Subaru's latest 2.5 litre flat four boxer engine would be the FB25D found in the most recent Forester and Legacy. It's not the FA24F from the new WRX, nor is it the FA24D used for the new BRZ. The compression ratios seem to be pretty similar between the FB25D and the FA24D though (12.0:1 vs 12.5:1). Bore is the same (94mm), stroke length is relatively similar (86mm vs 90mm). Naturally, Prodrive does modify the engine further (new cylinder liners, pistons, connecting rods, and a new valvetrain). But the fact that they can turbo the engine to over 400 hp might give some useful insight for people trying to turbo a new GR86 or BRZ (how to do it and how far one can go).The use of the latest 2.5 litre flat four ‘boxer’ engine producing over 400 bhp and 600 Nm of torque, combined with a six speed semi-automatic gearbox, WRC-derived launch control and turbo anti-lag, means the P25 will reach 100kph from rest in under 3.5 seconds.