Project Infiniti G20 Racecar: Transmission Upgrades

Project Infiniti G20 Racecar: Transmission Upgrades

By Steve Rockwood

It seems that every time we try to do something to make Project G20 faster, we find a faster way to break something.  This time around, we were certain that while the stock Nissan RS5F32V transmission was made of sugar glass, that our G20’s modest turbo power levels, stock clutch, and not-so-abusive driving habits should make it last through a season.  After three race laps, we were greeted with the all too familiar sound of a fork in a garbage disposal coming from the transmission.  Someday, we’ll learn a lesson the easy way.

Three laps.  Done.

To this day, we are still amazed that Nissan could pair such a ludicrously stout engine like the SR20 with the comically-weak RS5F32V transmission.  This is even more of a slap in the face when you consider that they installed this poor excuse for power transmission over their extremely stout RS5F50 (which has a limited slip variant) found in the lower-power first generation Altima.  Indeed, the path to a boosted SE-R is littered with the broken tears of SE-R transmission past: shattered cases, the dreaded fifth gear popout, bent shift forks, and, of course, the ubiquitous third gear failures.  Past SE-R enthusiast Chris “Shaggy” Allen became so adept at bombardiering third gear sets that the community quickly termed this failure mode “Shagging the transmission”.  You’d be hard-pressed to find an SR20 boost junkie who doesn’t know how to tear down his own transmission.  Not many other enthusiast communities can claim this dubious distinction.

There are a number of ways in which Nissan screwed the pooch when it comes to the design of this transmission.  For starters, unlike the block it’s attached to, the transmission has very little in the way of bracing or support, and seems to exist only to hold fluid in and keep the gearstacks sort of near each other.  Under wide open throttle, the case flexes enough to allow the input and countershaft to separate from each other, this problem is only exacerbated by the prodigious torque a boosted SR20 will belt out.  This separation reduces gear tooth overlap and puts the onus of corralling power to the very tips of the gears.  This problem is especially egregious in third gear, which is located at the center of the gearstack, and is the primary reason for failure under excessive power levels.

The case fails in its role of supporting the differential area as well, where a cutout we can only assume exists to drain fluid from a failed axle seal results in an unsupported section in the critical area between the diff and rear motor mount.  The easiest way to remedy this was to fill the void with epoxy, but cracks in the case are still common here, especially in cars that experience excessive wheel hop

It’s not all the case’s fault, however.  The SE-R transmission features an odd combination of stout gears (first and second) and high-wire-narrow gears (third and fourth) that will shatter as soon as you even think about putting your foot down.

So, what to do?  We could tear the transmission apart, buy a new input shaft (gears one through four are integrated with the input shaft), treat them in every imaginable way, and pray they last a weekend, or we can send the whole thing to the scrapyard where it belongs and start over.  We like starting over.

Luckily, we didn’t have to go far to start over.  Broken SE-R transmissions aren’t exactly new to us, and two options sat, covered in dust, waiting for us to do something with them.  Option one was Nissan’s own attempt at salvation: an RS5F70A transmission yanked from a 2001 Infiniti G20.  This transmission’s case no longer lets it down, its gears are 15% wider, and the arrangement of the gearstack reduces the length of unsupported input and countershaft strung between mounting points on the case.  A number of improvements in shift linkage support also help shift feel and prevent parts from failure as well.  One must wonder how the SE-R community would compare to the Honda community had Nissan installed a real transmission in the B13 to start with.

The RS5F70A  we used has thicker gears with larger teeth, a beefier case with more support, and a much improved shifting mechanism.