The Problem:
As most people know, when you run a Subaru 2.5l shortblock with 2.0l heads, the compression ratio is too high to run safely and efficiently especially on pump fuel. The common approach to resolve this issue has been 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 reducing the effectiveness of 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. While some of these setups can tune ok, they generally experience inconsistencies under sustained full throttle operation. Drivability and fuel economy are also adversely affected. Some have also tried using custom pistons to bring down the compression ratio, but this has a very similar effect to a thicker head gasket.

Our Solution:
Instead of using a thicker head gasket or low compression pistons, we decided to try something a bit different. We worked closely with our machine shop to carefully 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 and combustion chamber shape of a complete STI engine. This improves the effectiveness of the quench area and the overall efficiency of the engine.

The results with this approach shows big improvements 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 a 2.5l STI rather than a 2.0l WRX.

Here are some pictures of the process:








Price: $450 with bare heads. This service can also be combined with any of our other head services such as a valve job and re-surfacing.

Please download and fill out this order form when placing an order or sending in heads: http://eqtuning.com/forms/EQ-HEAD-FORM.pdf

Thanks
-- Ed