While all engine blocks are different and use different designs, webbing, reinforcement, and materials, there are a few things that all engines share. At the top of every engine block is the deck, and there are a few different designs that you’ll find: open, semi-open, and closed. Today we’re going to dive in, compare them, and find out which one is better.
With all modern engines, the cylinder head and the cylinder and created separately and later bolted together to complete the engine. The surface at the top of the block where the cylinder head mates the block is the deck. The job of the deck is to provide a solid surface for the head to bolt to, give the stability of the cylinder, and allow coolant to circulate the cylinders and eventually move up into the head or heads.
There are three different fundamental variations of a block deck: open, semi-open, and closed. The names refer to the coolant passages and how coolant is allowed to move around the top of the cylinders and into the heads.
An open deck, as the name suggests, is pretty much fully open between the cylinders and the engine block walls. Open deck designs have the coolant channel cast into the top of the deck. This design improves cooling and reduces hot spots in the cylinder but provides much less strength and rigidity at the top of the cylinder.
A closed deck, is the opposite of an open deck and it’s nearly fully closed. I say nearly fully closed because the deck surface is only drilled for the head fasteners, coolant passages, oil supply, and oil drain back passages. The tops of the cylinders are integral with the deck. This configuration is found in pretty much all cast-iron blocks and some high-performance aluminum blocks.
Semi-open falls somewhere between an open deck and a closed deck design, and every semi-open design is different. For the most part, it’s going to look like an open deck but with some bridging between the cylinders and the block walls. The area where it bridges is going to be different for every engine.
Looking at them from a top-down perspective, it’s pretty easy to see the differences. For reference, we can look at a Subaru EJ engine. You can see the open deck is very open around the cylinder walls, semi-open looks pretty similar but with some noticeable bridging, and closed is almost entirely closed except for a few coolant passages.
Which One is Stronger?
For the most part, you’re going to hear most people say that a closed deck is stronger than an open deck, and while this is fundamentally true since more material and more reinforcing will make a block stronger, that doesn’t mean that open deck blocks are bad. To demonstrate this, we can look at some modern performance engines and see that a lot of them use an open deck design.
The Honda K20A and K20C are both open deck. BMW N54 and N55 are both open deck. Ford’s 2.3L Ecoboost uses an open deck. On the other side of the spectrum, the BMW S55 and B58 are both closed deck, Nissan VR38 is a closed deck, and so on. As you can see, modern performance engines are kind of mixed bag whether they use an open or closed deck design.
Looking at some older engines, the Toyota 2JZ is a closed deck, Nissan’s RB26 and SR20 are both closed deck, Mitsubishi 4G63 is a closed deck, and so on.
The general trend here is that older turbocharged engines use a closed deck and modern turbocharged engines are a mixed bag of closed deck and open deck design. But, why is this? If traditional wisdom says that closed deck is stronger, why would manufacturers both using an open deck design, especially for a turbocharged engine?
The answer mostly comes down to manufacturing costs and thermal efficiency. One of the big downsides to a closed deck design is that it’s more expensive to manufacture a closed deck block compared to an open deck block. When you’re manufacturing tens of thousands or potentially hundreds of thousands of engine blocks, the cheaper manufacturing method can result in millions of dollars saved.
Not only is an open deck design cheaper to manufacture, with modern materials, casting, and machining, open deck blocks are much stronger than they used to be. A long time ago, it was common to hear people complain of blowing out cylinders when turbocharging an open deck engine, but that’s not nearly as much of a problem as it used to be.
If we take an example of some random manufacturer building a 2.0L turbocharged four-cylinder for example – if the manufacturer knows their open deck design is strong enough to withstand power levels of say 800 horsepower, why would they bother using anything stronger if their target power level is 300 horsepower?
The other reason manufacturers will use an open deck design is thermal efficiency. Heat management is key to producing an efficient engine, so an open deck is a preferred choice for manufacturers who want the best thermal efficiency possible.
How Does Cylinder Failure Occur?
Where you see the strength benefit of a closed or semi-open design is typically near the top of the cylinder. If you were to take an open engine and cram as much boost pressure as possible in it, the top of the cylinder is where you’re liking going to see failure.
This is because all the air pressure and the start of combustion occurs at the top. In an open deck design, the failure you’d be likely to see from excessive boost pressure is likely going to be a crack at the top of the cylinder, where a closed deck has that extra strength in that exact spot to help prevent the cracking from every start.
If not cracking, the other issue you could run into is cylinder distortion which causes a whole host of over problems and can quickly lead to catastrophic failure.
So, Which One is Better?
Yes a closed deck block is stronger than an open deck block. There are more material and more structural integrity, but with modern designs, materials, and manufacturing methods, the strength difference between the two isn’t as drastic as it once was. Many modern engines use a closed deck design and many of them also use an open deck.
The open deck is cheaper manufacture and for most applications it’s more than strong enough. If you’re building some sort of crazy high horsepower application where cylinder pressures are going to be way higher than they were ever designed for than a closed deck block will theoretically be the better option.
Somewhere in between a closed deck and open deck design is the semiopen deck which provides a little bit of the best of both. It’s important to note that if you’re comparing two different engines, the design of the deck is just one part of the puzzle when determining which engine is stronger.
A closed deck block isn’t always going to be stronger than an open deck block. There are plenty of other points where a block could fail, so don’t just look at the design of the deck when determining how strong a block is.
Closed Deck Conversion
That brings us to our next point, which is can you convert an open deck block to a closed deck block for extra strength, and the answer is yes. Converting an open deck to a closed deck typically involves the installation of a plate which sits between the cylinder walls and the block walls. This plate will feature holes to allow coolant to pass through just as it would with a closed deck.
The insert is typically pressed in so it doesn’t jiggle around since that would effectively eliminate the benefits of reinforcing those areas.
To answer the question of which one is better, it depends on what you want from your engine. If you’re building some insane high-horsepower monster, then a closed deck block might provide the extra strength you need, but some engines the extra strength is always necessary.
Open deck blocks have come a long way in the last 20 years and some of them are capable of withstanding massive amounts of power before failure occurs. But again, it’s important to remember that there are plenty of other points that your engine could fail from increased power and the deck isn’t always the issue.