From 2002 to 2014 the Subaru WRX had a 5-Speed manual transmission. Because there were many proven paths to modify the engine to make more power throughout these years, the limit of the transmission is also well documented at this point.


If you are interested in specific gear sets , please take a look at this post where I discuss the different gearsets that were either available for in the WRX, or could be fitted into the same case such as the STI Type-RA gears.


Fundamentally the gears themselves are the limiting factor of the 5-speed transmission, but this actually stems from the design of the transmission and the case itself. The WRX 5-Speed can be reliable up to roughly 350 wheel horse power with the Legacy GT/8-14 WRX gears, STI Version 6-gears, or the STI RA Gears, provided the car is not launched.


If you are trying to launch the car for maximum acceleration, this is what the 5-speed just doesn’t tolerate well for very long. If that is what you want to do with the car, the STI 6-speed is more reliable than any build of a 5-speed ever would be.


Beyond that power level, there just is no well proven way to keep the transmission intact for long term use. This is most likely due to first the size of the gears in the transmission, the long main shaft, and the split case design of the 5-speed transmission. None of those factors can be changed with a gear set, and that is most likely even the best gears available are not able to make the 5-speed transmission as reliable as a stock STI 6-speed transmission.


Because it is actually possible to spend more on a high grade gear set than a full 6-speed STI swap, we recommend moving from the 5-speed to the STI 6-speed if you either want to launch the car, or make more than 350 whp.

- Jon Cooley