Subject: Closed deck/open deck production dates and WRX/2.5 RS engine differences

First, a polite apology and a disclaimer. In checking on Trey Cobb's

inquiry, I found a chart that shows there were open deck WRX turbo motors

in production since late 1994. I had done major research on the WRX in the

winter of 1995-6 for my own amusement and reported on the incredibly

detailed, original, 1993 Japanese language production information available

to me at that time. The information provided was correct for the time

period reviewed. No new published information since then was found until

this summer when the HyperREV ver28 was available at my local Japanese book

store in Torrance. Even then, I did not read it cover to cover, as reading

Japanese for me is about like pulling teeth. As some of you know, my German

and Chinese is much better, while it is my wife who has the Japanese down

and she could care less about cars, so I only ask her when I need to.

At the time the 2.5 RS came out and when comparing North American engines

with Japanese engines, the open deck and closed deck issue is was the most

obvious, but not the only difference. While I see that open deck turbo

motors are used, it is my honest opinion other issues remain regarding any

decision to turbocharge the 2.5RS engine. It is obviously now the time to

list these issues, based on conversations with Subaru staff and Prodrive

staff, as I have done in the later portion of this article.

For those of you new to the list, I manage SPD Tuning Service in Redondo

Beach California. Because of our business plan, we are very conservative in

developing and offering performance parts. If one were to purchase a C43

Mercedes-Benz, one would simply not question the quality and reliability of

the AMG modifications. One might question the price, but that is different

issue. Quality is what SPD Tuning Service is aiming for. Not the cheapest,

not the most trick, just the most effective solutions to real performance

needs combined with complete satisfaction from every product offered. We

cannot ask our customers to plunk down $4000 dollars for a turbo kit when

the majority of them are making car payments and often do not have

alternate transportation, unless we are absolutely sure that such a kit is

going to work and work properly for a long time. Please realize that my

opinions and comments are always based on this business outlook. Others

are free to disagree with the opinions presented here, but not with the

principles on which the opinions are based. All I ask is that "we can agree

to disagree."

With this agreement in mind, I remain skeptical of major, reliable, turbo

power from the 2.5 RS, at least on a reasonable budget. I have also said in

my published comments at http://www.spdusa.com/ , "I would love to be

proven wrong." Wrong, if only for the reason that some of us may be willing

to spend a couple of thousand in engine related maintenance costs every two

or thee years. As a result, ones assessment of the risks involved with

turbo kits will be different. For some, a turbo kit for 2.5 engine will be

very desirable, no matter the costs involved. Here is my position: I want

to have a kick-ass motor I can run for not months, but for years of

reliable service. That is why, some 15 years ago, I went from my second 1.8

liter 145hp "SSS" engine straight to a 195 hp, fuel injected Nissan V-6

engine in my 1973 Datsun 510. Worked just fine. With my Impreza, I am going

from a 118hp 1.8 engine to a 260hp WRX engine in the same way. As Walter

used to say, "And that's the way it is." End of disclaimer! Please read on.

Because Trey asked, "Are you sure? Check HyperREV!" I started looking

closely through the entire HyperREV with my Japanese speaking wife at my

side and can report the following information for Impreza WRX Turbo engines

as of 97.9.1. This information can be found on page 176 of HyperREV Number

28, printed in 1998. Here is what I found:

Prior to 94.10.7 all engines are closed deck at 220hp (for auto trans,

w/hydraulic lifters, slightly smaller turbo w/milder cams), 240hp and 250hp

(STI). Note, I feel the autotrans engines are desirable as they are less

likely to have been trashed and SHOULD actually run on our 92 octane at

their designed horsepower and torque.

Standard production "WRX" EJ20G engines changed to an open deck design at

94.10 This is where I missed the boat, as I had only the original 1993

factory manuals. The WRX-RA, WRX-STI and STI ver II remained closed deck

with a high performance cylinder head, and other goodies, up to 96.6.6.

This closed deck "RA" engine is the "big power" motor that made Subaru

famous in WRC (world rally championship).

This RA version block, head and forged crank are still the ones to get if

"going for broke" in the power department. The RA head in particular has

different water jacketing, very expensive valve material and a host of

other details, while the block remains the strongest EJ series made, along

with its cousin, the EJ22 turbo block. The EJ20G "WRX-RA" motor was

designed for an era when Group A rally car engines were seeing 400 plus

horse power in the 8000 rpm range. The rule change to 36mm and then to a

32mm restrictor on the compressor inlet reduced the power output to the

point that special 1996 Prodrive road prepared WRX-RA and STI version cars

were "faster in a straight line than the Group A cars". This is a quote

form David Englishby of Prodrive over lunch in Banbury in 1996. The inlet

restrictor, with the lower rpm, lower maximum expected power figures along

with the Group A rally car rule change to permit special engine and chassis

developments, independent from actual production cars, lead to the redesign

in 1997 of a new "EJ20K" series engines.

The 1997 year model change over date, "96.6.6", saw a major internal and

external redesign of the engine. It is now the EJ20K series with re-routed,

resized and repositioned turbocharger, repositioned larger intercooler,

lower compression 8.8->8.0, bigger valves, more (relative) boost, and

slight cam changes. After this time, all production engines are open deck

design, including the STI and RA versions. This permitted a great cost

saving by changing all engines over to open deck die-cast cases while

keeping the basic strength for a 300hp design limit. One of the key

developments was a ceramic coating process that was first done by a small

UK company for the Prodrive assembled racing engines. A manufacturing

process was developed for this piston coating, making it possible for the

pistons of the K series engines to have greatly reduced friction surface

with the cylinder wall. That the "K" series engines have an open deck is

not the end of the story on whether to turbocharge the RS 2.5, as we shall

see.

In addition, From 96.6.6 a special EJ20G series at 250hp remained in

production all along with hydraulic lifters, 9.0 compression and a smaller

turbo. This engine was the basis of the Australian and UK Turbo models at

218hp. While not mentioned in HyperREV, they have now gone to a "K" series,

with its changed layout of peripherals, retaining the 218hp specification,

small turbo and higher compression. These are ideal candidates running on

US gas, if a low mile, salvage engine and transmission can be located. Note

that these transmissions have a 3.9 final drive compared to the 4.11 final

of the RS 2.5. However, their 5th gear is shorter, giving similar revs per

mile in 5th. This trans is "autobahn" gearing, much like the Audi 90, where

first and second handle everything up to 45-50mph and then three more gears

to 130 mph. We tend to want stop light to stop light performance and get

shorter intermediate gearing.

There is obviously a ground swell of interest (if not actual cash) in

finding some way of turbocharging the US engines. The fact remains that

although the redesigned K series turbo motor cases have an open deck

design, it does not follow automatically that the EJ25G DOHC engine used in

North America is cleared for take-off. There were and remain additional

issues to consider. Simply put, when first asked about turbocharging the

2.5, the open-deck issue was first on the list and made further discussion

to my mind pointless. Lest we give an answer in search of a question, let's

ask this question: Does the 1998 2.5 RS DOHC or 1999 SOHC engine have

common design features with the open deck or closed deck turbo motors? Here

is the information I have been able to assemble. It may not be the final

answer, but it is what I have been told.

We know that the issues that resulted in the open-deck K series motor were

answered as a result of lower demands being placed on the engine due to

changes in racing rules and from actual design changes made by Subaru

engineers. However, one can not assume turbocharging has been evaluated in

the EJ25 engine. The following remarks were made to me by a Subaru

engineering person, who should remain anonymous. They stated that the 2.5

DOHC engine was a "damn the gas mileage, give me some torque" responce to

US market needs and is a "stretched" design specification engine. As a

"stretched" design, it was produced with no consideration of additional

mechanical/thermal stresses of high power and/or rpm. For the sake of the

argument, its power output at 165hp is as near its design limit as a "K"

series WRX engine is at its 280hp output. In contrast, during the same

conversation, the closed deck Japanese EJ20G turbo block and the older

91-93 American closed deck EJ22 turbo block were considered "bullet proof".

This person had no personal knowledge of open deck WRX engines at the time

we spoke.

It was specifically mentioned to me that the thrust bearing area is known

to be marginal WHEN CONSIDERING MORE POWER. Note the emphasis. Not that the

engine has a design problem - that was not the point. The point was if one

were to take the engine out to some big torque and horsepower numbers, the

bottom end was a limiting factor. This implies that the bearing webs in the

case would "walk" under the torsional loads of large power output, where at

the designed 165hp this was simply not an issue. This type problem has been

true for many engine designs over the years, by the way. Second, the

cylinder head and water jackets were not designed for the thermal stress of

turbocharging. Third is the thickness of the cylinder wall. This is a

particularly important issue when it comes to the head gasket area in an

open deck design. From the brief conversations I have had with Subaru staff

it was made clear that no special attention was paid to potential stress

from high output in the design process of the EJ25 DOHC engine. In

contrast, the design of the EJ20G Turbo motor saw every area of design was

gone over in the "WRX-RA" series engines with a mind to 400hp output. This

is the most important point that one needs to know when assessing the 2.5

motor. Just where are the limits? I was left with the impression they are

lower, rather than higher than one would assume from the rally heritage of

the turbo EJ series Subaru boxer engines.

What else can I say? I was sitting there having this conversation about the

RS 2.5 engine and feel it should be made public. It does not mean one can

not supercharge or turbocharge the 1998 EJ25 engine to a nice 250hp. It

just means that SPD Tuning Service is not going to develop a kit for this

purpose, as I will also very seldom bet on less than a pair jacks when

playing five card poker. The odds are not in my favor. Especially with the

typical urge, myself included, to turn up the boost.

In the same conversation, I was told that 1999 2.5 SOHC engine has

addressed the bottom end concerns. Of the two engines the DOHC 98RS and

SOHC 99RS, I was told the SOHC engine is a much stronger engine. In

driving the SOHC engine, it does seem to be a smoother, tighter engine on

the top end, indicative of a stronger case, among other changes. The new

1999 EJ25 however, along with a 1.8 and a 2.0 version, is a family of

"lean-burn" combustion chamber/engine management engines. I do not know how

the this aspect will play into the high pressures and temperatures of

turbocharging, nor if the valve and seat material is up to the higher local

temperatures seen in turbo motors. So again, spending the big money and

developing a reliable turbo setup remains an open issue for me. On the

other hand, I suspect that if we see a turbo motor, it will be a 2.X SOHC,

non-intercooler engine from this new family, with the necessary engineering

changes. This part of the puzzle is starting to make sense, as it is

unlikely we will see the EJ20K with the extra cost of the DOHC heads and

intercooler. The fact that we have only 92 octane gas would effectively

limit this motor to 220hp on a hot, dry day, where a 220hp 2.5 would be

much less near the edge.

When considering the use of a turbo kit, there still remains the issue of

getting a strong enough pressure plate on the 2.5 as a turbo engine without

folding the clutch yoke in two pieces or replacing the take out bearing

every 10,000 miles. This is where the WRX clutch, flywheel, and

transmission case design comes into play. Without the turbo clutch setup,

there is a weak link built into the program from the start. Also remember

there is engine management to consider. Good "Turbo" engine management

systems were one of the design features that made the 2.0 turbo motors so

effective. I still consider the use of these complete ECU "Turbo"

management systems essential for a properly tuned turbo motor. It is

really another complete topic for discussion, but to keep it short, the

fuel and ignition curves of a turbo motor are quite different from the

standard motor. The turbo control electronics ECU systems, such as that

found on the SUBARU turbo motors, are also quite refined and effective

duplication is frequently beyond the capability of the turbo kit

manufactures, especially when trying to keep the costs affordable. For

these reasons, again, I shy away from turbo kits.

That is everything I know at the moment. I remain just as cautious about

just slapping a turbo on the 2.5, especially without changing the pistons

down to at least 8.5, if not 8.0 compression. If one must play around with

"the big power" as a friend of mine calls it, just remember, it is only

money. Thanks for taking the time to read such a long article. I hope this

information has proven helpful. Thanks to Trey for kicking me into gear! I

will try to ferret out more information on the 1999 engine. The big

question: Is the 1999 engine the base for the rumored "2001" US turbo

engine or are there yet other design changes due?

Best regards to all,

==m==

Mike