Type RA vs. STI Engine Updates

08 Jul 2018

Twenty-five years after the original Type RA rolled off the production lines in Japan the US market is finally getting it’s own. Apart from the monster 6-piston monobloc brakes and carbon fiber roof, the USDM Type RA comes with “upgraded pistons” and “more aggressive” ECU mapping.

This lead to many Subaru enthusiasts speculating on just how upgraded the pistons really are. So Jeff Sponaugle took it upon himself to order a fresh, new Type RA shortblock from Subaru to find out. This is what he found.

Type RA Block & Crankshaft

The block is the same casting as the latest engines, numbered “705.” The crankshaft is the same nitrided, full-counterweight design found in previous EJ25s. Unfortunately, the crankshaft doesn’t have the additional oiling ports found in other special edition EJs. The bearings and clearances thus far have been within typical late-model EJ257 specs as well.

Type RA Piston Differences

Upon initial impression, the Type RA and STI piston are both machined cast pistons. The dish volume measured out very close to each other and the valve reliefs are the same depth. Unfortunately the Type RA does not have forged pistons as some had hoped for, although we haven’t ruled out the possibility that the Type RA pistons are made from a different alloy that is less susceptible to fracturing (more ductile), as they are supplied by Hitachi rather than Aisin. The crown on the Type RA pistons appears to be 1 - 2mm thicker than the STI piston as well, which is hard to discern from the picture.

From the side profile you can see a few differences. First, the coating at the top ringland is extended to the crown on the earlier STI pistons. This is likely due to manufacturing differences (less tooling / different tooling) rather than any specific performance difference. Second, there is considerably more material supporting the wrist pin on the Type RA piston.

Compression height, ring position, and other dimensions appear to be the same. Meaning, there are no changes to the ringlands in position or thickness. Top ring gaps were 0.010”, 0.010”, 0.011”, 0.011”. and lower ring gaps were 0.014”, 0.014”, 0.014” 0.015”.

From the bottom you can see how much material was added to the wrist pin area (in red) as well as the reduction in the large center support (in green). The Type RA design is superior for piston rigidity since it reduces the amount of cantilever, as well as giving better support to the wrist pin itself.

Earlier STI pistons weigh in at 414 grams while the Type RA pistons weigh 424 grams. The extra 10 grams of weight is not a big deal when you consider the improvements made to the piston design. For comparison, forged pistons for the EJ257 typically weigh in the 370 - 400 gram range depending on bore size and design. The wrist pins weighed the same for both motors.

Type RA Con Rod Differences

The Type RA rods are identical to the GR STI’s rods as far as my eyes tell me. Not a big surprise considering the rods are definitely not the weak link in the motor. There are some differences in the late model rods compared to the earlier EJ257 rods and the EJ207 rods, notably the closed channel on the big end’s internal bore and the lack of a perpendicular reinforcement rib.

No visual differences between the GR STI rod, and in fact, the 2018 STI and Type RA rods share the same part number.