We've gotten used to seeing limited editions of the Subaru WRX STI launched solely for the Japanese market, with last year's introduction of the Subaru STI S209 one of the few exceptions. Subaru of Japan has now revealed yet another limited production STI, only this one previews a few things the Japanese automaker has planned for the next-generation model, due sometime later next year.

The WRX S4 STI Sport # is limited to only 500 examples and all have now been sold. Subaru Tecnica International performed the upgrades as usual, but these features are not found on any other WRX STI in Japan or elsewhere.

For example, it comes equipped with three flexible parts: a flexible tower bar up front, and a draw stiffener at both front and rear. What is a flexible draw stiffener? It's a component that applies some tension between the vehicle body and cross member to prevent the chassis from bending during serious driving. The result is more agile and responsive steering and improved overall ride.

Grip is further improved on all four wheels which leads to better stability when cornering. STI's general manager for development, Masuo Takatsu, said these stiffeners were developed specifically for the WRX S4 STI Sport # and that during testing "the occurrence of lateral G is reduced by 10 percent, and body roll is reduced by 4 percent." Expect to find these new components on the next-gen model.

The S4 STI Sport # also boasts a performance air cleaner and low resistance muffler in the intake and exhaust systems. Additional improvements include thicker sound-absorbing interior materials located under the floor and reduced road noise. The exterior has a few new aerodynamic parts such as a new front under-spoiler and outlet on the rear bumper for improved downforce at both locations. There's also a new side under-spoiler.

Interestingly, the S4 STI Sport # is CVT-only; no six-speed manual option is available. What does this say about the next WRX STI? To be determined. The EyeSight driver assist technology is also standard and it should be noted it's not compatible with a manual gearbox. Inside, the S4 STI Sport # comes with an all-black interior and the steering wheel and Recaro seats are trimmed in suede.

The bottom line is that the WRX S4 STI Sport # is sort of an experiment for next-gen body rigidity, aerodynamics, improved intake and exhaust flow, and reduced cabin noise.