SUZUKA CITY, Japan -- After turning a few laps at Suzuka Circuit in Japan behind the wheel of a 2015 Subaru WRX STI, we feel high. We need a minute to calm down, but the instant we pull into the pits, an engineer wearing a bright blue and pink jacket yanks us out of our lovely red WRX STI and ushers us toward another WRX STI.

See all 18 photos

This one is mac 'n' cheese yellow, and it has a carbon-fiber rear wing that might've been plucked off the trunk of a Honda Civic parked in front of some AutoZone parts store. We hate it. We're thinking of making a break back to the 2015 WRX STI when our man tells us this yellow car is a prototype for a WRX STI that Subaru Tecnica International will be building in the future. We get closer to the STI car and notice Brembo brakes with six-puck calipers up front, plus a cat-back exhaust peeking from beneath the rear bumper. And so we climb into this concept car's driver seat, a heavily bolstered bucket that doesn't match the standard passenger seat.

See all 18 photos

We ask for the rundown of what's new and different. The engineer from Subaru Tecnica International (STI) is mum for a minute before showing a little skin, and then he tells us that this car's turbocharged 2.0-liter flat-four resembles the engine from the U.S. version of the WRX, but it's been upgraded with blueprinted internals, a set of carefully weighed and balanced pistons, and a larger capacity ball-bearing turbocharger. He says it puts out well over 300 hp.

The 2015 Subaru WRX STI is one of the most impressive bang-for-your-buck track cars of all time. This prototype, though -- it's witchcraft. During our laps on the Suzuka F1 circuit, we pushed the production WRX STI as hard as we could, and the car drifted sideways in every corner, its clever differentials helping to keep us out of the weeds. But in the prototype, we can't push hard enough to make the car do anything wacky.

See all 18 photos

Damned if we're not trying, but the STI prototype is plastered to the track. The steering is sharp and tight, and the steering wheel whips back and forth when we put the right-front tire on Suzuka's curbing. The cat-back exhaust has a grizzly growl as the turbo-four revs up smoothly and freely, and the turbocharger's wastegate spits and whistles when we lift off the accelerator.

Where's the tube chassis? The fire extinguisher? The garish graphics? This WRX STI prototype is a fully fledged racing car, isn't it? Because it sounds and drives like one, that's for sure. Our high is back as we pop the six-speed gearbox into neutral and coast back into the pits. We get out, and the STI engineers quickly push the prototype into a garage, closing the door behind them.

There's a lot we don't know about this prototype. We're sure we won't see a Subaru WRX STI with a turbocharged 2.0-liter engine stateside anytime soon. It's more likely this sedan will debut in Japan as special-edition model, akin to the limited-run WRX STI S206 that Subaru brought out a few years back. But if the next-generation Subaru WRX STI benefits from even one or two of the updates done to this mac 'n' cheese prototype, we have a lot to look forward to.