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The Benefits of a Subaru Closed Deck Engine Block

Posted by IAG Performance on May 31st 2024

The Benefits of a Subaru Closed Deck Engine Block

The Benefits of a Subaru Closed Deck Engine Block

The power capacity of a Subaru engine relies heavily on the strength of its block. While a naturally aspirated engine can get by with an open deck block, a high-powered build calls for something more substantial. This is where the closed deck block comes into play. Designed to support high power applications, a closed deck engine block is strengthened by machined inserts that fill in the space around the cylinders. Whether you’re wanting more power for racing, or you just want more horsepower in general, a closed deck engine can get you there. If you’re thinking about increasing the horsepower in your Subaru’s engine, and you’re curious about the benefits of incorporating a closed deck block into your build, you’re in the right place. In this article, we’ll take a closer look at the pros of a closed deck, as well as the best way to build it out.


The surface of the engine block that the head gasket seals against is known as the deck. The channel between the cylinder wall and the block itself is basically what we’re talking about and whether it’s filled or not can make all the difference in how your Subaru engine holds up to increased power levels.

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Display photo of IAG 750 EJ20 Subaru Closed Deck Short Block For 2002-05 WRX - 750 BHP

Display photo of IAG open deck engine block


An open deck block has a lot of open space behind the cylinder wall, and while it’s ideal for naturally aspirated engines, it doesn’t have the strength to handle a higher powered rebuild.

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The next jump up in terms of strength and durability is a semi-closed deck, which is commonly found in EJ20 and EJ25 Turbo Subarus. The semi-closed variation offers a little more structural reinforcement from the addition of bridges that stand between the cylinder wall and the block. An unmodified semi-closed deck engine can reliably handle 400 to 500HP, but if you’re pushing for 700-800HP, a closed deck is the only way to go.

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Display photo of IAG semi-closed engine block

Display photo of IAG closed deck engine block


A closed deck block is fortified with inserts that fill in the voids between the cylinder walls and the block, leaving a channel for coolant to flow to the cylinder heads. The original factory closed deck blocks were part of the gen1 EJ20 turbo engines from Subaru, but these desirable blocks became increasingly more difficult to get a hold of. To satisfy the demands of Subaru enthusiasts looking for more power, closed deck modification came into being, making it possible to turn modern EJ20 and EJ25 engines into all-powerful closed deck blocks.

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Where the construction process is concerned, not all closed deck engine blocks are created equal. While the general idea is to fill in the voids with aluminum and machine the deck’s surface, we’ve gone further to insure against common problems that seem to haunt modified, high horsepower engines. The IAG closed deck construction process starts with brand new Subaru engine case halves that our machinists re-work to accept the IAG closed deck inserts. Once the case has been machined, we heat it so that it expands to allow the inserts, which have been frozen into a more compact state of contraction, to be installed. This provides a precise fitment of all the inserts, and leaves no unintentional gaps in the surface of the deck.

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Image of machinist manufacturing engine blocks.

Photo of engine blocks being machined.


Once the inserts are in place, the closed deck block is machined to a mirror finish. The short block is then bored and honed to a precise piston to wall clearance, ensuring that the engine will not only operate quietly, but will consume less oil. Our inserts feature small passages that allow coolant to flow to the deck surface, maintaining the block’s cooling ability without worry of overheating. Additionally, the holes in the block line up perfectly with the holes in the head gaskets for a precise seal. We’ve also used a near identical cast aluminum material as the block so that they both expand and contract with heat at the same rate. By eliminating the heat differential, the metals work in unison to prevent pressure imbalances and flexing between the metal surfaces. All these little details contribute to a well made closed deck block that can easily handle a big increase in horsepower, but how exactly does it work to protect?

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By increasing the strength of the OEM case halves, the fortified areas of the closed deck prevent cracking where the bolt reliefs are cut into the back of the cylinder wall. This is a really common issue with high horsepower builds. Additionally, the lack of reinforcement under the prolonged stress of additional power can cause the thin factory cylinder walls to flex, leading to issues with compression and oil consumption. The added strength and durability of a closed deck block solves all these problems, making it ideal for racing and high horsepower applications.

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Product display photo of closed deck engine block

Photo of machinery equipment used to machine engine blocks.


While a closed deck block lays a solid foundation for more power, it's essential to match it with durable, high-quality components throughout the engine, capable of managing higher horsepower. IAG Performance offers meticulously assembled closed deck blocks with top-notch components like IAG-Spec JE Pistons, Forged connecting rods, and trusted ACL or King Race bearings. With plenty of block and upgrade options to choose from, you’re sure to find the perfect block to reach your performance goals, while staying within your budget. Choosing a block from our lineup not only allows you to take advantage of a professional build, but it sets you up for success right from the start.

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If you’re serious about upping your horsepower game, a closed deck block can both manage the increased power and improve the overall durability of the block itself. Whether your target is 750, 950, or even 1150BHP, we’ve made it easy to get there while still maintaining the reliability of your modified engine. As always, if you have any questions, or you need a bit of friendly advice about your build, we’re happy to help. Feel free to give us a call at 1(410)840-3555 or shoot us an email at

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