JohnyHachi6 Reader
6/25/12 6:35 p.m.

I'm trying to decide which EJ25 engine I should put into my new impreza RS. I have the stock engine (still in the car) which is the DOHC EJ25D (I believe). I also have the later, Phase II, SOHC EJ251 (?) engine in a `99 parts car.

It sounds like they made some updates to the piston design and such with the Phase II engines, and they made a little more torque, but I hear people saying that the DOHC engines have a slightly better top end. I'd be using the car for a DD, auto-x and rally-x so the torque of the SOHC engine sounds good to me. If I were to swap in the later engine, would I need to swap in the MAP sensor, harness and ECU, or could the DOHC management system handle the SOHC engine without issue?

I don't know enough about them to make an informed choice and haven't had much luck finding trustworthy info on NASIOC. Anyone here have an opinion or some more info on these engines?

Thanks for the help.

Ojala Reader
6/25/12 8:19 p.m.

I dont think I have ever done that but it depends on which engine you get but the connectors, egr , intake manifold, heads are different and dont swap. You would need to do something about the knock sensor. The phase II use the JEC ecu while the phase I use the old Denso ecu which has a different pinout/number of pins. So it kinda comes down to either swapping the bottom end or swapping wiring.

Is the head gasket bad?

EDIT: I went out to the garage empty handed. I thought I had a phase 1 EJ25 out there but I must have gotten rid of it. So I am pulling the above information out of the dank recesses of my memory and you should take it for what very little it is worth

corytate Dork
6/25/12 8:45 p.m.

from the (I think) one time I drove each variant of 2.5RS, I lked the DOHC better. by alot.


6/25/12 9:17 p.m.

My understanding is that you would have to swap almost everything over electrically - ecu, sensors, everything. Those years are not "universal wiring" cars, as I understand, so you are likely to need some mods to the wiring.

The top end of the DOHC motors is not that much better and you have to spin the motor pretty good to get there.

I would make sure the HG's are good and put the DOHC back in it.

SlickDizzy UltraDork
6/25/12 10:30 p.m.

DOHC motors tend to have chronic head gasket issues, FWIW.

clutchsmoke Reader
6/25/12 10:48 p.m.
SlickDizzy wrote: DOHC motors tend to have chronic head gasket issues, FWIW.

Yeah this was the first thing that popped into my mind. If I were to go to the trouble of an engine swap on a 2.5RS I would go with an '02 or newer 2.0 or 2.5.

fornetti14 HalfDork
6/26/12 6:06 a.m.

The dual cam motors also had puck type shims which were a pain to deal with IMO. That said, I'd go with whatever is in the car already. No sense in causing yourself extra pain.

The single cam Phase II motors had a screw/jam nut set-up which made valve adjustment a breeze.

If you decide to build up a dual cam motor I have an ARP stud kit sitting in my garage for $100. I might have some old lifter shims somewhere as well. If you have to buy them they're about $5-$6 each at the dealer.

6/26/12 9:56 p.m.
SlickDizzy wrote: DOHC motors tend to have chronic head gasket issues, FWIW.

Mine had over 400k kms (250 k miles) on it when I sold it.

If they are fixed and fixed right, they will run for a long time. Most people blow them up because the HG leaks internally.

6/26/12 9:57 p.m.

If you are going to swap it - might as well go turbo.

ValuePack Dork
6/26/12 10:10 p.m.

If I were to pick one of your two engines, it'd be the single cam. The '99 is the EJ253, better long runner intake(good for turbos), stronger torque curve than the '98 EJ25D, slightly better head gaskets than the '98's 25D or the EJ251 of the '00-01 cars.

But it's not me doing the obscene amount of work for nt much gain. I'd just haul the 25D out and do the gaskets, it'll be good to go once sorted for quite a while.

NASIOC has been utterly worthless for normally aspirated knowledge since the WRX was released in the US; too much ore for too little iron. is the place to be, it's predominantly N/A-centric and is a pretty good crowd of informed folks. There's a few tidbits of useful info to be found these days at, as well.

Good luck with the car, looks like it'll be a lot of fun.

JohnyHachi6 Reader
6/26/12 11:26 p.m.

Thanks for the advice guys. I started on the engine this evening and have the manifolds and all accessories removed. I'm waiting for one of the engine mount bolts to soak in PB blaster overnight and I'll probably pull the engine tomorrow. If everything looks good internally, I think I'll just put the EJ25D back in (with new gaskets). If I run into anything more serious though, I'll probably pull the EJ253 (SOHC) from my parts car and see what shape that's in.

Just out of curiosity, any of you guys know if it's possible to put the EJ25D heads on the EJ253 block? Also, how would this setup do for compression ratio? I wouldn't mind having the updated thrust bearings and pistons form the EJ253, but I'd rather not swap over all the electrical crap right now.