Engine Internals

If treated correctly a Subaru engine is a very reliable unit. Over 400bhp is achievable on standard internals on certain models. It is however, wise to consider the mileage and previous use the engine has had before planning to tune to this level. 
On a 2.0, uprated pistons, rods and gaskets should be considered if higher power and/or track use is intended. This would cost in the region of £1.5K for parts plus the labour costs. 
For a similar sum you can buy a brand new 2.5 litre complete short engine (block,crank, rods, pistons and bearings). The 2.5 conversion with a mid sized turbo such as the AFP5/20 makes an incredibly fast car with excellent throttle response.

This will increase torque output by around 25% improving the boost response threshold by as much as 500rpm, with considerable improvements in midrange acceleration and off-boost driveability.

Any of our 2.0 level 1-5 upgrades are compatible with a 2.5 short engine upgrade, this complete set of block/crank/rods/pistons/bearings can be 
bought as a factory assembled 'short block'

This has become a popular addition where the car has had an engine failure or is of high mileage.

The 2.5 short engine shown above is directly compatible with the heads/sump/pumps etc of the older 2.0 model.

For use with all levels of upgrades consider adding an uprated oil pump and thicker steel head gaskets.
On 06/07 > WRX/STI 2.5 short engines the pistons run very tight bore clearances, this can cause issues when used at high power outputs on track or for sustained high speed runs as the pistons may tighten up in the bore causing scuffing and/or ring land breakage. 
On Level 4 and above we recommend a forged piston upgrade. The forged piston upgrade is also recommended for level 3 if used for 
sustained high speeds or if used regularly on track. 
For level 5 upgrades ,where the car is used for sustained high speeds or regular track use, we also recommend steel conrods and ARP head studs.

In all cases the original Subaru bearings and crankshaft are more than adequate.

The larger bore diameter of the 2.5 will benefit from cylinder head modification match the bore size. This can be done on request.

For outputs in excess of 500bhp we recommend the stronger closed deck block based 2.33 litre. This has thickwall liners as standard, and improved liner support from the closed deck which eliminates the gasket problem associated with the open deck blocks at high outputs. This type of block also has under piston cooling sprays which are adviseable on high bhp turbo engines.

Our own race car uses this type of block for outputs in excess of 800bhp. As testimony to the strength of the original Subaru crank, we have used the same crankshaft for 3 seasons racing, this includes using rpm up to 8500 and at speeds of up to 210 mph !