Subaru 2004 Model STI Performance 

 
   

"On the road, it all means the APS STI is a more relaxing drive because you’re not constantly hunting back through the six-speed for a ratio that’s actually gonna spool up some herbs. The thing will actually accelerate in sixth now, which is really saying something."

Motor
Magazine 

 
 

 

 

STI Pistons
The North American specification Subaru STI is truly unique - unlike any other STI on the planet. A popular discussion of late amongst STI performance enthusiasts has been over the construction of the US specification STI piston.

The pictures below show the visible differences between cast US Specification and forged International/JDM Specification STI pistons. Needless to say, APS takes into account the significant differences in piston construction, lubrication and piston pin loads when specifying design parameters on APS components for the different specifications of STI world-wide.

US Specification STI

Piston weight = 414 g
Piston Pin weight = 152 g

Australian Specification STI

Piston weight = 402 g
Piston Pin weight = 162 g

Note the number and spacing of oil holes. Cast piston (above-left) has 4 holes instead of 5 on the forged piston (above-right). The forged piston also has a wider field of oil flow around the circumference.
The forged piston (above-right) provides vastly superior material structure than cast pistons.
The dish volume of the cast 2.5L piston (above-left) is significantly larger than the 2.0L forged variant (above-right) in order to maintain the same 8.2:1 compression rato in both engines.
Piston Pins pictures above and below show the differences between the US Specification (left) and International/JDM Specification (right) variants. The US spec is essentially tubular section whereas the International/JDM spec are conical towards the ends with a great deal more material towards the centre. The later provides greater strength around the area of highest loads and greater lateral strength.