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Old 07-29-2001, 07:13 PM   #1
Marc Sawaya
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Join Date: Jan 2001
Chapter/Region: SWIC
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Out There Somewhere

Default SCC Impreza Project: EJ25 Engine dynamics.

Both of Sport Compact Car magazine's Impreza projects have contributed a great deal to my understanding of my car. I am forever indebted to the authors for the copious amount of information and expertise they have bestowed upon us, the reader. With the first Impreza project, SCC endeavored to make an RS-T, so this is very pertinent to this forum.

Many people are confused as to why Subaru powers the WRX with a 2.0-liter engine instead of the 2.5-liter one. Dave Coleman, the author of the project articles, explains a lot about the strengths and weaknesses of the EJ25 in the article below, and I consider this essential reading for those wishing to turbocharge their RS. This is part of the article that appeared in the May 1999 issue, and is long but very important.


Subaru’s powerplant is a unique package. Subaru is much like Porsche in that it began with a unique, but in many ways flawed layout (a flat six all the way in the back for Porsche, a flat four all the way in the front for Subaru). It proved that through solid engineering it could make the advantages of that layout shine through, and the disadvantages disappear.

The horizontally opposed layout of Subaru’s engine is good for chassis dynamics. First of all, it makes the engine lighter since there are no heavy main bearing caps or deep skirted blocks needed to keep the crank from being pushed out the bottom of the engine—the other side of the engine case serves that same purpose. The flat layout is also shorter than an in-line layout, minimizing the nose-heavy nature of a very front-engined car, and the center of gravity is lower, since most of the engine is in the same horizontal plane as the crank.

There are disadvantages, though, some of which are not immediately obvious. Because the engine is so wide, fitting it between the frame rails gets to be a challenge, especially as displacement is increased and the crank’s stroke gets longer. All else being equal, increasing stroke by 1mm on a horizontally opposed engine increases the width of the engine by 2mm. Push a four-cylinder all the way to 2.5 liters and you’ll have one wide engine if you’re not careful. Subaru engineers are careful, of course, and the 2.5-liter powerplant in the 2.5RS is no wider than the 2.0-liter turbo in the WRX. That in itself made us nervous.

To keep the engine compact, Subaru’s engineers gave the engine a huge bore (99.5mm) and a short stroke (79.0mm). Compare that to the 89mm bore and 96mm stroke on the 2.4-liter engine in the 240SX and you see how disproportionate that is. The problem is that knock resistance under boost tends to be related to, among other things, bore size. A larger bore means it takes longer for the flame front to travel from the spark plug to the further reaches of the cylinder. That’s more time for the heat and pressure of combustion to build until the fuel and air that haven’t met the flame front yet get impatient and have a spontaneous and uncontrolled explosion party of their own. Ping! Ping! Ping! Bang! End of project car.

The bigger bore also means there is more surface area for combustion gases to dump heat into, reducing the amount of energy left for shoving on pistons. Of course, there is a lot more to knock resistance and power output than bore size. The big bore, short stroke engine can be made to work just fine, thank you. Can you say 400-hp Porsche Turbo? We thought you could.

The other dimension that made us nervous was rod/stroke ration. The relationship between the stroke and the connecting rod length determines how fast the piston moves, how hard it accelerates, and how much of the forces pushing down on the piston actually get converted into crankshaft-turning torque. Because of the space limitations, we feared that Subaru had made the connecting rod extra short, forcing that huge, 99.5mm piston to accelerate extra hard. That could explain the low, 6250 rpm redline. To ease our minds, we decided to compare the rod/stroke ration of a few engines we are more familiar with.

The B18C engine in the Integra GS-R and the SR20DE in the Sentra and 200SX SE-R both have a relatively low (low means high piston acceleration) rod/stroke ration of 1.58:1 The KA24DE from the 240SX and Altima, surprisingly, has a much more comfortable rod/stroke ratio of 1.72:1. This can probably be explained by the fact that hood clearance requirements kept the SR20 and B18C engines short, limiting connecting rod length, while the KA24, which was originally designed for trucks, could get away with being taller. The KA24 also make up for its taller black with a two-stage timing chain that makes the cam sprockets smaller, keeping them from causing clearance problems. So what about the Subaru? The EJ25 engine in our 2.5RS falls in the middle at 1.67:1. The EJ20 in the WRX, interestingly, has a very good ration at 1.74:1. In pushing the EJ engine from 2.0 to 2.5 liters, the bore was increased from 75 to 79mm, but at the same time, rod length was increased from 130.5mm on the EJ20, to 131.7mm on the EJ25, meaning the engine should be 10.4mm wider (the extra stroke plus the extra rod length on both sides)- but it isn’t. The only explanation is that the wrist pin in the piston has been moved 5.2mm closer to the top of the piston. This might make the piston weaker, or it might not. The only way we’ll know is if we start breaking things- hopefully we’ll remain ignorant.

Our final concern is the head gasket. With the EJ25 pushing the limits of how big the EJ engine series can be, cylinder wall thickness is likely getting thin. Because the EJ25 has an open-deck block, the only cylinder sealing area is that thin ring around the top of the cylinder—the rest of the block surface from the edge of the bore all the way to the outside of the block. Again, all we can do here is be aware of the potential weakness.

Don’t let all this conservative whining about potential weaknesses scare you off, though. Cylinder head design is the most critical factor determining whether we will be able to make good power, and this is where the Subaru engine looks quite good. The standard pent roof combustion chamber with a central spark plug and the valves at a relatively shallow angle relative to one another seems to be the combustion chamber of choice for virtually every high-output, four-valve engine. That’s exactly what we have; in fact, the twincam 2.5-liter engine in our ’98 2.5RS shares its cylinder heads wit the 280-hp WRX. The US engine has different cams and different valve material, but the basic shape—a pent roof with a nice, shallow 30-degree valve angle—is exactly the same. If you have one of the single-cam Subaru engines, well, your results may vary.

Based on a hard look at the engine, our plan of attack is as follows: Because of the large, heavy pistons, unknown valve springs, and a rod/stroke ratio that is not as good as it could be, we are going to keep the redline where it is, at 6250 rpm, and try to make the engine as responsive as possibly in the lower part of the rev band. Because of a high, 9.7:1 compression ratio, our gasket sealing fears and pistons that are of unknown strength, we will keep the boost relatively low—under 10 psi, and maximize power output by ensuring minimum airflow restriction and the coolest, most dense air charge we can get. As a bonus, this approach should also give us a very easy-to-drive street car.
I hope this is as much of a help to you guys as it was to me. My question, as a 2000 RS-owner, is how is the SOHC EJ25 different? Is it stronger? Weaker? Are the ratios different?

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Old 07-29-2001, 08:18 PM   #2
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That's an old article. I think Dave's opinion of the EJ25 has changed a bit since then. Especially since the white MY99 2.5RS turbo project car is making more power than any EJ20-powered WRX he has ever driven. With gobs more torque too.

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Old 07-30-2001, 02:18 AM   #3
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Marc, that is exactly the kind of information that I was looking for about a week ago (still hadn't found it for the most part). Thanks for posting that, I'm trying to come up with somewhat of a good design for an ej25 block and the proper turbo to mate to it with what piping and whatnot, not exactly the easiest thing to do, and in doing my schematics I needed several of those figures!

Shiv, if you see this, would you mind asking Dave, if he has time, to consider revisiting his statement thats posted above, maybe in the next section of the Project 2.5RS as a little sidebar? Would be interesting to see what he has to say about the ej25 now.

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Old 07-30-2001, 08:07 AM   #4
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I would like for the Impreza project to be revisited. SCC is my favorite magazine but everytime lately I am dissapointed to find no Impreza. Please bring it back to me.
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Old 07-30-2001, 10:38 PM   #5
R Diamond
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Actually, I think Coleman deserves some credit for being pretty accurate. The main weakness on the MY98 EJ25 is indeed the head gasket.

Shiv, his recommendation about keeping boost around 10psi (without internal mods) exactly what you recommend today!

Sure, he's wrong about the EJ20 being superior to the EJ25, but you can't always get everything right.

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Old 07-30-2001, 11:45 PM   #6
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The reason Subaru is running a 2.0 litter to power the WRX is because the WRC rules don't let you run anything bigger
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Old 07-31-2001, 10:40 AM   #7
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Perfect maybe they can keep the EJ20 Turbo Engine in the WRX and more properly build an EJ25 Turbo engine for the Legacy. They could get away with widening the body a little to host a wider engine.

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Old 07-31-2001, 10:49 AM   #8
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Default Same sized

If I am not mistaken the engines are the same size, interms of how much room they take up in an engine bay. I don't for see them changing the size turbo engine on a legacy because of the main reason they use a 2.0 is because engines larger than that are heavily taxed in many countries most specificly Japan.
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