Subaru Cylinder Heads: Problems, Costs & Upgrades

Is your temperature gauge creeping up? Noticing a strange, sweet smell from your exhaust? Maybe your WRX just doesn't pull as hard as it used to. These aren't just minor quirks; they can be your engine's way of telling you something is wrong with its most critical components: the Subaru cylinder heads. Ignoring these signs can lead to catastrophic failure, turning a manageable repair into a full engine replacement. This guide will help you decipher the warning signs, understand the common causes of failure like a blown head gasket, and determine the best course of action to get your Subaru back to peak performance and reliability.

Key Takeaways

• Master Your Engine's Airflow for More Power: Your cylinder heads dictate how well your engine breathes, directly controlling its power potential and heat management. Upgrading them is one of the most effective ways to lay the groundwork for significant performance gains.
• Prevent Failure with Smart Maintenance: The most common cylinder head issues, like gasket failure, are often avoidable. Focus on your cooling system, use high-quality fluids, and monitor your temperature gauge to protect your engine from costly damage.
• Plan Your Upgrade from Parts to Tune: A successful upgrade requires matching the heads to your specific engine series (EJ vs. FA) and performance goals. A professional installation and a proper engine tune are essential to protect your investment and safely unlock your Subaru's potential.

What Are Subaru Cylinder Heads?

Think of your Subaru's engine block as the foundation of a house. The cylinder heads are the complex, high-tech roof that seals everything up top. Bolted directly to the engine block, the cylinder heads form the top of the combustion chamber, where the real power is made. But they do much more than just cap off the cylinders; they are the nerve center for your engine's breathing process.

These intricate components house the entire valvetrain—the system of valves, springs, and camshafts that lets air and fuel in and pushes exhaust gases out. The design and condition of your cylinder heads directly dictate how efficiently your engine performs, how much power it can make, and how reliable it will be under pressure. Whether you're pushing your WRX on the track or taking your Crosstrek on an overland adventure, the health and performance of your cylinder heads are absolutely critical. They are a key piece of the puzzle when it comes to building a powerful and durable Subaru engine.

How They Impact Engine Performance

The single biggest job of a cylinder head is managing airflow, often called "engine breathing." Just like an athlete, your engine needs to breathe efficiently to perform at its peak. The heads contain ports and valves that act as the engine's lungs, controlling the flow of the air-fuel mixture into the combustion chamber and the flow of exhaust gases out. A well-designed performance head improves this process, allowing the engine to take in more air and, consequently, generate more horsepower and torque.

Beyond airflow, cylinder heads are essential for heat management. They have internal passages that allow coolant to circulate, drawing heat away from the combustion chamber. This is especially important in turbocharged Subaru engines, which generate immense heat. An efficient cylinder head helps prevent overheating, which is a primary cause of the dreaded blown head gasket. Upgrading to better performance parts can significantly improve both airflow and cooling, giving you a more powerful and reliable engine.

Key Components Explained

A cylinder head is more than just a block of metal; it's a carefully engineered assembly of crucial parts working in perfect harmony. The main components include the valves, which open and close to manage airflow, and the camshafts, which orchestrate the timing of the valves. Most modern Subarus use a Dual Overhead Cam (DOHC) design, with two camshafts per head for more precise valve control.

Supporting these are valve springs and retainers, which ensure the valves snap shut at high RPMs without "floating." The head also forms the shape of the combustion chamber itself, where the spark plug ignites the air-fuel mixture. The design of these components determines your engine's character and power potential. Making any changes here, like installing more aggressive camshafts or porting the heads for better flow, often requires professional engine tuning services to recalibrate the engine and maximize your gains safely.

Cylinder Heads for Your Subaru: EJ vs. FA Series

When you start exploring cylinder head upgrades for your Subaru, you’ll quickly encounter two main engine families: the EJ series and the more recent FA series. These two types of engines have distinct cylinder head designs, and they are not interchangeable. Understanding which series your car has is the first and most critical step in choosing the right performance parts. Each series has its own strengths and a dedicated following in the enthusiast community, with plenty of aftermarket support to help you achieve your power goals. Whether you're driving a classic STI or a newer Forester, knowing the difference will guide your entire upgrade path.

Understanding EJ Series Heads (2002-2019)

The EJ series engine is a legend in the Subaru world, powering iconic models like the WRX, STI, Legacy GT, and Forester XT for nearly two decades. Known for their robust construction and incredible tuning potential, EJ cylinder heads are the foundation for countless high-horsepower builds. Their design has been tested and proven on tracks and rally stages worldwide. The performance community has embraced the EJ platform, leading to a massive selection of aftermarket performance parts designed to maximize airflow and power. If you own a Subaru from this era, you have a solid, well-supported platform for modification.

Exploring FA Series Heads (2012-Present)

Starting around 2012, Subaru introduced the FA series engine, representing a major step forward in technology. Found in models like the newer WRX, Forester, and BRZ, FA cylinder heads are engineered for greater efficiency and performance, incorporating features like direct injection. This modern design offers a different approach to making power, with a focus on improved throttle response and fuel economy without sacrificing performance potential. As these engines have become more common, the aftermarket has responded with a growing number of high-quality upgrades, allowing owners of newer Subarus to push their vehicles to the limit.

Matching Heads to Your Specific Model

Choosing the right cylinder heads is crucial, and it all comes down to matching them to your specific engine. You can’t put FA heads on an EJ block, or vice versa. Your selection should be based on your vehicle model, year, and what you want to achieve. Are you building a reliable daily driver with a bit more punch, or are you aiming for a full-blown track monster? Answering this question will help you decide between OEM-style replacements and high-performance aftermarket heads. For dedicated builds, exploring options for Subaru WRX upgrades or other models will ensure you get components designed for your specific goals.

Why Upgrade to Performance Cylinder Heads?

Stock cylinder heads are designed to meet the needs of the average driver, balancing performance with fuel economy and emissions standards. They do their job well, but for an enthusiast, they represent a major bottleneck. If you’re looking to get more out of your Subaru engine, upgrading to performance cylinder heads is one of the most impactful modifications you can make. This isn't just about adding a part for more power; it's about fundamentally changing how your engine breathes and makes that power.

An upgrade moves beyond the factory compromises, creating a more efficient and robust platform for all other modifications. Performance heads are engineered to work with bigger turbos, more aggressive camshafts, and free-flowing exhaust systems, allowing every component to reach its full potential. Whether you're building a competitive track car, a rugged overlanding rig, or simply a more responsive and exciting daily driver, performance heads are the foundation for a serious engine build. They provide a new level of capability and reliability, ensuring your engine can handle the demands you place on it.

Get Better Airflow and Breathing

Think of your engine as an athlete—its performance is directly tied to how well it can breathe. Stock cylinder heads often have restrictive ports and chambers that limit the flow of air and fuel into the engine and the exit of exhaust gases. Performance cylinder heads are meticulously designed with larger, smoother ports and optimized combustion chambers to maximize this flow. This process, often called "porting and polishing," allows your engine to take in a bigger gulp of air and expel exhaust more efficiently. The result is a much more responsive engine with smoother power delivery throughout the RPM range.

Increase Horsepower and Torque

Improved airflow is the key to generating more power. By enabling your engine to process a greater volume of air and fuel, performance cylinder heads create the conditions for a more powerful combustion event. This directly translates into significant gains in both horsepower and torque. You won't just see a higher peak number on a dyno chart; you'll feel the stronger pull and quicker acceleration every time you get behind the wheel. Whether you're modifying a Subaru WRX for the street or an STI for the track, a quality set of heads is essential for reaching your power goals.

Improve Durability and Heat Management

Power is useless if your engine can't handle it reliably. Performance cylinder heads are built from stronger materials and often feature improved coolant passages to manage heat more effectively. This is critical in turbocharged Subaru engines, where heat is the enemy and a leading cause of head gasket failure. By improving heat dissipation and providing a stronger casting, performance heads create a more durable foundation that can withstand the higher cylinder pressures and temperatures of a modified engine. Paired with a professional engine tune, this upgrade adds an essential layer of safety and longevity to your build.

Spotting the Signs: Is Your Cylinder Head Failing?

Your Subaru's engine is a finely tuned machine, and the cylinder heads are critical to its performance. When they start to fail—often due to a blown head gasket—the signs can range from subtle to dramatic. Ignoring them can lead to catastrophic engine damage, turning a manageable repair into a full engine replacement. Catching these symptoms early is key to protecting your investment and keeping your car on the road or track. Think of it as your engine sending up a flare; paying attention means you can address the problem before it gets out of hand. Whether you're pushing your Subaru WRX on the weekends or relying on your Forester for daily driving, knowing these warning signs is non-negotiable for any owner serious about performance and reliability.

Engine Overheating

If your temperature gauge is consistently creeping into the red, it's a major red flag. While many things can cause an engine to run hot, chronic overheating is one of the most common signs of a failing head gasket. When the gasket fails, it can allow hot combustion gases to leak into the cooling system, overwhelming it and causing the temperature to spike. It can also cause coolant to leak out, reducing the system's ability to cool the engine. Don't just top off the coolant and hope for the best; persistent overheating points to a deeper, more serious issue that needs immediate attention.

White Smoke & Coolant Loss

Seeing sweet-smelling white smoke billowing from your exhaust pipe? That's not a good sign. This is a classic symptom of a blown head gasket allowing coolant to leak directly into the combustion chambers. As the engine runs, it burns off the coolant along with the fuel, producing that distinct white smoke. This also means you're losing coolant, which explains why your reservoir might be mysteriously low. This problem won't fix itself and will only worsen over time, leading to more severe issues like engine overheating and internal damage. If you see white smoke, it's time to get your engine checked out.

Oil Mixing with Coolant

Oil and coolant should never mix. When a head gasket fails, the passages that keep these two fluids separate can be breached. The result is a contaminated mess that can wreck your engine. The most obvious sign is a milky, frothy substance on your oil dipstick or under the oil filler cap—it often looks like a chocolate milkshake. This "milkshake" is a poor lubricant and can lead to accelerated wear on your engine's internal components. If you spot this, stop driving the car immediately. Continuing to run the engine with contaminated oil can cause irreversible damage.

Rough Idling & Performance Dips

A compromised head gasket can cause a loss of compression in one or more cylinders. Since your engine relies on precise compression to generate power, any leak will disrupt the combustion cycle. You'll feel this as a rough or shaky idle when you're stopped, and you'll notice a significant drop in power and acceleration. Your engine might misfire, hesitate, or just feel sluggish. If your Subaru isn't pulling like it used to, a compression test can confirm if a bad head gasket is the culprit. Sometimes, what feels like a tune issue is actually a mechanical failure, which is why professional engine tuning services can also help diagnose these problems.

Common Problems Plaguing Subaru Cylinder Heads

While Subaru engines are built for performance, their cylinder heads can face certain challenges, especially when you start pushing for more power. Understanding the warning signs of a potential issue is the first step in preventing a minor problem from becoming a major engine failure. If you know what to look for, you can address issues early and keep your Subaru running strong on the track or trail. Here are the most common problems that can affect your cylinder heads.

Head Gasket Failures

The head gasket is a critical seal between the cylinder head and the engine block. When it fails, it can cause a cascade of problems. One of the most common signs of a blown head gasket is fluid contamination. You might find coolant mixing with your oil (giving it a milky appearance) or oil getting into your coolant. Other tell-tale symptoms include white smoke billowing from the exhaust, a rough idle, and external oil or coolant leaks, often visible at the rear of the engine. Ignoring these signs can lead to severe engine damage, so it's crucial to investigate them immediately.

Damage from Overheating

Your engine’s temperature gauge is one of its most important warning lights. Chronic overheating puts immense stress on the aluminum cylinder heads. When the engine runs too hot for too long, the metal can expand beyond its limits, which is a primary cause of head gasket failure and other serious damage. If you notice your temperature gauge creeping into the red, it’s not something to ignore. A failing water pump, a coolant leak, or a malfunctioning thermostat can all contribute to overheating, putting your cylinder heads directly in harm's way. Consistent high temperatures are a clear signal that your cooling system needs attention before the heads suffer permanent damage.

Valve and Combustion Chamber Issues

A compromised head gasket directly impacts engine performance by causing a loss of compression. The gasket is responsible for sealing the combustion chamber, and when it fails, that seal is broken. This means the controlled explosion that powers your engine isn't fully contained, leading to a noticeable drop in power. You might experience this as a rough, shaky idle, inconsistent acceleration, or a general feeling that the car is sluggish and unresponsive. These issues stem from the cylinder head's inability to properly manage the combustion process, turning your performance machine into a frustratingly slow ride.

Warping and Cracking

Warping and cracking are among the most severe forms of cylinder head damage, often resulting from extreme overheating. When a cylinder head warps, it no longer sits perfectly flat against the engine block, making it impossible for the head gasket to create an effective seal. In more extreme cases, the heat and pressure can cause the head itself to crack. This kind of damage often requires a complete replacement. The best defense is a robust cooling system and proper engine tuning services to ensure your engine operates within safe temperature ranges, especially in high-performance applications where heat is a constant enemy.

Repair or Replace? Weighing Your Options

When you’re facing a cylinder head issue, the path forward isn’t always clear. Do you stick with a simple repair, or is this the perfect excuse for an upgrade? The right choice depends entirely on your budget, your car’s condition, and what you want to achieve. Let’s walk through the main routes you can take, from a basic fix to a full-blown performance overhaul. Each option has its place, whether you’re just trying to get back on the road or aiming to build a track-ready machine.

Sticking with OEM Replacement Heads

If your main goal is to restore your Subaru to its factory condition, opting for OEM (Original Equipment Manufacturer) replacement heads is a solid, reliable choice. This is the most straightforward fix. You’re getting a part that’s guaranteed to fit and function exactly as the original did, which means no surprises during installation. This route is ideal for a daily driver where reliability is the top priority, or if you’re planning to sell the vehicle and want to ensure it’s in standard working order. While it won’t give you any performance gains, it effectively resets the clock on that specific component, giving you peace of mind.

Upgrading to Performance Cylinder Heads

For those who see a problem as an opportunity, a failing cylinder head is the perfect reason to upgrade. Performance cylinder heads are engineered to dramatically improve your engine’s airflow, which is the key to making more power. These heads often feature ported and polished runners, larger valves, and stronger valvetrain components to handle higher RPMs and boost. This upgrade is for the enthusiast looking to get more horsepower and torque out of their WRX or STI. It lays the foundation for future mods like a larger turbo, turning a necessary repair into a serious performance investment.

Considering Rebuilt Solutions

A rebuilt cylinder head can be a great middle-ground, offering a cost-effective alternative to buying brand new. In this process, a used head is professionally inspected, cleaned, and machined back to factory specifications. Any worn components, like valve guides or seals, are replaced. Some rebuilders even offer mild performance enhancements as part of the package. This can be a smart move if you’re on a tighter budget but still want quality. The key is to source your rebuilt head from a reputable machine shop with Subaru engine experience to avoid inheriting someone else’s problems.

When to Go for a Complete Engine Build

Sometimes, a cylinder head failure is just a symptom of a larger issue, especially in high-mileage or heavily modified engines. If you’ve experienced catastrophic damage or your goal is a high-horsepower build, simply replacing the heads won’t cut it. A complete engine build ensures every component is matched to handle your power goals, involving a new short block, forged internals, and performance heads. It’s the most intensive option, but it’s the right way to build a reliable powerhouse and requires professional engine tuning services to get everything dialed in perfectly.

Breaking Down the Cost of Cylinder Head Repair

When you’re facing a cylinder head issue, one of the first questions is always, "How much is this going to cost?" The answer isn't a single number; it's a range that depends on whether you're sticking with stock parts or going for a serious performance upgrade. Understanding the different costs involved—from the parts themselves to the labor required to install them—helps you budget properly and make the best decision for your Subaru and your performance goals. Let's walk through the key expenses you can expect.

OEM Replacement Pricing

If you're looking to simply replace a damaged cylinder head with a factory equivalent, you'll be looking at OEM (Original Equipment Manufacturer) parts. The price for these can vary quite a bit depending on your Subaru's specific model and year. For example, a head for a newer STI will likely cost more than one for an older Impreza. While this is often the most straightforward path to getting your car back on the road, it’s a baseline cost. It restores your engine to factory spec but doesn’t add any performance gains, which is a missed opportunity for many enthusiasts who are already paying for the labor.

Performance Head Price Ranges

For those who want more power, upgrading to performance heads is the way to go. These parts are engineered for better airflow and durability under stress. The investment for high-quality performance parts reflects their advanced engineering and materials. Prices can range from a few thousand dollars for a street-oriented package to well over $8,000 for a set designed for competitive racing. This cost covers modifications like porting, polishing, and upgraded valvetrain components that allow your engine to breathe better and produce significantly more horsepower and torque. It’s a serious investment in your vehicle’s potential.

Labor and Installation Expenses

The price of the cylinder heads is only part of the equation. Professional installation is critical, and labor costs can add a substantial amount to your final bill. This is a complex job that requires tearing down a significant portion of the engine. A skilled technician will ensure everything is assembled to precise specifications, which is essential for engine health and performance. Don't forget that after installing new heads, you'll almost certainly need professional engine tuning services to adjust your car’s ECU to the new hardware, ensuring it runs smoothly and safely.

Factors Influencing Your Total Cost

Several other factors can affect your total repair cost. The biggest one is the "while you're in there" work. If the technician discovers other worn components during disassembly, like a failing water pump or timing belt, it’s wise to replace them at the same time to save on future labor costs. More importantly, you need to address the root cause of the initial failure. If a blown head gasket was caused by an overheating issue, you must fix the cooling system. Ignoring underlying problems will only lead to another expensive repair down the road.

How to Prevent Cylinder Head Damage

The best way to handle cylinder head problems is to avoid them altogether. While some issues are unavoidable, proactive maintenance is your strongest defense against overheating, warping, and gasket failure. A little attention to your engine's core systems goes a long way in preserving its health and performance, saving you from expensive and time-consuming repairs down the road. By focusing on a few key areas, you can significantly extend the life of your cylinder heads and keep your Subaru running strong, whether you're on the track or a backcountry trail.

Stick to Cooling System Maintenance

Your cooling system is the first line of defense against the heat that can destroy cylinder heads. Sticking to a regular maintenance schedule is non-negotiable. This means performing routine coolant flushes with Subaru's recommended fluid and conditioner, and inspecting your hoses and thermostat for wear and tear. Over time, coolant loses its protective properties and hoses can become brittle. Replacing these components before they fail prevents overheating and ensures your engine operates at its optimal temperature. Think of it as simple, affordable insurance against catastrophic engine damage.

Use High-Quality Coolant and Oil

The fluids you put in your engine matter immensely. For your cooling system, always use Subaru Super Coolant or an equivalent specifically formulated for aluminum engines. The right coolant contains additives that prevent corrosion and protect the delicate passages within your cylinder heads and engine block. The same principle applies to engine oil. Using high-quality oil reduces friction and wear on all internal components. Investing in premium fluids is a small price to pay to maintain the integrity of your engine and support the high-performance parts you've installed.

Monitor Your Engine's Temperature

Get into the habit of watching your temperature gauge. It’s one of the most direct ways your Subaru communicates its health to you. While modern cars are good at managing temperature, any sudden spike is a major red flag. If you see the needle climbing into the red, don't try to push through it. Pull over safely as soon as possible and shut off the engine. Continuing to drive an overheating car, even for a short distance, can turn a minor issue like a coolant leak into a warped cylinder head, leading to a much more serious repair.

Get a Professional Engine Tune

An improper air-fuel mixture can create excessive heat and pressure in the combustion chamber, putting immense stress on your head gaskets and cylinder heads. This is especially true for modified vehicles. A professional engine tuning service ensures your engine runs efficiently and safely under all conditions. A skilled tuner will optimize your fuel and ignition maps, balancing power with reliability. This not only protects your engine from detonation and overheating but also ensures you’re getting the most out of your performance upgrades.

Where to Find Quality Subaru Cylinder Heads

Finding the right cylinder heads is a critical step in any engine build. The market is full of options, but not all parts are created equal. Knowing where to look and what to look for ensures your investment translates into real power and reliability on the road or track. Whether you're repairing a daily driver or building a track monster, sourcing high-quality components is the foundation of a successful project. Let's break down your best options for finding cylinder heads that meet your performance standards.

Crawford Performance: Your Source for High-Performance Heads

When you're aiming for top-tier performance, precision is everything. At Crawford Performance, we know that the small details make the biggest difference in an engine build. That’s why we emphasize that "double-checking your work and using the dowel pins on the engine block to guide the heads into the correct position" is a non-negotiable step for a perfect installation. This meticulous approach is core to how we build and prepare our high-performance heads, ensuring you get the power and reliability you expect. You can learn more about our process in our complete guide to an affordable Subaru cylinder head upgrade.

Choosing Between OEM and Aftermarket

One of the first decisions you'll face is whether to stick with OEM parts or go for aftermarket performance heads. Genuine Subaru heads are a reliable baseline, designed to meet factory specifications and deliver dependable, everyday performance. However, if you're looking for significant power gains, the aftermarket is where you'll find heads engineered for superior airflow and durability under stress. These options cater to specific performance goals, from spirited daily driving to full-on track builds. This variety lets you choose parts that align perfectly with your budget and how you plan to use your Subaru, giving you control over your build's final outcome.

What to Look for in a Quality Part

Regardless of where you source your heads, quality should be your top priority. Look for heads made from high-grade materials with precision machining. But remember, even the best parts need proper care. Good preventative maintenance is crucial for longevity. This means regular coolant changes using demineralized water and the correct Subaru additive. To protect your investment, always use a high-quality coolant like Subaru Super Coolant or an equivalent specifically formulated for aluminum engines. This simple step helps prevent corrosion, manage heat effectively, and ensures your engine runs cool and strong for years to come.

Choosing the Right Cylinder Head for Your Build

Selecting the right cylinder head is a critical decision for your Subaru engine build. It’s not just about bolting on a part; it’s about creating a balanced system to achieve your power and reliability goals. The heads dictate how well your engine breathes, which directly impacts performance. Making the wrong choice can create a bottleneck, limiting your other upgrades, or lead to compatibility issues. To get it right, you need to consider your specific engine, your performance targets, and the practical steps required to bring it all together.

Match the Head to Your Engine

First, the cylinder head must be compatible with your engine block. Subaru engines from 2002 to the present fall into different series, primarily the EJ and FA, and their heads are not interchangeable. Whether you drive a WRX, STI, Crosstrek, or Forester, getting the correct head for your model and year is the foundational step. This guarantees that bolt patterns and fluid passages line up perfectly. A compatible head ensures all the performance parts you add later will integrate seamlessly, creating a reliable and powerful engine package. Always verify the part is designed for your exact engine.

Define Your Performance Goals

What do you want your Subaru to do? The answer will guide your cylinder head choice. A build for a daily driver has different requirements than a dedicated track car built for high-RPM abuse. If you’re building an overlanding Crosstrek, you’ll prioritize low-end torque and durability over peak horsepower. Performance heads are often offered in stages, from street-focused packages with mild porting to full-race versions with oversized valves and aggressive camshafts. Choosing a head that aligns with your goals gives you a car that’s powerful, enjoyable, and reliable.

Consider Installation and Tuning Needs

Upgrading cylinder heads is a major project. You’ll need new gaskets, seals, and likely upgraded head studs to handle the increased cylinder pressure. Depending on the head package, you may also need to upgrade valvetrain components like springs, retainers, and camshafts. Once the hardware is installed, a professional tune is non-negotiable. The new heads drastically change your engine's airflow, and the ECU must be recalibrated. Proper engine tuning services will optimize fuel and ignition timing to maximize performance safely, protecting your investment and ensuring your engine runs smoothly.

Related Articles

• Affordable Subaru Cylinder Head Upgrade: A Complete Guide
• Everything You Need to Know About Your Subaru Engine
• Complete Subaru Engine Guide

Frequently Asked Questions

Is upgrading my cylinder heads really necessary if I'm just adding a bigger turbo? Think of it this way: a bigger turbo is like giving your engine a megaphone to breathe through, but stock heads are like asking it to breathe through a straw. You can force more air in, but the engine can't use it efficiently. Performance heads open up those pathways, allowing the engine to take full advantage of the increased airflow from the turbo. Without them, you're leaving a lot of power on the table and creating unnecessary stress on the engine.

My car is overheating. Does that automatically mean my head gasket is blown? Not necessarily, but it's a serious warning sign you shouldn't ignore. Overheating can be caused by simpler issues like a failing thermostat, a coolant leak, or a bad radiator fan. However, a blown head gasket is a common cause, especially if the overheating is persistent or accompanied by other symptoms like white exhaust smoke. The best course of action is to have the cooling system professionally inspected to find the root cause before any permanent damage occurs.

Can I just replace the head gasket without replacing or machining the cylinder head? While it might seem like a cost-saving measure, it's generally not a good idea. The extreme heat that causes a head gasket to fail often warps the cylinder head's surface, even if it's not visible to the naked eye. Placing a new gasket on an uneven surface is a recipe for another failure down the road. At a minimum, the head should be professionally checked for flatness and machined to ensure a perfect seal.

You mentioned EJ and FA series heads. How do I know for sure which one my Subaru has? The easiest way is to check your vehicle's model and year. Generally, WRX models from 2002-2014 and STI models from 2004-2021 use the EJ series engine. The FA series was introduced in models like the 2015-present WRX and the Forester XT from 2014-2018. A quick search online with your specific year and model or a look at your engine cover will give you a definitive answer and ensure you're looking at the right parts for your car.

Why is a professional tune required after installing new cylinder heads? Installing performance cylinder heads fundamentally changes how your engine breathes. It takes in significantly more air, which means the factory engine computer (ECU) settings for fuel and ignition timing are no longer correct. A professional tune recalibrates the ECU to match the new hardware. This not only maximizes your power gains but is also crucial for engine safety, preventing dangerous conditions that could damage your new investment.