Supporting Your Cylinders – Open, Semi-Closed, Or Closed Deck?

You’ve more than likely heard the terms open deck, semi-closed deck (semi-open), and closed deck before. But, do you know how to identify which design your block uses or why an open-deck that works well for a naturally aspirated build won’t likely last very long if it’s boosted or has nitrous? Which design your engine has will play a major role in the maximum peak cylinder pressure your block can handle, which can be simply translated to maximum power.

Peak cylinder pressure is much greater at the top the of the cylinder, where the combustion event happens. This is in addition to any secondary pressure sources the cylinder might be exposed to, such as forced induction, nitrous, and detonation. Naturally, this is the location that most failures happen, the more pressure you introduce into the cylinder, the more likely the cylinder wall is to bubble, crack, or fail entirely.

The video above, posted by time attack team Jager Racing and featuring Outfront Motorsports, presents the advantages of using a closed deck block on their time attack Subaru WRX STI. The video focuses specifically on the EJ25 motor, but the advantages can be generalized.

Open Deck

An open deck Ford EcoBoost 3.5L V6. Notice the lack of any type of structural support around the top of the cylinders (although some open deck designs utilize a support point at the end of each outside cylinder).

An open deck is one of the most common designs found in lower horsepower aluminum blocks. It is the easiest to manufacture and provides the overall best cooling efficiency, due to the coolants ability to make full contact with the surface area of the upper portion of the cylinder. Some will argue that this block has the benefit of weighing less than its two siblings because less material is used, but this weight difference is marginal for most blocks.

The downside to an open deck is that it provides the least amount of structural support at the top of the cylinder walls, where they need it most. This reason alone makes an open deck block the least reliable choice for almost any type of forced induction or nitrous application, and should be left to low compression, naturally aspirated engines and low boost platforms.

Semi-Closed Deck

A semi-closed deck flat-four EJ25, found in the Subaru WRX STI. Notice the bridges used to help prevent any distortion around the top of the cylinders.

A semi-closed deck is the most common design found in modern factory turbocharged aluminum blocks, and is stronger than an open deck by adding structural support to the top of the cylinders at four points. Using modern casting techniques, this design can handle respectable boost levels into the mid 30 PSI range and a much higher peak cylinder pressure than an open deck (depending on the application).

Because of the added material around the cylinder walls, this style is more difficult to manufacture and requires more machine work before the cylinders can be installed. Some will argue that the added support joints of a semi-closed deck reduces cooling efficiency and is more prone to cylinder hot spots by limiting the surface area for the coolant to contact. At higher horsepower levels, a semi-closed deck is still prone to failure at the points in between the supporting joints, especially in endurance racing or road racing applications where sustained high temperatures are common.

The most common solution for semi-closed and open deck blocks is installing aftermarket sleeves that can support higher cylinder temps and pressure. This process is pretty expensive, but it is highly recommended that you find a reputable machine shop that has experience with your platform. Sleeving requires a lot of precise machine work to not damage the block, so only the best in your area should be trusted.

Left: A factory 3.5-liter EcoBoost engine prior to being machined for aftermarket sleeves. Right: A sleeved 3.5-liter EcoBoost engine.

Closed Deck

A modified closed deck flat-four EJ25, found in the Subaru WRX STI. Notice the lack of any open space around the water jackets (only coolant ports), providing full support around the top of the cylinder walls.

A closed deck design is generally left to iron blocks and aluminum racing engines. It is the perfect design for fully built, high compression, high revving race engines that require the highest level of structural support available. Common on high level drag race motors, this design is also able to withstand prolonged periods of high heat and cylinder pressure commonly seen in endurance racing and road racing.

A closed deck block is much more expensive and requires an entirely different casting procedure if done from the manufacturer, but can also be modified using pieces that are press fit into position around the water jackets. The latter process requires a custom head gasket, along with precise calculations and machine work to verify that the pieces fit properly and that the water ports are drilled properly to provide adequate flow to efficiently cool the heads.

Many will also note that a closed deck block provides the least amount of cooling capability out of the three. Although this is true to an extent, many of todays closed deck blocks have been put through extensive testing and are designed to guarantee efficient cooling.

Conclusion

Depending on the goals for your build, you may need to sleeve your block or upgrade to a closed deck design. Most modern semi-closed deck blocks can handle very respectable power levels before needing to be sleeved or upgraded, and open deck blocks can handle just about anything you throw at it in naturally aspirated form. But, if you are building a race engine and want the ultimate reliability out of your block, your best option is a closed deck design or aftermarket sleeves (depending on the application and your budget).

About the author

Kyle Kitchen

Born and raised in Southern California, Kyle has been a gearhead ever since seeing his first Mitsubishi Evo VIII in 2003. He is almost entirely self taught mechanically, and as an inexperienced enthusiast always worked on his own vehicles, regardless of the difficulty, just to learn how to do it himself. Prior to becoming a freelance writer for the company, Kyle started his automotive performance career with Power Automedia as a shop technician, where he gleaned intimate knowledge of LS platforms and drag racing builds; then later joining the editorial team as the Staff Writer for EngineLabs And Turnology. Today, Kyle is an experienced EFI calibrator; hot rod builder; and motorsports technician living in the San Jose area. Kyle is a track junkie with lots of seat time. You can usually find him racing his Mitsubishi Evo X in local time attack and road race events.
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