Subaru five-speed transmissions aren’t known for being the most robust or tolerant of abuse, with the early 02-03 WRX transmissions being affectionately referred to as a “glass box”. Lucky for us, our gearbox isn’t quite so fragile, but failures are still known to happen when driven aggressively with a higher horsepower than stock. Shearing the teeth off our gears wasn’t something we wanted to risk, nor did we want to spend the thousands of dollars it would cost to put an STi six-speed into our Legacy. We opted to go with the Moore Performance blast plates to increase the rigidity of our five-speed transmission case.
What are blast plates? Matt Cole from Moore Performance can it explain it better than I can, “The Blast Plates were created out of necessity for an alternative, less expensive solution to the 5 speed gearbox's weaknesses in motorsports and daily driving. The idea for the Blast Plates was first put together at the SCCA Solo Nationals event years ago by our team after repeatedly swapping out gearboxes trackside after failures. It's well known that the 5 speed transmission is made up of two case halves and these aluminum case halves will flex under heavy load. When the case halves flex outward the gears are actually pulled out of factory spec and that is when we see failure occur. Our fix to this would be simple – prevent the case halves from flexing. To do this, two CAD designed and FEA analyzed reinforcement plates are bolted through the transmission (not to the transmission – the hardware extends through the gearbox) so that when the aluminum case halves attempt to flex away from each other the steel reinforcement plates prevent that movement keeping the gears engaged one hundred percent, one hundred percent of the time.”
In order to have enough room to run the long bolts through the case, you have to either lower the transmission or remove it from the car. Once you have enough room to work the install is very straight forward. Simply remove the factory bolts, mount the plates, and then torque the new hardware to the spec listed in the instructions. Now both our clutch and transmission should be up to the task of containing our pending power gains.