Project Infiniti G20 Racecar: Transmission Upgrades

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Option two was the even stronger RS6F51H gearbox found in a B15 Sentra SE-R Spec V (clumsy name, ain’t it?).  This gearbox has a lot going for it: it comes stock with a helical limited slip, it’s found in a couple of V6-powered cars (Altima, Maxima, Tiburon), so it was designed from the factory to handle the power levels our car was making, and it also has 6 gears, making for less RPM drop between shifts.

The RS6F51H Spec V transmission (left) has a considerable beef advantage over the stock unit (right), has six forward gears, and a helical limited slip differential stock.  Unfortunately, it’s not a bolt-in affair.

All of the above parts, plus an SR20 bell housing from a P12 G20, a custom axle bracket, custom motor mounts, and other small parts are needed to convert to the RS6F61H.  (These goodies will be going in another FWD SR20 turbo project you’ll have to check back to read about.)

We decided to go with the P11 transmission.  Now, we’d love to tell you we picked it because it’s lighter weight meant less weight gain on our car’s already heavy nose, but that’d be lying.  In the end, the deciding factor was not only our laziness (the Spec V transmission requires a fair bit of work to install compared to the relatively drop in P11 unit), but also our familiarity with the now-discontinued NISMO differential we had in the previous trans.  We’ve experienced helical limited slip diffs in FWD cars before, and weren’t as impressed with their performance as we were with the NISMO clutched unit.  That, and if you’ve ever shifted a Spec V in anger, you’d remember that it’s more recalcitrant than a 2 year old way past his nappy time (or half the MotoIQ staff in a technical argument).  The SR20 valvetrain is not appreciative of “money shifts”.  The RS6F51H would’ve been the more reliable replacement, but we felt it would be overkill for our power levels.

Side note: deciphering Nissan’s transmission codes isn’t too difficult.  The first two letters indicate shifter location, RS being “rear shift” in our case.  The next number indicates number of forward gears, and the next letter is the drive type: “F” being FWD.  The next numbers refer to the gear stack code, and the last letter refers to the differential type: A is open, V is viscous limited slip, and H is helical limited slip.  So, our P11 box’s RS5F70A code means it’s a rear-shifted five speed FWD box with the 70 gear stack and an open diff.  If only Nissan had made the bell housings the same on all of their engines that shared drivetrain layout.

The P11 G20 isn’t the only vehicle this new and improved transmission came in, but there are some things unique to the G20 which make it better-suited for our use.  First, the gear ratios are longer on the P11 models than on B15 Sentra SEs.  This pairs nicely with the torque-laden and ultra-wide powerband our boosted SR20 puts out, allows second gear to be useful without a smoke-show, and also reduces unnecessary shifts (most tracks will only need 1 gear change).  The longer gearstack is shared with the QG18DE-powered B15 Sentras, making replacements cheaper and easier to locate.  Lastly, we went with a non-LSD case because we already had a NISMO clutch-type diff for this application, making the need to find a unit equipped with the box o’ snot VLSD unit moot.  Because SE-R fans everywhere gobbled up nearly every LSD-equipped transmission from junkyards, being able to put the non-LSD units to use will make finding a replacement that much easier, and most importantly, cheaper.

Our “70” gearstack is identical to the stack found in the QG18DE-equipped B15 Sentra with a manual transmission, making finding replacements much easier.

Of course, we’ve taken steps to reduce the possibility we will need to find a replacement in the first place.  Improved does not mean perfect.

Before we even thought about installing our new transmission into the car, and since we had to install the NISMO diff anyway, we made sure to give it a complete tear down and inspection.  Ruining an expensive race weekend because of a failed $20 bearing is bad joojoo, so we had Howard Watanabe of Technosquare tear it down and see what needed replacing.  Luckily, after over 100,000 miles in a street vehicle, our transmission was in decent shape.  However, all but two synchros were worn.  Our only guess for wholesale synchro failure is that the previous owner did not use the correct manual transmission fluid (GL-4) or didn’t have a left leg.  While the case was apart, Howard welded in an aluminum insert to fill the void around the passenger side differential seal.

Because the differential webbing has a lot more support around it, welding this area is not as necessary as it is on “32” gearboxes.  However, since we had it apart, we had Howard weld in this aluminum insert.