Closed Deck vs. Open Deck Engines: What Subaru Owners Need to Know

What Are Open Deck and Closed Deck Engines?

When Subaru enthusiasts talk about "open deck" and "closed deck" engines, they're referring to the structural design of the engine block—specifically, how the cylinder walls are supported at the top where they meet the cylinder head.

Open Deck Design

Factory Subaru EJ and FA engines use an open deck design. In this configuration, the cylinder walls (or "sleeves") are only attached to the block at the bottom. The top of the cylinders is essentially exposed to the water jacket, with no material connecting the cylinder walls to the outer block structure at the deck surface.

This design offers several manufacturing advantages:

• Better cooling – Coolant can flow freely around the entire cylinder
• Lighter weight – Less material means reduced overall engine mass
• Easier casting – Simpler and more cost-effective to produce

For stock or mildly modified applications, the open deck design works perfectly fine. Subaru's engineers designed these engines to handle factory boost levels and power output reliably.

Closed Deck Design

A closed deck design adds structural material that bridges the gap between the cylinder walls and the outer block at the deck surface. This creates a rigid, fully supported cylinder that resists deflection under pressure.

Think of it like this: an open deck cylinder is a tube standing freely with support only at the base, while a closed deck cylinder is reinforced at both the top and bottom—making it significantly more resistant to flexing or cracking under stress.

Why Closed Deck Matters for High-Horsepower Builds

As soon as you start pushing serious power through a Subaru engine—whether it's a built EJ257 in your STI or a boosted FA20 in your WRX—the open deck design becomes the weak link in the chain. Understanding why this happens will help you make smarter decisions when planning your build.

The Problem: Cylinder Wall Flex

When you increase boost pressure, you're dramatically increasing the combustion forces pushing down on the pistons. These forces also push outward against the cylinder walls. In an open deck block, the unsupported cylinder walls can begin to flex and deflect under this pressure.

This cylinder wall movement creates several cascading problems:

• Head gasket failure – Cylinder flex breaks the seal between the block and head, allowing combustion gases to escape or coolant to enter the cylinders
• Ring seal issues – As the cylinder bore distorts, piston rings can't maintain proper contact, leading to blow-by and lost compression
• Cracked cylinder walls – In extreme cases, the repeated stress causes the cylinder sleeves to crack, requiring a complete block replacement

This is why you'll often hear about high-horsepower Subarus suffering from blown head gaskets or cracked ringlands. While tuning and supporting mods play a role, the fundamental block structure is often the root cause at elevated power levels.

The Power Threshold

So when does closed deck become necessary? While there's no magic number—results vary based on tuning, fuel, boost control, and driving style—most builders agree on general guidelines:

• Under 400 WHP – A well-built open deck block with forged internals can survive, but you're pushing the limits
• 400-500+ WHP – Closed deck is strongly recommended for reliability and longevity
• 600+ WHP – Closed deck is essentially mandatory, along with additional block reinforcements

How IAG Solves the Problem

IAG Performance has become the gold standard for Subaru closed deck conversions. Their process involves precision CNC machining of the factory block to accept custom-designed inserts that bridge the cylinder walls to the outer block structure.

What sets IAG's closed deck process apart:

• Liquid nitrogen installation – Inserts are frozen to shrink them, then installed into the heated block for an interference fit that's stronger than the parent material
• Maintained coolant flow – Strategic coolant passages are machined into the inserts, ensuring the engine still cools properly despite the added material
• OEM streetability – Unlike some race-only solutions, IAG's closed deck blocks are designed to be daily driven with proper cooling and reliability

IAG Closed Deck Short Blocks: Choosing the Right One for Your Build

IAG Performance offers a comprehensive lineup of closed deck short blocks designed for different power goals and platforms. Whether you're building an EJ-powered STI, an FA20 DIT WRX, or a boosted BRZ, there's an IAG solution engineered for your specific needs.

EJ25 Closed Deck Short Blocks (WRX/STI/FXT/LGT)

The EJ25 remains the most popular platform for high-horsepower Subaru builds, and IAG offers three tiers of closed deck short blocks to match your power goals:

IAG 750 Series Closed Deck Short Block

The 750 Series is the entry point into IAG's closed deck lineup, designed for builds targeting up to 750 horsepower.

• IAG closed deck block with CNC-machined inserts
• Forged pistons and rods
• 75mm forged crankshaft
• King XP Series race bearings
• Ideal for street/strip builds with upgraded turbo setups

Shop IAG 750 Series EJ25 Short Blocks

IAG 950 Series Closed Deck Short Block

The 950 Series steps up the internals for builds pushing toward 950 horsepower.

• IAG closed deck block
• IAG Fire-Lock head gasket system (prevents head gasket failures at extreme boost)
• Pinned main caps for additional block rigidity
• Upgraded forged rotating assembly
• Perfect for dedicated track cars and serious street builds

Shop IAG 950 Series EJ25 Short Blocks

IAG 1150 Series Closed Deck Short Block

The 1150 Series is IAG's flagship EJ25 offering, built for 1,150+ horsepower applications.

• IAG closed deck block with Fire-Lock system
• Pinned main caps
• Billet crankshaft for ultimate strength
• Premium forged pistons and rods rated for extreme cylinder pressures
• Designed for professional racing and no-compromise builds

Shop IAG 1150 Series EJ25 Short Blocks

FA20 DIT Closed Deck Short Blocks (2015-2021 WRX)

The FA20 DIT engine in the 2015-2021 WRX responds incredibly well to modifications, but its factory open deck block has similar limitations to the EJ when pushed hard. IAG's 800 Series FA20 DIT Short Block solves this with a purpose-built closed deck solution.

• IAG closed deck block conversion
• Forged pistons and rods
• O-ring and 1/2" head stud prep for secure head gasket sealing
• Designed for 500+ WHP builds on the FA platform

IAG also offers an 800 Series FA20 DIT Long Block option that includes IAG's 800 CNC Pocket Ported cylinder heads for a complete top-to-bottom performance solution.

Shop IAG 800 Series FA20 DIT Blocks

FA20 Closed Deck Short Blocks (BRZ/FRS/GT86)

For naturally aspirated or boosted BRZ/FRS/GT86 builds, IAG offers the 800 Series FA20 Short Block in two compression ratio options:

Choosing the right compression ratio depends on your forced induction plans. If you're running a supercharger or a conservative turbo kit, the 12.5:1 option maintains strong naturally aspirated response. For larger turbo builds with higher boost targets, the 10.5:1 compression provides necessary detonation resistance.

Shop IAG 800 Series FA20 Short Blocks

Final Thoughts

The difference between open deck and closed deck engine blocks isn't just technical jargon—it's the difference between a reliable high-horsepower build and an engine that's living on borrowed time. Factory Subaru blocks are well-engineered for their intended power levels, but once you start chasing serious boost and big turbo setups, the structural limitations of the open deck design become a real liability.

IAG Performance has spent years refining their closed deck solutions for the Subaru community, and their lineup covers everything from weekend warriors to professional race teams. Whether you're building a 500 WHP daily-driven STI, a track-focused FA20 WRX, or a full-tilt 1,000+ horsepower monster, there's an IAG closed deck short block engineered for your goals.

Key takeaways:

• Open deck blocks are factory standard and work great for stock to moderate power levels
• Closed deck adds critical cylinder wall support needed for high-boost, high-horsepower applications
• Plan for your future power goals—it's cheaper to build the right block once than to rebuild twice
• A closed deck block is just one piece of the puzzle; proper tuning, fueling, and supporting mods are equally critical

Ready to start planning your build? Browse the full IAG Performance lineup at SubiMods or reach out to our team if you need help selecting the right short block for your project.