Randy Pobst “Marvels” at the TPC Racing GT4TPC Racing
Randy Pobst “Marvels” at the TPC Racing GT4
On May 17th, TPC Racing had the pleasure of hosting acclaimed driver Randy Pobst of Motortrend; independent journalist Matt Farah of The Smoking Tire; and automotive editor of Ars Technica, Jonathan Gitlin, at Summit Point Motorsport Park for a private track day.
Before dawn, the TPC Racing crew loaded up a fleet of DSC Sport-equipped cars; including a Porsche Cayman GT4, Dodge Viper TA, and a Corvette C7 Z06; and shipped out to Summit Point. By the time the crew had pulled into the paddock, the rain was falling and puddles were accumulating on the track. It was looking like the day was going to be a total bust.
As we hopefully waited out the weather, Pobst and TPC Racing owner Mike Levitas shared stories of their racing days together in Grand-Am. After that, Mike gave tutorials on how to tune an active chassis using the DSC Sport tuning software. Finally, at about 1pm the sky began to clear up. By 1:30pm, the track was hot and the real fun began.
Randy Pobst began (and spent most of) his day in the GT4. He and Mike had exchanged notes relentlessly on their experiences in the GT4 leading up to this track day, and this was Randy’s first time in the driver seat of a TPC Racing GT4 fitted with DSC Sport. Mike’s knowledge for chassis tuning paired with Randy’s ability to drive the wheels off anything that has four, made it appear as though Randy had been driving this car his whole life.
Even as an extremely well-sorted 700hp Viper TA violently ripped around the track, cooking a set of Hoosier A7s, somehow the GT4 on street tires, piloted by Randy Pobst, looked like the fastest car on the track. Of course, in reality it wasn’t (the Viper was). However, the amount of grip and speed he carried out of turn 10 onto the main straight was enough to make your head spin. Based on unofficial hand timing, Randy was consistently pulling 1:20s (with a passenger!) on the main course; an impressive feat for a nearly stock GT4 on half-worn OEM tires.
Of course, the TPC Racing Cayman GT4 wasn’t totally stock. It was very lightly modified with some suspension goodies in order to correct some of the shortcomings from the factory. Let’s start by identifying said issues before we discuss TPC Racing’s solution for them.
It’s been well documented, by Randy Pobst and others, that the GT4 suffers from a case of understeer from the factory (see here, here, or here). Pobst also mentions that, on track, the Cayman GT4 feels “slightly underdamped,” particularly over bumps. Additionally, many experienced drivers (many of whom have expressed this here on Rennlist) have noted issues with corner entry oversteer, tire wear issues on track, and excessive nose dive/bodyroll. So how did Michael Levitas of TPC Racing and DSC Sport go about correcting these characteristics on his personal GT4?
Reducing the understeer to a reasonable amount is a fairly easy task. In fact, Randy already shared his trackside quick fix in his second Motortrend review of the GT4:
As his experience suggests, adjusting the sway bars doesn’t fully solve the issue with understeer the car has coming out of the factory. The other inexpensive – and vital – step in reducing the GT4’s understeer is a proper alignment, which Randy also addresses in the same article. It’s something the TPC Racing team does very, very well.
As most of us here already know, a proper alignment is everything for a performance car. As we took our car in for an alignment, we used shims to widen the rear track for increased lateral grip. As the majority of our customers use their cars both on street and on track, tire wear was also a factor in determining our alignment specs. However, as we aligned again and again, we found ourselves walking a very fine line between understeer and corner entry oversteer. It seemed as though the more we attempted to dial out the understeer, the more trouble we were running into with oversteer. So we turned to the damping.
We knew that a lot of that oversteer was a result of the geometry of the car when under heavy load. As the suspension travel increased, we were experiencing more and more oversteer. We decided to see what we could do with the damping. Whereas years ago we might have used stiffer springs to address this issue, we now turned to the DSC Sport tuning software. We dialed in more damper into the rear of the car to mask the oversteer. It helped, quite a lot in fact, but we still knew we were just masking the problem, rather than solving it. So we turned back to the rear geometry.
On a similar note, we were also able to greatly reduce nose dive and body roll, allowing us to threshold brake deeper into the braking zone, by improving the damping via the DSC Sport controller. We feel on such a high tech modern sports car, such as the GT4, the DSC controller and tuning software matches the sophistication of the car.
After multiple outings at the track, we determined that the rear toe curve was the major culprit, and we were masking it with stiffer damping via DSC mapping. More specifically, the OEM rear toe link – as well as any other link we’ve been able to find on the market – produce a greater amount of toe deviation as the suspension travels. After multiple progressions of track days, street testing, and parts development, our engineer developed an improved toe link design specifically for the rear of the GT4 that reduces the amount of toe deviation throughout the suspension travel range; during compression and rebound.
TPC Racing GT4 Toe Link Kit
The TPC Racing GT4 Toe Link Kit eliminates entry oversteer by solving the geometry problem at its root, rather than simply masking the issue with stiffer damping or springs. As a result, we were able to create a custom DSC calibration map for a GT4 fitted with the package that allows us to soften the rear dampers for even more grip coming off the corner.
While TPC Racing already offers a toe link package for the 981 that fits the GT4, it does not feature TPC’s proprietary offset bushing design, intended specifically for the GT4 upright. The TPC Racing GT4 Toe Link Kit, paired with the proprietary TPC Racing setup specs, allows for an improved toe curve on the rear Macpherson strut design. What this means is even at the highest of g loads, the toe settings are still optimal for grip. Pair this package with the DSC Controller, and the GT4 becomes an incredibly well-balanced car to drive.
As Randy Pobst himself later reflected, “I marveled at the balance and control of the TPC-tuned Porsche GT4 I drove at Summit Point. TPC racing makes this great car better to drive.”
While the TPC Racing GT4 Toe Link Kit and the DSC Sport V2 controller are available separately, we refer to the combination of these two products as our stage 2 suspension package for the 981 GT4, which also includes the full setup support that has become synonymous with TPC Racing products.