How much power will this gearset handle? It is difficult to provide a straight forward answer to this question because there are many contributing factors that will influence the gearset reliability. However, we have found that it is difficult to achieve good reliability in applications over 350whp that are driven aggressively. Additionally, heavy shockloading created by hard launching and flat shifting and accentuated by harsh clutches will tend to lower the reliable torque capacity of the gearset.

How does this compare to the stock WRX gears? Subaru has increased the strength of the standard gears used in the USDM WRX models over the years so it really depends on the model year used for comparison. The Type STi and Type RA gear sets are factory based gearset options that we pieced together into a complete kit and improved on with the REM® superfinishing process. When we first introduced these kits the 2002 WRX was the current model and these sets were a fairly substantial improvement for the weak standard gearbox. As the years progressed the strength difference became progressively less. With the introduction of the Legacy GT gearbox and later the MY2008 WRX (which had the same gears as the Legacy GT) the primary difference between the Type STI and the factory gears became simply the superfinishing process.

In terms of specific numbers the percentage increase in strength looks approximately like this for MY2002 WRX vs. Type STI/RA based on tooth size and surface treatment benefits: 1st = 15-20% increase, 2nd = 33-38% increase, 3rd = 42-47% increase for RA / 28-33% increase for STI, 4th = 5-10%, 5th = 5-10% although sometimes in extreme cases 5th gear will fail by splitting from the base of the tooth to the inside diameter of the gear and the RA gear has much greater material thickness here. In comparing MY2003 to MY2007 WRX the difference in 1st, 2nd, and 3rd is reduced noticeably becoming: 1st = 10-15% increase, 2nd = 17-22% increase, 3rd = 35-40% increase for RA / 15-20% increase for STI. In comparing MY2008+ WRX the difference becomes only the surface treatment which we estimate somewhere in the 5 to 10% range. The only exception being that the RA 3rd gear will provide an additional 10% due to its larger tooth pitch.

How do the gear ratios compare? When comparing to the MY02-07 USDM WRX the Type RA and Type STI are closer ratio gear sets. However, the Type STI set has identical 4th and 5th gear ratios so only 1st-3rd gears are closer than stock. The RA ratios are closer from 1st-5th. Specifically 1st is a taller ratio and 5th is a shorter ratio.

What is a close ratio gear set? Close ratio sets mean the ratio difference between adjacent gears is smaller so the rpm change with each shift is reduced. This allows the engine to operate within a narrower rpm range which may be beneficial for acceleration. In general a close ratio set will be more fun to drive on tight twisty roads. However, the taller 1st gear demands that the clutch be slipped a little more in order to get the car moving and the shorter 5th gear will raise the highway rpms and potentially the fuel economy.

I have heard that transmission case flex is the big problem on the 5-speed, is this true? Overall there is insufficient rigidity of the bearing supports and there is flexibility in the main shaft. When you subject the gearbox to heavy loads this overall lack of rigidity allows the gears to deflect away from each other disrupting the proper gear mesh and dramatically increasing the bending loads on the gear teeth. External case reinforcements or braces will probably make little difference because the distortions are primarily of the internal structure. The bottom line is that the transmission design was never intended to support the power outputs that people are easily achieving these days.

What are my options if I need something to handle higher power outputs? It will be necessary to select an aftermarket gear set option with a much stronger tooth profile and/or wider tooth pitch such as one of the PPG options we offer. Or an upgrade to a 6-speed gearbox would be another option. The 6-speed gearbox uses an 85mm gear centerline distance vs. 75mm for the 5-speed. This allows for a wider tooth pitch (10-20%) all other things being equal. The 6-speed also uses much wider gears face to face(15-20%). Additionally, the case design, bearing sizes, and bearing support structure creates a much more rigid assembly.

Why do I need to use the speedometer gear supplied with the kit? The speedometer gear is driven off a series of grooves cut into the 3rd/4th driven gear cluster. There is a difference in the groove pitch on the 3rd/4th driven cluster supplied in the kit that makes it incompatible with the original speedometer gear. The mismatch in the gear mesh will tear up the plastic speedometer gear over time and eventually the speed sensor signal will be lost (the speedometer will stop reading and the ECU will set a lowered rev limit thinking that the car is stationary). We supply standard in the kit a custom speedo gear calibrated for a 3.9 final drive ratio and with a proper gear mesh. Alternatively, by special arrangement we can supply a 4.11 or 4.44 speedometer gear of proper mesh. The 3.9 and 4.11 speedo gears are metal construction, however, the 4.44 is a factory plastic gear as this was the original factory ratio that these gear sets were designed around. We cannot supply a 3.7 speedo gear ratio (for MY06-07 WRX) and suggest using the 3.9 gear and accepting the small speedometer error (about 5% lower than actual) or electronically processing the speed signal with some type of external correction unit.

What is included in the installation kits? At minimum, the install kits all include the following parts: 3 different shaft lock nuts, case gasket, main shaft oil seal. These are parts that should never be reused when rebuilding the transmission. Additionally, normally aspirated kits also include: main shaft rear bearing, 5th gear bearing race which are required for proper fitment. Some kits also include various additional parts necessary to properly adapt the gearset to that particular application. The actual parts vary.

Do these kits include synchros and bearings? For the most part no. Some install kits may include certain synchro parts and the normally aspirated install kits include the main shaft rear bearing. But for the most part they do not include synchros or bearings and those parts, if required, should be purchased separately (see related parts for links to these parts). These gearsets work with the original factory synchros. If the install kit does not include the particular synchro, then the original replacement synchro is the correct one to use.