Installing SOHC or DOHC EJ25 heads on an EJ22T
Hands on test by Larry Witherspoon

Finally, the real poop on the cold hard realities of a 2.5 head swap onto a 2.2 turbo block - reading time 7 - 8 minutes

Yeah, troops. All I heard and read at first was that the late model (1999 - 2001) 2.5 "Phase II" SOHC heads were a bolt on swap for the 2.2T motor that came in the 1991 - 1994 Turbo Legacy. A bolt on sounded like a good thing because the majority opinion had the 2.5 head flowing a lot better than the 2.2T head. I have also seen flowgraphs on the Cobb Tuning website backing up the majority.

I stumbled across a pretty good deal on a pair of Phase II heads, so I leaped before I looked. Took them to my buddy Dan Paramore of DPR Racing Development Dan did the heads on my last car six or seven years ago. He's been around a long time and he knows his stuff. Just got back from the Salt Flats setting World Land Speed Records in a car and on a bike. We talked about what I want out of these heads, and some of the things we still needed to know. Homework for me. I think I'm finished with my homework.

So straight up, yes. The heads do bolt right on. The head bolt pattern is the same. However, depending on what you're after, just bolting them on is not necessarily the whole enchilada. True, the 2.5's do pass gas a lot better on a static flow bench than the 2.2T's. And certainly static flow is a big key to what we can expect under dynamic (read real world) conditions.

Head flow testing is usually done with equipment which can duplicate standard conditions of pressure and valve lift, to give a result in volume per unit time, commonly cubic feet per minute. The stock 2.5 SOHC Phase II heads flow about like you could expect to achieve from the 2.2T heads after a good rework. Most common type of rework is a procedure called porting, where material is ground away from passages called ports in the head which are not shaped for optimum high speed gas flow. After the ports are reshaped they are polished. But there is a lot more that can be done with valve seats, combustion chamber shape, and other stuff maintained as top secret for reasons of national security.

Now that I've digressed a little, let me do it some more. I'll get back to the bolt on thread in a bit. After Dan and I decided on what would be a good selection of head operations for my purposes and pocket change, I found a couple of contacts that had the info I was looking for. But first you have to understand why I was looking for it. This project started out as a desire to bolt on an intercooler so I could dial up a little additional boost without any problems. My ride is a daily driver and grocery getter. I'm not planning a trip to Bonneville. I just want to have a little fun scooting across the intersection ahead of a few rice burners and an occasional Mustang. My objectives are to keep what we call the "ilities" in the aerospace industry within reason...reliability and maintainability, serviceability, get what I mean; dependability. I'm not after all the horsepower at any cost. I just want to "enhance" what I have a little bit. If you're racing or record setting, that's different.

Ok bunky, back to your fancy heads you're ready to bolt on, but first I would like to call your attention to a few details. First of all the 2.5 head has a bore of 99.5 millimeters, which is 2.6 mm larger than the 96.9 mm 2.2T block bore. You'll have a 1.3 mm step all the way around the 2.2 cylinder bore where it mates to the 2.5 head, to negate who knows how much of the flow advantage and/or head work unless it is dealt with. Not too bad, but not really what I call bolt on. Strike one.

Next, the 2.2T block coolant passages do not quite match those in the 2.5 head the way they should, though supposedly enough to be effective. We'll, ok, maybe so, but still, another not quite bolt on but not too bad thing I don't like. And While we're talking about mismatches, the intake manifolds have different bolt patterns, and the spacing between the ports may be different. Different or not, now you need a 2.5 manifold or maybe some kind of work done on the 2.2 manifolds. But the heads bolt on you say again? Yup, sure do. And I say strike two.

Big item of concern is the cam. Remember that static flow that looked so good? A turbo engine, a "REAL" turbo engine, uses a cam profiled specifically "designed to match the characteristics of the turbo engine", to paraphrase a Subaru Technicians Reference Guide. Using the 2.5 normally aspirated cam in our turbo engine will result in some loss of that static flow advantage under dynamic conditions. According to one of my sources, the 2.2T cam will not fit the 2.5 head. If I'm gonna do this swap, I really feel like I need a new cam. Now I picked up the heads second hand from a salvage yard. Reground turbo cams aren't likely to be so easy to stumble across. Fact is, cams would cost more than the salvaged head which I'm already thinking about putting money into. This kind of isn't what I had in mind when I was expecting a bolt on. But they would still bolt on you say? Yes, it's true, but I say strike three.

Then finally, the right side 2.2T cylinder head has 3 holes for the turbo oil supply, oil return, and coolant return. Also a right side camshaft support has been added to the 2.2T turbo engine in order to supply oil to the turbo and return coolant from the turbo. Guess what's not on the 2.5 head. Right, it could be made to work though. There's an oil plug on top of the block towards the back, and you could tap into the heater hoses or something for the coolant return, and the heads still bolt right on! But remember, I'm wanting to enhance my turbo engine, not bolt on something and then have to adapt ten other things to make it work. To me that's not an enhancement it's a regression. Strike four.

Hey, if you're after all the power, I say more power to you, go for it. For me, after getting all this way into these heads, I'm backing off and bailing out. I'll stick with the 2.2T heads for whatever they're worth. They come with a turbo cam, the manifold is made to match, and all the places for oil and coolant are cool. So does anybody have a pair of 2.2T heads they want to sell or trade for some 2.5's, or does anybody want to buy a pair of 2.5 SOHC Phase II's ??? The bidding is now open, do I hear half a million from the gentleman sitting at the keyboard reading this expose' ?

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Offline contact is fine

Larry Witherspoon
Southern California