"Hybrid" EJ25 Block with EJ20 Heads

[lsfourwheeler]

I have just recently bought my donor car for my 818 and this car has all sorts of funny business going on at the moment that I'm hoping to resolve before I get my kit. One of the biggest quirks is that it has a EJ257 STI short block and EJ205 heads which seems to be a fairly common thing for WRX swaps as it doesn't require a new wiring harness an ECU. Is anyone on the forum here using a similar setup or have any suggestions for things to look for in this kind of swap?

[q4stix]

You can essentially treat it as any other EJ series engine. The EJ205 heads flow a little less and will raise the compression slightly compared to the EJ255 heads. Think of it like putting a built stroker kit in a small block V8. It will tune roughly the same as a 2.5L WRX would.

[Wayne Presley]

But either the heads need to modified for the 2.5 shortblock or JE hybrid pistons need to be installed for the combo to work well. If you have a 2.0L harness, then no other changes will be needed.

[STiPWRD]

^ what he said. I'm running an EJ257 block with EJ205 heads on my 818 and installed the JE pistons to get the compression ratio back where it needed to be. You can't really inspect for this sort of thing unless you take the engine apart.

[SixStar]

Hybrid builds are very common in the Subaru community. You have an aging population of 2.0L cars that have been out of production for 12 years and have a very high failure rate.

Wayne is correct that you will need to go with the JE pistons or have the heads combustion matched. If you're in the minority of the population running the car at a high altitude you don't technically need to do either. They make great power too for the simplicity of the build.

Here's mine.
GfOqKMx.jpg

[lsfourwheeler]

Thanks for all the help! Would y'all recommend the JE pistons or machining the head to match? Most opinions I've heard are, if you have the money, machine the heads to match and use STI head gaskets.

[EODTech87]

Since you already have the short block put together I would just machine the heads rather than swapping pistons.

[awd-turbo]

There is a company out in CA I can find the., But they machine the heads to flow for that block. I used them once when I built a WRX about 8 years ago. They did all the mapping of the port and heads for best air flow.... Very reasonable.... I think it was 800 total..

[lsfourwheeler]

There is a company out in CA I can find the., But they machine the heads to flow for that block. I used them once when I built a WRX about 8 years ago. They did all the mapping of the port and heads for best air flow.... Very reasonable.... I think it was 800 total..

Hmmm, I'm a good ways from California but share the name of the shop as I'd be interested in knowing about that. I'm assuming $800 doesn't include changing the cams to STI cams or anything.

[EODTech87]

I would suggest checking with IAG especially if you are only looking to match the combustion chamber as they are very reasonably priced($350) and do great work on EJ series engines. If your wanting to change cams it will require checking valve lash which runs ($250). You might also check out Headgames Motorworks. Their site doesn't list prices for combustion chamber matching but I've heard good things about them.

[q4stix]

A first question I have before you start... is this a running donor with the entire engine already assembled? That was the impression I had from your first post.

[lsfourwheeler]

A first question I have before you start... is this a running donor with the entire engine already assembled? That was the impression I had from your first post.

Yes, the engine is currently fully assembled and running. But it has numerous issues at the moment. To sum it up, I'll just say it does not run particularly well and I plan to do a lot of maintenance work to the engine. The swap seems as if it was done by a monkey with a wrench. Hoses everywhere, threads stripped, leaks, and god knows what else.