There's a good reason we're rolling on dubs in a bone stock G35. It's part of an overly-complicated attempt at testing different types of limited-slip differentials on the same car, on the same day, to gather a bunch of data we're not quite sure will truly reflect the differences between various types of limited-slip differentials (LSDs). But here goes.
Although we've described the inner workings of various types of LSDs ad nauseum (and occasionally installed and trialed one specific version on a project vehicle), we've never really tried a back-to-back test to figure out how each one performs and feels. Granted, it's a matter of taste and driving style-like drifting, aggressive track lapping or just being stupid on the streets-but, in theory, different diffs are better suited to different situations. The only way to prove this is to install each one and run it on a set course while data-logging wheel speeds, throttle position, steering angle and vehicle yaw-all under the same conditions.
The premise goes like this: get an open diff, an OEM viscous, a popular 1.5-way clutch type and a Torsen gear-type LSD for the same car, using the same set of tires. We've been planning to do this for years, but the monumental task of installing reliable independent rear-wheel speed and steering angle sensors made it a back-burner project.
It wasn't until we stumbled onto UpRev's Cipher data and diagnostic software for the Nissan 350Z, Infiniti G35, and most VQ35DE platforms that it became possible. The software runs on a standard PC and allows the computer to communicate with the vehicle ECU via an USB-to-OBDII dataport cable. Through this, the software can read in real time, most engine and stability control sensor channels at up to 20Hz, or 20 data samples, per second. In addition to recording and saving data to an Excel file, Cipher also allows temporary access to most CAN-BUS channels for anything from turning on wipers and lights to changing idle and injector maps.
We originally intended to use a 2005 Nissan 350Z with its independent rear suspension and single mechanical differential, but ended up using the more advanced G35. This car comes equipped with the Vehicle Dynamic Control (VDC) system which incorporates a steering angle and yaw sensor on top of the standard individual wheel-speed and ABS sensors found on the Z's Traction Control System (TCS.) Using OE sensors saves us from noise, calibration, accuracy or durability issues.
To get better data, we also slapped on a staggered set of 20x9 (front) and 20x10 (rear) Axis MOD7 wheels wrapped with Bridgestone's new top-of-the-line RE050A Pole Position tires. The larger rolling radius of the 20s adds some resolution to our data as a noticeably bigger wheel speed difference (if the outside and inside tire are turning at different rates when the diff isn't locking). The super-sticky Bridgestones also give enough resistance to load up the diff and make them work on a 100-foot radius turn. And they provide the most grip without stepping up to R-compounds and overloading the stock suspension and roll rates.
Off to the track then, with four 75-pound pumpkins (each housing a different LSD) prepped by Steve Mitchell at Nissan, our Racepak G2X telemetry recorder, and a support vehicle full of tools and an understandably disgruntled mechanic. Each differential was subjected to eight laps on our 200-foot figure-eight which simulates a decent mid-speed turn and gives ample opportunity to test turn-in, constant radius balance, and track-out. Along with the Cipher data, we also kept tabs on lap times and consistency. The fasted and average lap times for each diff are also listed. For you statistics buffs, we also calculated one standard deviation for the laps we ran. The consistency of the laps are more important than the fastest or average lap times as track conditions improved as the day progressed. We tested the diffs in the following order: Nismo Clutch type, Quaife, open and viscous.
Nismo 1.5-Way Clutch-Type LSDMost G35 or 350Z owners prefer a clutch-type LSD, on account of the large selection available and the huge hype in drifting. A 1.5-way LSD locks under acceleration and only partially locks off throttle. They are known to clunk and pop as the clutch plates grab and release, which is a drawback for some, as is the relatively abrupt engagement. Depending on how tightly it's packed or preloaded, this type of diff will cause a car to understeer at turn-in, since both wheels are already locked and pushing the car straight instead of letting it rotate. The advantage is that the touchy locking nature makes it very sensitive to throttle modulations-drifters like that.