the spaces between the piston ring gaps fail.”
This is mostly false. That idea has been pushed for years to sell aftermarket internals and tunes claiming to prevent ringland failures. In reality, Subaru’s structural design and factory tunes aren’t inherently defective. Failures usually come down to how the engine is maintained and used.
Here’s the truth in 2025:
-Ringland cracks and bearing failures almost always stem from detonation (knock) or oiling issues.
-Causes include neglecting oil changes, running low on oil, poor fuel quality, or installing performance mods without proper tuning.
-Even with modern FA/FB/FA24 engines, which are more refined than the old EJ, the same principles apply: proper maintenance and smart tuning prevent most failures.
How to reduce risk today:
-Install an Air/Oil Separator (AOS) to keep oil vapor out of the intake system.
-Use a quality tune if you add performance parts—factory ECUs are more advanced now, but aftermarket changes still require recalibration.
-Follow Subaru’s recommended oil weight and intervals, and monitor oil levels regularly.
-Use the right fuel—especially important for direct-injected and turbo models like the FA24.
Understanding Subaru Short Blocks: Open, Semi-Closed, and Closed Deck
When people talk about Subaru “short blocks,” you’ll hear terms like Open Deck, Semi-Closed Deck, and Closed Deck. These refer to how much structural support surrounds the cylinder walls where coolant flows.
-Open Deck
Found in older, non-turbo Subarus and the first two generations of the WRX. These had very little cylinder wall reinforcement, making them the weakest design.
-Semi-Closed Deck
Since 2005, almost all Subaru production engines use this design. It adds critical support points around the cylinder walls, significantly improving strength without the cost of a full closed deck. This remains the standard in today’s FA and FB engines.
-Closed Deck
Typically used in built or motorsport engines. Provides maximum reinforcement around the cylinders, making it ideal for high horsepower builds (500+ HP). Subaru doesn’t ship closed-deck blocks from the factory in consumer cars, but aftermarket builders and tuners still use them for racing and big turbo setups.
Modern updates:
-The FA24 turbo engine (used in Ascent, Outback XT, and Legacy XT) and FA24D in the BRZ are semi-closed deck designs, capable of handling more stress than older EJ engines.
-Subaru’s newer CB18 turbo engine (in the Levorg and global Forester) is also semi-closed deck, optimized for efficiency but with enough reinforcement to handle turbo pressures.
-For hybrid boxer engines rolling out from 2024 onward, Subaru continues to rely on semi-closed deck designs for durability while balancing cooling efficiency.
Closed Deck was once used long ago (early 90’s) by Subaru but quickly discontinued due to its lack of reliability. Today the concept is entirely aftermarket, where the coolant passageways of semi-closed and open deck blocks are filled with metal making a solid connection between the cylinder wall and crankcase. Sure, it’s strong as hell, which is why it was first practiced in the racing world with extreme heat and high horsepower builds. It kept the cylinder walls in one piece for a longer period of time, extending the amount of race miles each engine lasted.
The downside to the added strength is the fact that the coolant passageways are sealed off, causing the engine to run at higher temperatures and frequently overheat. This makes the internals fail quicker, despite the block itself staying intact. There are other ways to cool an engine with aftermarket modifications, but they cost and take away cohesion in the engine bay. It’s a cheap bandaid approach to those wanting to avoid a new engine.
Shops that sell these blocks actually take old, retired blocks and repurpose them. This is why the cost is so low for people looking for a strong EJ engine, making them subsequently popular. They are also popular due to some misleading marketing; that the OEM semi-closed deck blocks are prone to failure and cannot support high power builds. This is absolutely not true if you maintain our recipe for a healthy engine in the section below.
What Do I Need to Keep My Subaru Engine Reliable? (2025 Update)
At Crawford, we use this phrase a lot—it’s practically our motto: “Don’t fix what’s not broken.”
When it comes to modifying your Subaru, this mindset is crucial if your top priority is reliability. The single most reliable “build” is keeping your Subaru 100% stock, aside from premium fluids and a meticulous maintenance schedule. That may contradict what you’ve read on forums, but remember: a lot of that noise is opinion—or worse, marketing.
With nearly three decades of Subaru-specific R&D under our belt, here’s our proven recipe for efficiency and reliability:
1. Don’t Fix What’s Not Broken
We get it—you love a big mod list. But unless a part has verifiable benefits, don’t replace components that already work as designed. Subaru OEM parts are engineered to work together as a system. Once you start swapping them out unnecessarily, reliability often goes downhill fast.
That’s why so many Subaru “money pit” stories exist—owners break this first cardinal rule.
2. Always Get a Quality Tune
If you add power-increasing components, the single most important investment is a tune from a reputable, Subaru-experienced tuner.
This is true at every level: stock, bolt-ons, or full race builds. Poor tunes—whether an amateur’s custom map or even generic off-the-shelf (OTS) tunes—can kill an engine quickly.
-OTS maps are only safe for light break-in or driving gently to your tuner. They’re not tailored to your car, your fuel, or your climate.
-Reputable tuner means someone with a strong track record and years of Subaru experience, not a newbie practicing on your car.
-Today, remote tuning is more viable than ever, thanks to advanced logging and ECU reflash tools. But even then, don’t settle—find the best.
3. Stay on Top of Maintenance
Still as important in 2025 as it was in 2019.
Oil: Subaru engines hate running low. Always use the factory-recommended weight and spec. Turbocharged engines require full synthetic.
-Intervals: With premium oils, 4–5k miles is safe, but check levels often. Change filters every ~2k miles if possible.
-Brands: We’ve trusted Motul for years, but any high-end full synthetic that meets Subaru’s requirements will do.
-New factor: Direct-injection engines (FA24, FB25D) benefit from regular intake valve cleaning (walnut blasting or chemical service) to prevent carbon buildup.
4. Run a Proper Air/Oil Separator (AOS)
This remains one of the most effective reliability mods. A true AOS reduces detonation and oil consumption while letting the engine operate at its best.
Beware: many cheap catch-cans are marketed as AOS units but don’t function the same. Crawford’s own AOS design continues to be our best-selling product for a reason.
My Engine Failed. Now What?
First step: diagnose the cause. Common culprits include detonation, oil starvation, poor tuning, or neglect. Compression and leak-down tests plus oil analysis will help narrow it down.
Rebuild vs. New Block
-EJ Engines: Almost completely phased out of new models, but still common in older WRX/STIs. Rebuilding EJs is risky because cylinder walls are semi-closed and prone to going out of round. Honing makes them weaker. Best option: new block if you want reliability.
-FA/FB Engines: Stronger cylinder walls and better design. Rebuilding is usually safe and cost-effective, especially for FA24 and FB25 engines. Still, a new block is always the most reliable choice.
-CB/Hybrid Engines: Newer designs (CB18, upcoming hybrid boxers) aren’t as widely rebuilt yet, but semi-closed decks give them strong reliability for stock and mild power builds.
Cylinder Heads
9 times out of 10, Subaru heads are reusable. Exceptions: overheating (warping/cracking), broken valves, or severe cam scoring. Always resurface reused heads for proper gasket sealing.
Building Heads?
For turbo engines, the answer is still no. Gains are small compared to simply turning up boost. For NA builds, cams and headwork can make sense, but for turbos, the money is better spent elsewhere.
Supporting Parts to Replace
Any part that oil touched must be cleaned or replaced after a failure:
-Oil pump (replace, not clean—especially in FA/FB front-cover designs).
-Oil cooler (replace).
-Seals & gaskets: At minimum, replace intake, exhaust, water pump, and head gaskets.
-ARP 11mm head studs: Reusable, stronger than OEM, and originally designed by Quirt Crawford.
-Spark plugs: Upgrade to NGK one-step colder Iridiums when installing a new short block.
-Oil & filter: Always start fresh with proper break-in oil and filter.
Power Levels & Built Engine Considerations (2025 Standards)
Thanks to better designs, Subaru engines handle more power today than they did in 2019—when maintained and tuned properly.
-EJ25 (stock block): Safe up to ~400whp / 480bhp.
-FA20 NA (BRZ/FR-S/86): Up to ~300whp / 360bhp with supporting mods. Rod swap to FA20 DIT rods still recommended for boosted builds.
-FA20 DIT (2015+ WRX): Capable of ~400whp / 480bhp.
-FA24F (Ascent/Outback XT/Legacy XT): Newer turbo block handles similar power to a built EJ, with better reliability margins. ~450whp achievable with proper tune.
-FB25D / FB20D: Non-turbo, not ideal for big power—reliability drops sharply when boosted.
-CB18 (Levorg/Forester overseas): Still being explored, but early tuning shows solid reliability in the 300–350whp range.
Crawford Built Stages (unchanged but updated for 2025)
-S2: 450whp / 540bhp
-S3L-X: 550whp / 660bhp
-S3L-X with FSR Upgrade: 600+whp / 720+bhp
Click here to check out our built blocks.
That’s right folks: you don’t need a closed-deck block for high HP builds!
What makes Crawford Built Blocks unique?
All components engineered and manufactured by Crawford Performance have been tested for ultimate performance and maximum dependability on and off the race track. For over 2 decades we have tested all combinations of upgrades for the turbo-charged EJ20, EJ25, and more recently the new FA20 Subaru engines. We have pushed these engines to their breaking points to determine which components fail at what level. From that research we have come up with forged internals to keep your engine strong enough to withstand your desired power level.
Our builds include the following upgrades over OEM:
S2
-CrawfordSPEC S2 JE Forged Pistons. These Pistons are approved by the SCCA and stamped accordingly
S3L-X (formerly S3L-i)
-CrawfordSPEC S3L JE Forged Pistons. These come with an option $250 upgrade for FSR, which is an even stronger billet forging. FSR stands for Forged Side Relief.
-CrawfordSPEC X-Beam Pauter Billet Rods
About CrawfordSPEC Pistons
Every CrawfordSPEC piston features a unique dish design, engineered by and exclusive to Crawford Performance. Our lead engineer, Quirt Crawford, has worked side-by-side with JE Pistons to create a design with the following results:
-More manifold vacuum at idle, which makes for a tighter, healthier, more efficient engine
-Lower crankcase pressure
-More power at same boost level with same supporting mods
-All CrawfordSPEC pistons are sized for the OEM bore of 99.5.
-About FSR Piston Upgrade
Piston forging developed by Quirt Crawford. Although the forging style is available to the public, our skirt length and dish design is exclusive to Crawford Performance. This forging upgrade features:
-100% CNC machined from high quality 2618-T6 wrought aluminum bar stock
-Specifically designed for high horsepower road and drag race engines
-Internal and external bracing (bridge-like design) provides a more rigid construction while minimizing overall weight
-Billet means pistons are machined on every surface allowing for all unnecessary weight to be removed during manufacturing, reducing excess friction/heat from high horsepower applications
-Weight reduction of up to 32 grams
-Includes high quality 9310 steel wrist pins
-Reduced skirt width and shorter wrist pin when compared to traditional “full round” style forgings. The narrow skirt helps minimize piston contact with the cylinder wall that can cause friction and power loss while the shortened wrist pins reduces the overall weight.
About CrawfordSPEC Rods
Every CrawfordSPEC rod is designed at a specific length, engineered by and exclusive to Crawford Performance. Our lead engineer has worked side-by-side with Pauter Rods to create a design with the following results:
-Longer than stock length not only increases strength but also increases the rod ratio which allows for smoother and more efficient power delivery
-Billet vs. Forged, made with the strongest steel available in the industry: 4340 chrome-moly
Why doesn’t Crawford use aftermarket cranks, bearings, rings, or gaskets?
It goes back to one of our motto’s: Don’t fix what’s not broken. Our blocks are expensive due to all of the OEM parts that are included, which are premium. A lot of people are puzzled as to why our prices are higher than another companies blocks that include more upgrades. We’ll say it again: you get what you pay for. Aftermarket parts are cheaper because they do not have the same quality standards as OEM. They are cheaper for a reason. And we do not and will not replace parts in an engine that do not fail. We have used and tested aftermarket cranks for example that are stronger than OEM. They have consistently had a higher failure rate than OEM cranks at the same power levels.
But I spun a bearing. The bearing must be the issue, right?
To answer this question, we must first explain how bearings work, so that why they fail makes more sense. This particular writeup on bearing failure was written by Quirt Crawford himself:
General engine bearing failure 101
FYI, this is a general letter to anyone that has had a bearing failure in their Subaru motor.
The Subaru motor has two sets of bearings in it, one set on the main journals of the crankshaft and another set on the rod journals of the crankshaft. The oil that lubricates and floats these bearings comes from the oil pump via the engine block and is fed into the crankshaft via the main bearings, so they see the oil first while the rod bearings see the oil secondly. With this fact in mind it is easy to diagnose if the rod bearing failure was caused by a lack of oil pressure, or from detonation / excessive loads.
If there is a lack of oil pressure then the main and rod bearings will be damaged from scoring with the rod bearings seeing the most damage as they see the highest loads. The damage to the bearings is usually even across all four rod and main journals, so you will not see just one bearing damaged from a lack of oil pressure.
Detonation / excessive loads on the rod bearings is very easy to diagnose as the rod bearings themselves are the only bearings that are damaged, leaving the main bearings fully intact. Detonation is like a giant hammer pounding on the top of the piston and this impact goes straight down the connecting rod and into the rod bearing which pushes out the very small film of oil keeping the rod bearing and rod journal on the crankshaft from making metal to metal contact with each other. Detonation can and will damage only one rod bearing at a time where a lack of oil pressure damages all four at the same time.
Statistics show that the large majority of Subaru motors that have damaged just one rod bearing happen to cylinder #3. There are many different opinions as to why this occurs and I will not go into this as it is not relevant to the topic of this letter.
When you have a bearing failure in your motor, cleaning out ALL of the bearing material inside your motors components as well as the turbo is of utmost importance! If this is not done correctly then your replacement motor and or turbo will fail from ingesting this leftover debris. Sadly this is also a high statistic in our industry… And the person that suffers from this costly mistake is always the vehicle owner as the shop that did the work usually does not even realize the mistake they made, so they blame it on something or somebody else. At Crawford we always throw away any and all oil coolers on the vehicle as getting the bearing debris out of them is impossible. The small filters in the cylinder heads must also be replaced, if not then the turbo will fail within 1000 miles of the new motor install. The oil pump will also be damaged from the bearing debris as will the oil pressure relief valve which is located in the body of the oil pump; at Crawford we never reuse the old oil pump either.
- Quirt Crawford, Founder of Crawford Performance
Have any further questions after reading this article? Feel free to reach out to us any time:
855-67-SUBIE (78243) Ext. 1
technical@crawfordperformance.com