If you have read any of our discussion about the WRX 5-Speed transmission to this point, you have probably noticed that there is a somewhat low threshold at which we recommend moving to the STI 6-Speed transmission because of how much more durable it is than the WRX 5-speed. But that then brings up the question of how durable is the STI 6-Speed then?


For better or worse, the STI 6-Speed has not changed very much at all from its introduction to the us in 2004 right up to the 2019 model. They are all very durable transmissions, and because they have been in service for a long time, their limits are fairly well documented at this point.


From everything we have seen, the STI 6-speed is reliable for just about anything that you can throw at it from launching to drag racing to circuit racing up to about 600 wheel horse power. The design of the case, the size and width of the gears, and the support bearings on the main and pinion shaft all make this a very robust transmission. If you are driving the car on the street or on a race track, the transmission will most likely prove to be fairly reliable even well beyond this point.


If you are launching the car, and especially of your are running your car at the drag strip with sticky tires, you will find the weak link of the STI transmission first. Remember that because an All Wheel Drive car is sending drive force to all 4 wheels, and it takes a lot more to break the car lose to relieve driveline stress in a launch, and that puts a lot more stress on the transmission and driveline.


So if you are doing all of that, the first weak link that you will find are the transfer gears that go to the center differential. Those seem to be particularly susceptible to fail from hard launching. There are a number of replacement gears available, and IAG has a couple of good options in fact.

IAG Transfer Gear Pic

Beyond that, there are some gear sets available, but they are fairly expensive with a typical cost for a gear set starting around $9,000 and going up from there. These sets will give you the ability to change gear ratios, and to go with a dog box if you like, and you can go all the way up to a sequential gear set if you like. But because of their cost, and the fact that the stock gears can reliably hold a lot of power, many people just stick with the stock components. And if you want a little more reliability, you can REM treat stock gears for example.


There is one more potential issue that is worth mentioning and that is the shift forks. The stock STI forks are made of aluminum, and it is possible to bend them, especially if you miss a shift. There are more durable steel shift forks that are available, so that might be worth considering if you have this type of failure.

Shift Fork Pic 1   Shift Fork Pic 2


Beyond that, a well maintained STI 6-Speed should be reliable for years, and that is why we like them so much.
Thanks for reading and Stay Tuned!

- Jon Cooley