The engine is the beating heart of a car. It is the key component that transforms air, fuel, and spark into usable power. Not all engines are designed and built the same way.
One of the most important design differences is the design of the deck, situated at the top of the engine block where it meets the cylinder head cover. Open-deck and closed-deck refer to the design of the aforementioned deck. How do these two types of engines differ?
In this article, we are going to compare open deck vs. closed deck engines and tell you what are the fundamental differences between the two, and how they compare against each other. So, if you are interested in a short yet sweet automotive engineering lesson, you’ve come to the right place!
Open Deck vs. Closed Deck Engines: How do they differ?
The main difference between these two engine designs is that an open deck engine offers more space between the engine block walls and the cylinders. Through these openings, more coolant can flow which improves engine cooling. On the other hand, a closed deck engine is more rigid and thus is stronger and can withstand more abuse.
However, modern-day advancements in engine design have come to a level at which an open-deck engine isn’t far behind a closed-deck engine regarding structural rigidity. A closed-deck design is more expensive and is typically associated with high-horsepower performance cars which need to endure extreme cylinder pressures for extended periods of time, something you’d often find in racing.
Open deck engines tend to be cheaper, easier to make, more efficient, and have better cooling. On the contrary, closed deck engines offer more cylinder pressure support which gives you a higher power ceiling, ability to push more boost, and better long-term durability.
Open deck engine advantages:
1. Reduced manufacturing and development cost
In the world of automotive manufacturing, an open-deck engine is ideal as it requires less raw materials and is easier to produce. Development costs are also generally lower which means less research and development expenditure for a company.
Inherently, open-deck engine design is dominant these days as it brings down production costs through fewer development expenses and saving on material costs.
2. Cooling system efficiency
With more space between the cylinder walls and the rest of the engine block, coolant can flow closer to the combustion chamber due to the open design. Coolant can flow right on the cylinder walls close to the friction and heat of the inside of the cylinder. This helps the engine stay cooler in virtually all scenarios.
An open deck engine can perfectly function with a coolant system that would be insufficient for a closed deck engine.
3. Reduced weight
The differences in weight are marginal between both of these engines design. An open-deck engine will generally have less iron or aluminum which in turn means a lighter block. The difference is small; for example, the open deck N55 weighs around ~30 pounds less than the closed-deck B58.
However, racing enthusiasts with a focus on weight reduction and a lower center of gravity are sure to greet weight loss with arms wide open, especially as the engine tends to be one of the heaviest, if not the heaviest part of the entire car.
Closed deck engine advantages:
1. Higher engine block strength
With added structural rigidity, a closed-deck engine is able to withstand high boost pressures much better than an open-deck engine. A closed deck engine can cope with higher amounts of stress without causing catastrophic engine failure. This means that you can run a closed-deck engine with very high boost numbers.
Closed-deck engines can generally be tuned to levels of power and torque way above factory numbers. It is not uncommon to see closed-deck engines tuned to make twice or even three times the amount of their stock horsepower.
2. Better long-term durability
With added reinforcement between the cylinder walls and the cylinder block, the sheer amount of metal keeps the engine structurally solid for a longer time. This also means a higher cylinder wall strength which goes hand-in-hand with the reliability of high horsepower setups. Closed-deck engines should be able to outlast a comparable open-deck engine quite easily.
3. Better for the track and high-demanding scenarios
A closed-deck engine is perfect for built racing engines that endure high levels of stress all the time. This is why most performance engines are indeed closed-deck engines. This design is able to withstand prolonged heat and continued high strain better, even though it does not manage to cool the engine as well as an open-deck design.
Semi-Open Deck Engine
If you simply can’t make up your mind about which of these two engine designs you like more, you should also note that semi-open engine designs also exist and are intended to be the middle ground between open and closed deck engines.
A semi-open deck keeps the large openings as in the open-deck design but adds additional support (bridges) in a few spots throughout the openings.
This gives a semi-open-deck engine a higher maximum PSI capability which means that it can endure pressure better than an open-deck engine. However, it does not maintain the open-deck cooling efficiency as it is also somewhere in the middle in this regard. Semi-open deck engines are widely used by Subaru and other manufacturers.
Well-known open-deck engines:
- BMW N54 – The N54 engine is a 3.0L twin-turbocharged six-cylinder engine. Even though it uses an open-deck design, it is still popular with tuners and is known to be able to push more than 600hp on a stock engine block with stock internals.
- BMW N55 – The N55 engine replaced the N54 and is also a 3.0L turbocharged six-cylinder engine. It uses an open-deck engine design. A stock block N55 can push up to 550hp.
- BMW N20 – The N20 engine is a 2.0L 4-cylinder turbocharged engine later replaced by the BMW B48. In contrast to the B48, the N20 features an open-deck design.
- BMW N52 – The N52 is the successor to the BMW M54 and also uses the same 6-cylinder configuration, with a displacement between 2.5L and 3.0L. Despite being an open-deck engine, the N52 is known as a dependable, under-stressed engine.
- Nissan VQ35DE – One of the best performance engines in the early 2000’s. This engine powered the 350z among other Nissan vehicles.
Well-known closed-deck engines:
- BMW B58 – The B58 replaced the N55 and is also a 3.0L straight-6 with a single twin-scroll turbocharger. With stock internals, the B58 can withstand more than 750hp.
- BMW S58 – A new M Performance engine powering the G80 M3 and other M-cars. It is the successor to the S55, and it also features a closed deck.
- BMW B48 – The B48 is part of BMW’s new modular family of engines. It is a 2.0L 4-cylinder single turbo engine powering most of BMW’s lineup.
- BMW S63 – The famous S63 engine is a 4.4L twin-turbocharged V8 used on many BMWs top-spec large M cars such as the M5. With a closed deck design and an appetite for destroying tires, the S63 offers between 547hp and 617hp. With stock internals, the S63 can output well over 800hp.
- BMW S85 – This gloriously-sounding 5.0L naturally aspirated V10 is found in the 2005-2010 BMW M5 and M6. With exactly 500hp on tap, the S85 is one of the most exciting, but difficult to live with BMW engines.
- Toyota 2JZ-GTE – One of the most iconic engines of all time. A turbocharged straight-6 capable of making over 1000 hp on stock internals. It is no surprise that this engine has a closed-deck design.
- Nissan RB26DETT – Another well-known Japanese engine from the late ’90s. This engine powered the R34 Nissan Skyline GT-R.
Frequently Asked Questions
Are open deck engines weak?
If we were to compare two exact engines the only difference being that one is open-deck and the other is closed-deck, yes, an open-deck engine is comparably weaker. However, employing a closed-deck design is one of many different ways how you can make an engine more durable.
With today’s advancements in materials, precision machining, and overall manufacturing, some open-deck engines like the BMW N54 are really strong. The same story goes for most turbocharged open-deck engines.
Are closed-deck engines better?
If you want an excellent performance engine capable of receiving continuous abuse, yes, a closed-deck design is better. Closed deck engines can be more easily tuned and usually withstand higher power and boost pressures. From stock, a closed-deck engine has a much higher power/pressure ceiling compared to an open-deck engine.
Can you make an open-deck engine closed-deck?
Making a closed deck engine out of an open deck engine is possible, but conversion will be needed. Track and dragstrip enthusiasts came up with the idea of filling the block with epoxy filler. This is one of the ways of converting an open deck engine into a closed deck, but this process isn’t guaranteed to leave you with a functional engine.
Coolant flows must be rerouted, and there are many additional considerations such as heat expansion, the melting point of the filler material, etc.
The best thing you can do is use a CNC machine to precision cut the area around the cylinders and install a precision cut plug/insert. This is typically finished by further deck processing which involves machining components such as the coolant ports onto the deck and decking the mating surface, the bore, and the hone.
Closing thoughts
Automotive engineers have almost perfected the internal combustion engine. There are design differences that offer advantages and disadvantages.
Closed deck engines are stronger and can withstand the added boost of an aggressive tune much better. That said, open deck engines with forged internals such as the N54 can also have extraordinarily robust internals. You shouldn’t discard them for your next build solely due to engine deck design.