Subaru 6 Speed STI Gearbox Transmission for 5 Speed Models The 6-speed STI box was designed by Subaru with motorsport in mind. It is by far the best and strongest gearbox Subaru have ever made, and with the highest level of operability. The six-speed gear set is nearly twice the size of the 5-speed and twice as strong. It can easily handle up to 300 Kw of power and a high degree of driver abuse. Subaru Timing Belt Replacement - Subaru Cam Drive Timing belt recommended service intervals. All Subaru EJ series engines from 1991 to current model shapes use a high quality rubber timing belt that is not only durable but efficient as well, and Subaru have recently adjusted their timing belt service mileage upward. from 1991 to 2001, it was still a straightforward repair despite the two