Project Infiniti G20 Racecar: Transmission Upgrades

,

To finish off the rebuild of our transmission, we installed NISMO’s 1.5 way limited slip differential.  This differential was originally designed for the RS5F32A transmission, but drops into the RS5F70A without modification as the diff and ring gear carried over to the new design (probably because it was the only reliable part of the transmission).  With the differential, we installed new bearings, plastic speedometer gear, and new axle seals (which we replace any time the transmission is removed from the car as cheap insurance).  We chose the NISMO diff over a gear-type unit like the one built by Quaife because we like the more positive lockup of the clutch units.  Most SoCal tracks have extensive curbing that needs to be hopped to take the fastest possible line.  With a helical unit, putting a wheel in the air while hopping the FIA curbing unloads the differential, causing it to behave exactly like an open unit and you end up with wheelspin when the car lands.  This moment of wheelspin not only slows you down and causes unwanted understeer, it’s also inconsistent with the normal handling behavior of the car.  With the clutch-type units like the NISMO diff, an unloaded wheel does not cause the diff to behave like an open unit, allowing us to hop curbs without any funny business.  We don’t like funny business.

Now that the transmission was finally ready to install, there were a couple of items that needed addressing in order to install the P11 transmission in our P10.  The RS5F32V’s clutch engagement arm needed to be swapped over, and the bottom of it needed to be ground down to fit properly in the bell housing.  This was fairly easily accomplished with a bench grinder and some pressure, just be extremely careful to keep it round and centered or you’ll put unnecessary pressure on the arm.  The weight on the bottom of arm itself needs to be ground down to clear the vent tube, and the shifter stabilizer bushing needs some minor clearancing to resolve some interference.

Part of our shifter stabilizer bushing interfered with the shift rod.  Nothing a grinder can’t fix.

Since we doubted our Nissan Key Value clutch’s ability to cope with the 230lb-ft of torque our mighty SR20 belted out, we upgraded to a Jim Wolf Technology pressure plate and stock B15 Sentra clutch disk.  This clutch has always had a very special place in our hearts since we first installed on in an SE-R over 10 years ago.  Like most aftermarket units, the JWT pressure plate offers more clamping force, but it does so with lighter pedal effort than stock.  This is welcome not only in rush hour traffic (which this car will obviously never see), but also on the track, allowing us to shift faster and reduce effort.  Like the newer transmission, the B15 clutch disk solves a lot of the broken spring issues the older units saw.  While it’s still not perfect, it is a vast improvement in reliability over the old unit.  We opted to stay with a mild disc for our clutch setup to reduce drivetrain shock.  An unsprung hub with pucks would’ve increased holding power and reliability of the clutch assembly, but would’ve put unnecessary stress on the rest of the drivetrain.  Since we weren’t drag racing, a little sacrifice in ultimate holding power was worth it in the name of reliability.

All that time spent selecting a reliable setup would be wasted if we decided to throw the old clutch and flywheel hardware and throwout bearing on there.  To keep things fresh, we installed a new OEM throwout bearing, clutch cover bolts, and ARP flywheel bolts from GSpec Performance.  All hardware got a small dab of red Loctite to ensure they stayed put.  Flywheels and clutch covers that come loose are not a good time.

We also replaced the clutch cover bolts with new OEM parts.

ARP flywheel bolts ensure the flywheel stays where it belongs.  Don’t forget the Red LocTite!  You don’t want to end up like this guy below…