What effect does changing the final drive have on the car's performance? A shorter final drive (higher numerical ratio) increases the torque multiplication across all gears improving acceleration. However, it also increases the engine revs per mph in all gears which has the effect of requiring upshifts sooner and reducing the top speed (assuming it was gearing limited to begin with). A 4.44 ratio is best suited for applications such as rally, short tracks, and winding canyon roads where top speeds are low but require short bursts of acceleration from corner to corner.

What is the transfer ratio? The transfer gears are a set of "drop gears" inside the transmission that connect the center differential to the propshaft in order to send power to the rear diff. These gears are a multiplier only for the rear diff ratio. Subaru uses only 1.0 or 1.1 ratios for the transfer gears depending on the application. The 1.1 ratio has been used with certain applications using a 3.90 final drive ratio. With the 1.1 transfer ratio a 3.545 rear diff ratio is used (3.545 x 1.1 = 3.90).

Do I need to change both the front and rear diff ratios to convert to 4.44? Yes, the front and rear diff final drive ratios must match each other while also taking into account the transfer ratio. A 1.0 transfer ratio has no numerical effect on the rear ratio. If there is a mismatch in ratios then the center differential will be forced to absorb this constant speed difference which will overheat and destroy it fairly quickly. To convert to 4.44 you must change both the front and rear final drive sets (the front being inside the transmission case and the rear being inside the rear diff). If the transfer gear ratio is 1.1 then you must also convert that to a 1.0 ratio.

Where can I purchase the proper transfer gears? Please email us if you require transfer gears for this conversion. In some cases the only option available is aftermarket. For example, Subaru does not make a 1.0 transfer gear set for the MY2006 and newer STI but we can supply a high performance aftermarket set.

What effect does the REM ISF® treatment have on the final drive gears? ISF® processing significantly reduces the surface roughness of the gears in a non-directional manner. This high degree of "polishing" reduces frictional losses which is important with this type of final drive due to the fact it is a hypoid gear design. Hypoid gear teeth endure a high degree of sliding motion under heavy loading so the benefit is significant. Additionally, because the microscopic surface irregularities have been removed there is a reduction in the break-in requirements and the associated metal particulates circulating in the oil that result from that break-in process. The strength of the component is also increased because there is a reduction in stress risers on the surface of the parts which might otherwise serve as a starting point for failure.