South Australian company Willall is developing the world’s ultimate Subaru WRX STI and, best of all, you can buy one.

Dubbed the ‘25BB’, it’s conceived as a modern-day successor to the iconic WRX STI 22B and uses Willall’s extensive machining knowledge, proven at drag strips and racetracks around the world, in a coachbuilt widebody shell.

The heart of the 25BB is Willall’s billet block EJ25 (codenamed WREJBB), capable of handling up to 3.5bar (50psi) of boost and producing 800kW. “We got involved in the MY16 STI because they’ve an excellent layout but are susceptible to durability issues at high power levels,” says Willall representative Martin Donnon.

“We got involved in the high end of racing them with Willall designing a billet crankcase so people can run 800-1000hp without having recurring problems.”

The 25BB came about thanks to the traction limitations of Willall’s hard-working development car. “We started hillclimbing it and having a bit of fun and thought ‘we can’t get enough tyre under it’,” explains Donnon.

“So we thought ‘let’s make our own widebody for it!’. You can’t get anything wide for these cars other than the front fenders, so we thought we’d metal finish an entire car. It’s WRC blue so we’re going to make it faithful to the whole 22B concept.”

The bodywork modifications involve custom-made front guards, rear doors, rear doors skins and rear quarters, all made from factory steel, with front and rear bar modifications to suit the wider guards. Despite being “very tricky and time-consuming to make” according to Donnon, the extra width allows Willall to run a 305-section tyre all ’round on an 18-inch wheel.

Under the bonnet is a WREJBB capable of 800kW, with Donnon explaining “We have a magic set of cylinder heads and camshafts to make that sort of power, we have a special rotating assembly, which is pistons, crank and rods that take it out of 2.63 litres [and] a specially-made turbocharger system.”

Obviously this sort of power would turn the standard driveline into molten lava, so Willall installs upgraded diff gears and driveshafts, a carbon fibre tailshaft and triple-plate clutch.

Though Willall’s development car runs a PPG sequential gearbox, Donnon maintains the standard H-pattern manual is deemed strong enough for the road: “The six-speed’s a beautiful thing – it’s a very, very strong gearbox.”

Prior to the widebody mods that are currently being applied, Willall’s development car ran a 9.8sec quarter mile at the Snowy Mountains 1000 (see video below) while its sister car ran a 10.1sec quarter at 233km/h at Adelaide International Raceway. Donnon estimates the 25BB will be good for 0-100km/h in around 2.7sec and 0-200km/h in 6.2sec.

Currently, Willall is building a handful of cars for export, however Donnon says, “we would entertain building one for a local client.” Cost is to be confirmed and would depend on the exact specification of the car, however according to Donnon: “We’re talking GT-R money.”

With the bodywork currently being finalized, the finished 25BB will appear at the Subinats at Phillip Island on July 2, after which MOTOR is promised its own turn at the wheel. Stay tuned for images of the finished car and a full review.