Subaru EJ25-EJ20 Hybrid Builds

Subaru EJ25-EJ20 Hybrid Builds

Bolting EJ205 cylinder heads onto an EJ257 block can be quick and fairly affordable way to get an old 2.0L car back on the road. It can also create some complications due to the mismatch in combustion chamber sizes. We’ll show you the possibilities, procedures & caveats of this often misunderstood & misinformation filled swap.

STi shortblocks seem ubiquitous these days. Everyone & their cousin seem to be hocking built, closed-deck, EJ257 shortblocks capable of supporting 9900 RPM, 1200 bhp, guaranteed to gain all the cred at the local vaped-flavored parking lot. TLDR- there are plenty of EJ257 cases available to replace a tired or broken EJ205. They beauty is they bolt together in a fashion not unlike legos.

EJ205 heads have a smaller chamber volume (49cc & 92mm-ish diameter ) compared to the (often 56cc & 99.5mm-ish diameter) EJ257 block. This effectively increases the compression ratio when these parts are combined. Higher compression can be beneficial for power output, but on an engine originally designed for 91 octane fuel at 8:1 CR, it can lead to detonation (knocking) or pre ignition. Detonation occurs when the air-fuel mixture ignites in more than one place in the chamber, the multiple flame fronts collide creating a large pressure wave resulting in an often audible know that sounds like an early 1980s American car in the heat of summer. Pre ignition occurs when the flame is propagated prematurely due to the pressure and heat within the cylinder- often from a sharp edge or unburnt hydrocarbon ember. This abnormal combustion can damage the engine relatively quickly.

To prevent detonation with the higher compression caused by the EJ205 heads, the engine control unit (ECU) would need to retard the ignition timing. This means the spark plugs fire a bit later in the compression stroke, reducing peak pressure and temperature inside the cylinder. While this prevents detonation, it sacrifices some power output and engine efficiency. In severe cases, the ECU may not be able to retard timing enough for safe operation on 91 octane fuel. In that case, you'd be forced to use higher octane fuel, which can be more expensive.

The fix works three ways. 

1. Thicker Head Gasket- this is the least favorable in my opinion. More room for dead area away from the combustion, quench pads on the deck of the head near the block are mismatched to the bore & piston. Tuner will likely need to run less timing that optimal for MBT (maximum brake torque).

2. Larger Dish Piston- this option is the most expensive unless you’re already building a block. Quench pads & bore interface are still less than ideal but compression ratios are down around 8.2-8.5:1 depending on exact setup.

3. CNC Chamber Profiling- this is the most effective option in our opinion. We’ve scanned the B25 casting, EJ257 combustion chamber and created tool paths to match the profile shape and volume using a CNC mill. Your tuner will agree, this option will make the most horsepower, run the smoothest & live the longest out of these three options.

If this sounds like something that could benefit your build, please reach out.

Back to blog