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Discussion Starter #1 (Edited)
This thread is a work in progress. There are a number of members with legitimate questions about building their Subaru motor. My goal is to disseminate information so members can make informed decisions for themselves and not be misled because they lack critical information or are ignorant about motors in general. If you would like me to cover a particular concept, please post in this thread. I may not get to it immediately, but hopefully over time I will cover the basics you should know about before taking steps to substantially increase the power output of your particular engine application. In the meantime, perhaps another member or vendor can answer your question.

I intend to define frequently used terminology, explain different concepts I come across in my own research, and what things you should keep in mind in your quest for more power. I will ask vendors and tuners different questions about creating more power, building the 2.5L Subaru engine, and tuning it for optimal power. Any topics or concepts where there isn't universal agreement will be detailed here. Each week, I hope to add another topic. While I am working on this project, you can follow my work below:

Current Status: Finishing research on open, semi-closed, and closed deck blocks and looking into the next topic.

1. Terminology
2. What is an open deck block? What is semi-closed deck block? What is a closed deck block?

***Sources for the information contained in this thread include different vendors, wikipedia, and different people I consider authorities about general engine topics.
 

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Former Site Owner
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Discussion Starter #2 (Edited)
GENERAL TERMINOLOGY

Long Block: A replacement engine including the crankshaft, connecting rods, pistons, heads and head gaskets. [

Short Block: A replacement engine block containing the crank, connecting rods and pistons, but without heads, manifolds or external components.
 

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Former Site Owner
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Discussion Starter #3 (Edited)
What is an open deck block? What is semi-closed deck block? And what is a closed deck block?

I hear these terms tossed around so I decided to learn more about them first.

To understand what the difference is between these blocks, it is helpful to understand how some Subaru motors are designed while looking at some images.

You will notice three images below of an open deck block, a semi-closed deck block, and a closed deck block (borrowed from another Subaru forum). Motors create heat. To keep a motor operating at its optimal temperature and from overheating, the cooling system circulates coolant through the block in the channels that sit between the outer cylinder walls and the inner sides of the block--the open space around the cylinders.

In the pursuit of more power, enthusiasts increase boost and run higher octane fuel (among other things), which causes increased stress loads on the cylinders. The cylinders may move around as these stress levels increase beyond the power tolerances of the motor's original design.

Under significantly higher power loads, the cylinders can move around enough to wear out the head gasket, which is a critical gasket that seals the cylinder heads. If the head gasket wears out, then the coolant leaks into the cylinders and evaportes, which greatly increases the risk that the motor will overheat as more and more coolant evaporates until its gone (a very bad thing). Other parts in the engine may also fail as the temperature of the oil and coolant increase outside their normal operating termperatures for extended periods of time.

To my knowledge, the North American STI comes with a semi-closed deck design.

A semi-closed deck Subaru motor is modified to add strengthening support to the upper part of the cylinder wall.

A closed deck Subaru motor is modified more than semi-closed deck motor. Large sections of the open area around the cylinders is filled in to prevent the cylinders from vibrating. The engine builder leaves holes so coolant still flows through the heads. Filling in this area prevents the head gaskets from blowing in high performance applications.

I am still conducting research to learn at what power level the stock STI motor begins to see this stress, how much is tolerable, and at what power level do you risk blowing a head gasket. In the end, most of us do not race so we want something that is reliable, or a solution that will last a specified period of time--like at a time in the future when we will have funds to upgrade to a bigger, better motor.

OPEN SUBARU BLOCK:





SEMI-CLOSED SUBARU BLOCK



CLOSED DECK SUBARU BLOCK:
 

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GENERAL TERMINOLOGY

Long Block: A replacement engine including the crankshaft, connecting rods, pistons, heads and head gaskets. [

Short Block: A replacement engine block containing the crank, connecting rods and pistons, but without heads, manifolds or external components.
Kevin,

A long block can mean drastically different things to different people and I think that's important to note.

To some a long block includes the shortblock, head gaskets, head fasteners, heads (complete). Others also include valve covers and gaskets, cam gears, tensioner, idler pulleys, timing belt, oil pump, water pump, etc. etc. Some also include manifolds and beyond. For example, junkyards often offer longblocks that even include accessories like the alternator, a/c compressor, power steering pump, fuel rail(s), injectors etc. etc.

As far as the shortblock goes, it includes the bearings and case bolts for sure. Sometimes it also includes piston access covers and a few other odds and ends too. It depends who you get it from.

I wish all of this was cut and dry, but it's not.:tup:

I'll check out the next part of your FAQ when I have a minute.
 

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awesome, the semi-closed deck thing was puzzling me till now. thanks for the info, can't wait to learn more.
Do the walls look thinner on the closed deck block or is it my imagination? Also, it looks to me that the closed deck has much less cooling surface area, is this true? And if it is doesn't it affect reliability?

What are the 2.0 blocks like?

Thank you for all the interesting information.:tup:
 

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Very interesting, I hope this thread keeps going :D Thanks!

Could you explain what happens when the cylinders move around? Do the actual cylinder walls move and that is why a more closed deck would help alleviate it?
 

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Very interesting, I hope this thread keeps going :D Thanks!

Could you explain what happens when the cylinders move around? Do the actual cylinder walls move and that is why a more closed deck would help alleviate it?
To my understanding, the cylinder's wall will flex side to side from the intense pressure in the cylinder, simple because the wall is not strong enough. This will cause head gasket to wear and thus blow. Coolant starts leaking into the cylinder and all hell breaks lose... This is why people get their block sleeved- it provides much more support.
 

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I'm still curious on how is builders modify the Semi closed deck and make it a fully closed deck...
They "CAD" (make computer models of) the opening patterns in the decks, then make aluminum inserts which fit closely in the openings. Then weld and resurface the decks.

To my understanding, the cylinder's wall will flex side to side from the intense pressure in the cylinder, simple because the wall is not strong enough. ...
Under high boost pressures and harsh treatment the tops of the cylinders can move in an open-deck configuration because they are not well supported. They are probably well cooled however. The closed deck provides the support needed for high-boost and other stresses, and probably doesn't cool the tops of the cylinders as well as the open deck. The semi-closed deck is a kind of compromise. It give some additional support to the cylinder tops, and still allows quite a bit of water flow near the tops of the cylinders.

I've heard an argument made for open decks with beefy cylinder liners with a wide top flange. The argument being that detonation events, if they occur, start at the upper portion of the cylinder, and so better cooling in that part of the cylinder helps in that regard. See:
YouTube - Subaru SVX Sleeved Block Resurfacing
 

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Not much going on here lately...

I know when we were apart of this discussion in 'private' there was alot of information floating around... Its a shame it didn't get put up.

I know it was Kevins' intention to bring each engine builders perspective to the table to share with all the members of IWSTI.

So I will do my part in sharing some of our information with the community that has seemed to disappear into the archives of IWSTI.

Follow this link and you will find out our engine design and assembly thoughts...

This is just a small tidbit so far...

Upgrade Packages
 

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I just finished doing a 1K mile break-in on my Closed-deck STI 2.5L. So far so good, motor gets GREAT vacuum (~-22.5 ) and basicaly sounds like a stock motor, even on cold starts.. If it's REALLY cold out ill get a tiny bit of piston slap at first start, but thats to be expected on a build like this.. Mods on my build include;

EJ257 Closed-deck Shortblock
Forged JE Pistons
Forged H beam rods
08+ STI nitrided crank
ACL race bearings
1/2" ARP headstud upgrade
11mm subaru oil pump
Supertech dual valvesprings with Ti-retainers
Kevlar race timing belt

At first i was worried about cooling issues being that i live in socal, but i was reassured by my builder that it wouldnt be a problem, and i trust him because the place is mainly a sandrail shop so most of their motors are in sandrails that are run out in the desert in 115+ degree heat getting romped on, so IMO if it can take that it should be no problem in my car, and so far so good! Did first oil change at ~650 miles, and now at ~1200 miles. Will prolly do another oil change at 1500, and then ill go to at least 3500-4K miles before next oil change.. Ive done a bunch of reading and alot of times people change oil TOO frequently, and the motor wont seat the rings well becuase the oil never get a chance to break in and therefore the rings never seat.. But im getting good vacuum so i think i did a pretty good job so far, started building a bit of boost after the first oil change (wastegate basically) and gradually revved it higher and higher.. So far ive taken it to 7K only a couple times.. But luckily i live in a hilly area so i got a good amount of engine breaking in which i heard is good to do to help seat rings..
 
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