2017 Subaru WRX 2.5L Petrol · S7 UCS

The 2.5-litre EJ257 turbocharged engine features hypereutectic cast aluminium pistons with relatively brittle ringlands. Due to unequal exhaust manifold heat soak and localized cooling characteristics, cylinder four runs significantly hotter than the other cylinders. Under spirited driving, track use, or when poor-quality 95 RON fuel is used, engine detonation causes the top or second ringland to crack. This leads to heavy blow-by, rapid oil burning, compression loss, and potential cylinder wall scoring. Installing forged pistons and an upgraded cylinder four cooling mod resolves the issue.

The secondary air injection pump directs atmospheric air into the exhaust ports on cold startup to accelerate catalytic converter warm-up. The two solenoid check valves mounted beneath the intake manifold are vulnerable to condensation corrosion and exhaust soot accumulation, causing them to jam open or closed. When a valve fails, the engine ECU disables the SI-Drive mode selector, disables cruise control, and logs trouble codes P0410, P2440, or P2441.

The aluminium Brembo brake calipers (black 4-piston on pre-facelift, neon yellow 6-piston on facelift and Final Edition) suffer from clearcoat lacquer peeling and corrosion around the steel bleed screws and piston seals due to exposure to wet UK road salt. Furthermore, if brake fluid changes are neglected, internal galvanic corrosion binds the pistons within their bores, causing brake drag, excessive disc heat, and steering judder under braking.