Originally Posted by EQ Tuning

We are excited to announce that we have a great new option for the 2.0/2.5 hybrid folks! As most people know, when you run a 2.5l shortblock with 2.0l heads, the compression ratio is too high to safely run. The common approach to resolve this is to use a thicker head gasket to try to bring the compression ratio down. While this can bring the overall compression ratio down, it is by no means ideal. The resulting combustion chamber shape remains inefficient since you're leaving the 2.0l chamber in the heads untouched. You are in essence shrinking the quench area and introducing several sharp edges to the combustion chamber that can build up heat and cause knock. This makes the engine less efficient than a full EJ257 with EJ257 heads.

Instead of using a thicker head gasket, we decided to try something a bit different. We worked closely with our machine shop to actually map out the combustion chamber of the EJ257 STI head and create a CNC program to replicate the exact same chamber every time. Using this program, we then set up the 2.0l head castings in the CNC and machine out an exact copy of the 2.5l combustion chamber into each 2.0l head casting. We are then able to use a quality OEM head gasket and maintain the exact compression ratio of a complete STI engine. This greatly improves the quench area and overall efficiency of the engine because you now have the exact same combustion chamber as a full EJ257.

We have done several hybrid builds with this approach now and have seen great results not only in overall power output, but especially in terms of crisp, smooth off-boost drivability and torque. We've also seen a significant increase in fuel economy as compared to a standard hybrid build. From a tuning standpoint, the difference is so significant that we had to spend quite a bit of time building new base maps for these builds. Under low load conditions, these setups can run as much as 15-20 degrees more timing advance than a hybrid build with a thicker head gasket. This was a great thing to see as we end up with a timing curve much more similar to an STI rather than a 2.0l WRX.

This service is available for any EJ205 heads and its $375 for a set of heads.

Thanks
-- Ed

Originally Posted by Concillian

Aside from matching the bore without need for machining, and having higher lift stock cams, the 2.5L heads flow about the same as ej205 heads.

If you had your combustion chambers opened to ~98mm when you did your hybrid, as is proper, then there is little reason to swap castings.

If not, you really should, as this will significantly improve your resistance to det, which will allow a tuner to get more power at virtually every RPM. Cams make a difference too, WRX cams will start the torque rolloff a little sooner than the STi cams, and the STi cams sooner than aftermarket 264 or 272 cams.

If the heads have some miles on them, it can be worth a valve job just to refresh the seat & keep a good seal, especially if you plan to keep them on for a long time (100k+)

Opening the combustion chambers or swapping to B25 / V25 / D25 castings is WELL worth it if you haven't done so. I think it's around $300-400 to have chambers opened. valve job is probably closer to $5-700, but might be cheaper if combined with chamber opening since they have to disassemble / reassemble the valves & springs anyway.

Timing belt, water pump, etc... is hard to answer without knowing mileage and past maintenance history. Things to replace around 90-105k are
- timing belt
- all timing belt idlers
- water pump
- drive belts
- hoses

Originally Posted by blue04WRX

i also have a hybrid build done by eq, and my heads are the same as mentioned above, and my car runs super smooth, it feels like great and its not even anywhere close to being the same price as if you were to buy 2.5l heads.

Originally Posted by mr_silas

first of all thank you so much for all the information.

as far as the 98mm combustion chambers, do you mean liked honed out for forged pistons? because im pretty sure i dont have forged pistons.

kinda on a budget, but if its a "must do" than ill consider it. im hoping this motor should last me at least 70k or more. im not gonna beat on it to hard but i know the ringlands on #4 are pretty re noun for breaking.

alright so ill have to look into cams. if i dont have sti ones than ill have to consider getting some.

as far as the valve job goes i guess the only way to tell if i need new ones is once i get down there right?

Originally Posted by Concillian

The combustion chambers in the heads. Where the valves are. 2.0L engine is ~92mm, 2.5L engine is ~98-99mm. If you don't get 2.0L heads enlarged in there, you're leaving power on the table... at every RPM. You get a little corner right where the piston comes up. Lemme find that Tomei document, it has a good picture in Chapter 3...
http://www.tomei-p.co.jp/news/Report-EJ25-V1.html#C3.4

The issue being that the stock 2.5L bore is bigger than the 2.0L bore, so you get "cap" that's too small for it. Like a mushroom, but the other way (mushroom is big cap, small stem. Hybrid is small cap big stem)

STi cams, watch the used market, they go for like $150 or so. They require some modification to be used in non-AVCS heads, but it's easy. It's not as big a deal, but if you're going to do it, when the heads are off is the time.

Valve job is machining the seats so they match your stock valves. Also gives some performance benefits. This sometimes goes out a bit after 100k+ miles.

My current engine, for example, is 150k, and I'm expecting 190k before I rebuild. This is a reasonable expectation from a stock block if you don't run it at the limit. Now if you're expecting 150k out of your block and your heads already have 100k on them, well, you may want to refresh the seats. If you're expecting 70k and they only have 50k out of them, well then it's not a big deal.

Originally Posted by stupidchicken03

If you put in the 2.5 heads wont you have to put in an STi ECU and do a lot of rewiring under the hood as well?

Originally Posted by Concillian

There are two issues with the 2.5 heads:
1) oil. AVCS cams have holes for oil (tap them, insert setscrews, JB weld it closed). AVCS oil lines and solenoids must be in place to keep oil from going everywhere, and only going where you want it. Even if AVCS is not used, you need to have AVCS lines and solenoids. Using non-AVCS cams (ej205 stock or aftermarket) will solve the cam holes, but stock AVCS cams are an upgrade over stock ej205 cams.
2) Cam sensor position is different.

Solution to 1 is as mentioned, plug the appropriate holes in the appropriate ways.

Solution to 2 is to either run the cam sensors from a 2.5, or to fabricate a mount for the 2.0 location. Cosworth makes a piece of aluminum that does just that.
http://www.rallysportdirect.com/shop...et-p-3244.html

Then you use your stock cam gears with cam seals for the 2.0. Any Subaru performance shop is likely to know all this.

The alternative is to wire up AVCS and use it, but that's significantly more complex.