Just completed STI 2.5L Shortblock in WRX - Details inside
I just got done putting the USDM STI block in my 2002 WRX. Circumstances worked out for me to do this - it wasn't something that I was planning on, but when a buddy who was suppossed to buy it off me fell through, I decided to give it a whirl. There are many others that have discussed this subject, and I want to return the favor that they have shown to detail some of my findings. (Thank you all for the input on this swap - it has been very helpful so far.) I am posting this to hopefully help someone else with their decision to make this swap or to help others with some details of the swap so they know a little more about it. I am not trying to portray myself as an authority, nor as a "tech guru", just trying to help with what I have found.

This is going to be long, but that is because there is much to present.

The Goal
The goal of the car is to make a 400-450 crank HP on pump 91 octane fuel. The car must be streetable and be used for daily driving. The car is not designed specifically for the 1320, auto-x, or a dyno sheet, etc. Car modifications must be "bang for the buck mods" with a limited budget. Based on my goals, I felt the STI block would fit well and could handle the HP numbers. It fit the pocketbook very well.

(I am not saying that it cannot be used for dragging, or Auto-X, or even more power, those just aren't the goals. Those that are specifically looking to see if this swap will accomplish those items for themselves may feel unsatisfied by this thread. This is a thread to explain the details of the swap more than for HP and 1/4 mile bragging rights.)

The Block
USDM STI 2.5L short block. No changes to internals, etc.

The Heads
Based on the discussions had before, it was noted that the WRX head will not flow well for the HP numbers I did want to accomplish at high RPM. I have a peer that has a built 2.5L engine in his car edged from a N/A 2.5 block with forged internals, closed deck, etc. and he found that getting over 400 crank HP on his stock head was harder to do as well. (Although getting 400 HP was possible...and probably more is possible...over that value was more difficult than under 400 HP.) I decided that the head needed some work.

I sent my heads to Performance Race Engineering (PRE) in Portland, Oregon. They did a great job on the heads - it was the most beautiful thing I, or the mechanic, who has built many a domestic and import race engines, had ever seen. They put in titanium valves, dual springs, retainers, etc. Then they enlarged the ports on the head, polished it up, and port matched on the exhaust and intake sides. The head was desgined to flow for 5-600 HP with a 8500 RPM rev limit. But of course, we will never go to those limits with it.

(I set this in motion with the information I had at the time. Through some great contributers like Sponagle, Quirt, and Nathan at TXS, it is now being discussed the idea that the WRX heads flow fine, it is the cam that is the problem. I know that sponagle proposed the idea of just switching out the wrx cams for the Sti cams. I hope this is discussed more because this would be a great alternative for many reasons. I just like the idea that you could buy $600 or so in STI cams and it would do the job rather than a lot of expensive head work. Throw in a $1400 block , and for $2000 in engine parts you could have a great upgrade for wrx owners.)

Parts Of Note for the Swap
- I used a WRX gasket kit from Subaru, although it was missing a few gaskets needed. We ordered those from Subaru separately.
- I used a Cometic aftermarket STI head gasket.
- I used a new WRX timing belt.
- I used ARP head studs.
- I ended up using an aftermarket exhaust manifold. I was worried about exhaust flow out on the stock WRX manifold, so I bought a new one. I am not sure if it is necessary. I will let the more experienced make that conclusion. I thought it couldn't hurt, so I added it.

Notes on the installation:
- There is an extra cooling port on the bottom of the block that is not "natural" for the WRX configuration. There is probably a way to use it, but we ended up plugging it up.
- I had to get some new shims (sp) for the new head.
- I used my "WRX" clutch and flywheel. They are an Exedy flywheel and a Exedy stage 2 clutch
- The plastic piece that sits below the engine didn't fit anymore due to the exhaust manifold.
- All other parts, tubes, lines, sensors, oil pan, etc. were used from the WRX engine with no need for replacement or modification.

Previous Parts of Note:
- I already had a UTEC, PE 1820 turbo, 650cc injectors, Perrin Fuel Rails, and MRT FMIC. I wouldn't have tried to do this swap without those componenets.
- I obviously had the turbo back, shorty intake, etc.

Instalation Difficulty:
The mechanic that did the swap did not note any major issues and it took the "normal" time to do a swap/rebuild like this. I don't even remeber him having any real minor issues
other than the items listed above. We did have an O ring rip on an injector which temporarily sprayed fuel, but this is more of a "user error" than a problem with the swap.

Anything else?
I was surprised how well the WRX ECU adapted to everything so well. It acted as if it was so comfortable with the new dispalcement and knew exactly what to do. I have noticed that it likes running much more/higher timing than the WRX did before like it was prepared for the change. It has been very easy to tune with the UTEC with the proper wideband O2 sensor and EGT guage. (We don't have an optimum tune yet...still trying to behave with the motor.)

So How is it?
As of this writing, I only have about 250 miles on it. I will try to get more as I try to give you a better idea. But it is great, I really enjoy it. I haven't been in the high RPMs much, but the low end torque is great. I have had no detination on 91 octane, or anything like that, although I should note that right now we are running conservatively rich and have not pushed the motor hard. But the PE1820, on rich fuel, spools very quickly. But the drive is nice, very smooth, exactly what I wanted. It flows well, and the motor doesn't feel "choked" like I sometimes felt on my 2.0L engine. I have no doubts that I will be able to achieve my goals.

OK, What about the disadvantages?
Well, the engine itself is fine. All the issues that I will call "disadvantages" have to do with my driveline/transmission. (what a surprise, huh?). The clutch is having a hard time with the new torque, and I can feel it skipping along with the normal "clutch chatter". I didn't think the clutch would be giving me trouble, but I know that I am probably going to need to find another solution - wether it be a different clutch or a stronger clutch, I don't know.

It goes without sayting that the transmission will be an issue. Anyone who thinks they can do this swap without a war chest set aside for the transmission should reconsider. Everyone that I have known that have put more displacement in their WRX has gone through their OEM tranny in 3-6 months. I don't expect myself to be an exception.

Thanks NASIOC:
Like Sponagle said once, "let's keep this board techincal". I need to thank Nasioc for this board, and that it is technical and that we can share details. I really need to thank sponagle, Nathan at TXS, and Quirt for their help on the STI shortblock that helped me greatly. Especially Quirt and I-Speed, who has no financial interest in sharing knowledge that could help the rest of us, but provided it anyway for the benefit of the community about the STI shortblock.

Thanks