Our goals for this project are to build the ultimate street performance engine without using forced induction or nitrous. As we plan on building this as an example of the potential of this engine for the street, we are going to focus on a reliable setup that can be driven on a daily basis, run on pump gas, and still provide good idling characteristics and gas mileage.

As this setup is to used for everyday street use, we are going to work on making improvements in the RPM range where the engine spends most of its time. Since torque is such an important element to a good performing street car, we will be focusing our street engine buildup on creating a strong torque curve instead of a high revving engine that might fall on its face unless revved unmercifully.

Before we get started, lets take a general look at this engine. The 2.5L SOHC Phase II engine utilizes an aluminum, liquid cooled cast block and is currently being used in the late models Subarus including the 1999-on Impreza 2.5RS, 2000+ Legacy and 2000+ Foresters.

Now that we have the basics out of the way, lets get our hands dirty and find out just what this engine is all about. While we've read tons of speculations about how to make power from these engines, very few people have actually pulled one apart for the purpose of making improvements. And those that have are extremely secretive about it. It's time to stop guessing, lets go into this motor and get some good hard data!

Engine Disassembly - Cylinder Heads
First thing we're going to do is pull off the stock cylinder heads for some careful analysis. In nearly every production piston engine made, cylinder heads are where you can make the largest improvements. Since the Subaru 2.5L engine is an Over Head Cam engine, the cylinder head also houses the 2nd most important component to making power which is the camshaft. Also remember that since we're using a horizontally opposed engine, we actually have two separate cylinder heads and cams for each bank of cylinders similar to a OHC V-style motor. Finding the right combination of cylinder head modification and camshaft profile are going to be our key to making our street motor a true beast!

First thing we need to do to remove our cylinder heads is to pull our intake manifold. We also need our intake manifold for some important flow bench testing so we can properly match it to our cylinder head and cam. Removing the intake manifold is easy as it comes on as a complete assembly.
Next, we'll pull our timing belt. Simply remove the main crank pulley using a 22mm socket and remove the timing belt covers. The timing belt comes off easily by removing the #2(red) idler pulley. Also make sure to mark the alignment points on your timing belt for when it comes time for reinstallation.
With the timing belt off, it's now time to remove the valve covers to gain access to the six bolts on each side of the engine that hold the cylinder heads to the short block. Also make sure you remove the small bolts which hold some accessories to the head such as the dip stick tube and rear belt cover.
Loosen the cylinder head bolts in a criss-cross pattern starting on the outside. Leave the top outer bolts hand tight to prevent the head from falling off after you remove the other bolts. Once loose, carefully remove the cylinder head and gasket. Be careful not to scratch the mating surface to the block!
Check out the nice cylinder heads Subaru has built. These heads have technology in them that would make most domestic V8 guys cry in envy. As you can see we have an aluminum head with very sturdy valvetrain components. Buried underneath the valvetrain and camshaft cap is the actual camshaft. These heads also feature a trick roller rocker/cam assembly with screw-type valve actuation adjustment. Very nice!
Here you can see the open deck design. We will be taking a closer look at the insides of the block in the near future to see any areas of weaknesses that might be addressed. We will also use this time to investigate any options to make this block more bulletproof for higher cylinder pressures (ie: turbo/nitrous).