Having addressed most of our cooling concerns in part 1 we continue with the transmission upgrade for our Infiniti G37S. Ten years ago it would have been a fringe case to perform an automatic transmission upgrade for a project car. These days many manufacturers don’t even offer a manual for their high-performance models. Even enthusiast cars like the new BMW M5 use a standard torque converter automatic like our car instead of a DSG gearbox. We understand this isn’t the sort of upgrade for everyone, but the purpose of this car has shifted from daily driver to weekend project and we’ve invested a lot of time and money in the car already. The thought was then, what can we do to get rid of the laziness our car had due to slow shifts and slipping between gear changes to continue to upgrade our sports sedan.
Reading through the Nissan press releases on the G37 forums a term was used frequently for this gear change slipping called rev-flaring. Specifically on the early 7-speed automatic transmissions. This at least made us feel better that the slipping we were experiencing was normal but as an enthusiast it was still unwanted behavior. We were also less confident to proceed with any more power upgrades until the transmission was addressed. Those who have gone down the forced induction route state that any power over 400 lb/ft destroys the automatic transmission when boosting at low RPM’s in 5th, 6th or 7th gear. While it was great that Nissan wanted to provide driveability along with the performance and efficiency achieved with full lockup torque converter, the execution at times resulted in sluggish and worn out lockup torque converter clutch. Our UpRev tune did help firm up the shifts a bit by increasing the line pressure in the transmission, but at times the rev-flare slippage would creep in between shifts and sometimes acceleration just felt soggy due to the transmission.
We found a few upgrade options out there for our transmission, but to be honest rebuilding an automatic transmission isn’t something we’ve really done at MotoIQ and there weren’t a lot of references to call for upgrades. We did remember way back when there was a turbo FWD SR20 guy out there who had an automatic transmission modified by Level 10 for his B14 Sentra. With some quick internet searching, it turns out Level 10 is still around and they have an upgrade for our 7AT transmission (Part#INFIN-G1800-RE7). They offer the rebuild parts separately as components where you can have a local expert install them or they can send a complete transmission upgraded and ready for install. The individual components consist of:
- The rebuild kit, that includes all the clutches, gaskets seals, O rings, and upgraded clutch friction material to hold more power and provide improved gear engagement. Some of the clutch baskets are modified to take more discs to give more clutch friction area and torque capacity.
- An upgraded valve body which decreases shift times and improves the overall shifting quality of the transmission.
- An upgraded torque converter. The original torque converter was designed for the stock power levels of the car but we’d argue that ours wasn’t that great in handling the basic bolt-on increased power in our car. The Level-10 torque converter is stronger and has a higher stall speed. This is the speed that it engages, equate this to increasing the rpm that you let the clutch out at. The torque converter lock-up clutch material is changed from something like cardboard (no kidding) to a semi-metallic material that is stronger and grips harder.