In the last installment we deduced that there was likely a mechanical problem with T-Rex. The GPS data acquired at the track indicated reduced grip in many of the medium- and high-speed corners. Also, on the odd occasion during the initial phase of extreme braking at the track the car became very unstable in the rear, providing the driver an unwelcome wake-up call. Fortunately, this provided an additional clue to the mystery of lackluster lap times.
With the suspicion that the OEM limited-slip differential (LSD) had worn out on my ’10 GT3 with some 5,500 miles on the clock, I started the physical investigation of the problem in my garage. By jacking a rear wheel off the ground with the other wheels blocked I was able to perform what is affectionately know as the Spin Test. With the e-brake released and the transmission in Neutral, you try to spin the wheel that’s off the ground. If the preload on the LSD is reasonable, the wheel will not spin at all, indicating that the diff is likely in good working order. However, if the wheel spins more or less freely (taking in to account the fact that the brake pads will be dragging slightly on the rotor), then the LSD could be worn out, or the preload very weak. Both rear wheels spun rather freely, indicating that this may be the problem.
I’ve been told by a couple of sources (my Porsche dealer being one) that the Spin Test is not accurate because the factory static load breakaway (preload) torque is very low, some 9 to 11 ft-lb on a new 997 GT3. A more precise estimate of the preload can be obtained with the rear wheels off and using a range-proper torque wrench, but there are still problems. Regardless of the measurement or how it’s done, since the LSD is not actually under load at the time when such test measurements are taken, any measurement could be irrelevant. Also note that the Porsche shop manual procedure to check the LSD preload is done with the unit removed from the transaxle (i.e., a bench test). So what does a poor guy do?
I took it to the dealer to get an opinion, providing them with my experiences and findings. Their testing was done with the LSD in the car and resulted in a preload torque measurement of 8 ft-lb; I was told that Porsche would not replace a unit unless it was below their 2 ft-lb minimum specification. I was also told that I’d need to pay up for the labor costs if I wanted a bench test. If the LSD unit measured below the 2 ft-lb minimum, Porsche would replace the unit and cover the labor cost. Otherwise I was on the hook for all labor regardless, and parts should I want a new diff installed. I’m not a gambling man, so I declined their kind offer.
After bit of web research, I decided that a fresh, OEM LSD was not up to the task of enthusiastic tracking because its friction discs are made from brassgood for longevity, but rather poor for friction and bite (it would need to be replaced again in 5,500 miles, or less). A stronger and more reliable solution was definitely in order. There are many ways to go if one desires a completely new aftermarket LSD unit, but installation requires the transmission to be removed for proper installation and setup, a very expensive proposition. The solution I found to be the simplest, at a reasonable price, was having the OEM unit rebuilt and upgraded to a stronger specification. This method saves quite a bit of shop time because the LSD can be removed with the transmission in the car, and no special setup is needed when reinstalling because the OEM diff will fit perfectly. Even better, if you’re so inclined, you can remove and replace the LSD in the comfort of your own garage and save a little more.
I sent my unit to Matt Monson at Guard Transmission in Boulder, Colo., for a complete rebuild and upgrade. He offers a few rebuild options, all with improved plain plates and friction discs (clutch packs), and the choice of retaining the OEM ramps 28/40, Guard 40/60 ramps, or Guard 50/80 ramps (the most aggressive setup). I opted for the rebuild with Guard 40/60 ramps as Monson recommended this as an excellent street and track solution. FYI: The cost for the Guard LSD clutch pack rebuild with upgraded moly-coated/heat-treated friction disks was $950, plus a further $650 to upgrade to Guard ramps, and $100 ring gear removal and installation if you aren’t willing to mess with this yourself. Note, when the first Porsche GT3 hit North America by storm in 2004/5, marking the climax of production for the 996 series, it came equipped with a 40/60 limited-slip differential, so interesting that this specification was not used on the 997 GT3. I did learn that the steeper 28/40 ramps were used so the traction and stability control systems can interact more transparently and to reduce the car’s inherent understeer.