This is the final article in our Porsche 911 GT3 project series, so thank you for following along. For this installment we've decided to document a final solution for the handling issues. The problem was inconsistent grip on the rear-end in high speed or high load corners, causing the driver to be more tentative and thus slower on lap times.
In Part 4 of the series (EC 4/11), the factory limited-slip differential was blamed for the looseness, so the worn-out LSD was rebuilt with more aggressive Guard Transmission internals. This greatly improved grip but the car remained "loose".
After further deliberation, one additional option was considered: heat from the engine and forces on the rear suspension were causing the OEM rubber suspension joints to degrade more quickly than expected after numerous track events.
Once degraded, the weakened rubber joints allow a degree of slack under load, thereby causing the rear geometry to physically and momentarily change the alignment of the rear wheels. The movement is very small but creates a condition where the rear-end is no longer perfectly predictable because it's effectively steering itself.
The solution is solid joint links from the aftermarket. A company such as GMG Racing in Santa Ana, CA has various options to fix this issue, including a rear bump/toe steer kit, rear dogbone set and solid control arm thrust bushings. These are the same parts GMG uses on its championship-winning World Challenge Porsche 911 GT3 Cup cars, so the quality is impeccable.
Fortunately, the parts are relatively easy to swap, requiring about one hour labor per corner. However, you must have a four-wheel alignment done immediately afterwards to fully enjoy the benefits.
We had access to a racecar string alignment system, so decided to tackle the job ourselves. We also had some other tweaks in mind and since GMG offers front bump/toe links and thrust bushings plus adjustable front and rear sway bars, we decided to test everything in one go.
When taking on such a large job, we approached one corner at a time to ensure the new GMG links and bushings were fitted in the same orientation as the OEM pieces. We also built a tool to press out the factory rubber thrust bushings while the control arms were installed on the car - this saved lots of time, but was still fairly difficult.
As for the hollow GMG sway bars, they're 30-50% stiffer (depending on the setting) than the original parts, with a thicker 29mm diameter bar up front (25.5mm stock) and the same 25.5mm in the rear. This sway bar kit includes polyurethane bushings and lube.
The rear bar is straightforward to install on a lift or axle stands. It uses four bolts and the two end-links, with easy access to everything. The front bar is another story, however. It uses a similar fixing arrangement, but the front subframe must be loosened and lowered to fit the new bar. This procedure added another hour or two to the install, and you need a safe method to lower the subfame with the car in the air...
With the string alignment in place, we used a hybrid street/track alignment. Using a digital level and a straight edge you can measure camber to 0.10 when on a flat surface. So we set zero toe and -2.20 camber up front, while the rear had 1.5mm of toe-in with -1.80 camber. The sway bars were set in the middle, which meant five holes on the front and three holes in the rear.
Rear link set and thrust bushing for one side
GMG solid dogbone compared to stock Porsche part
GMG two-piece thrust bushing compared to the stock part, with the home-made extraction tool shown
GMG two-piece thrust bushing compared to the stock part, with the home-made extraction too
Once complete it would allow us to make minor adjustments along the way to find the best geometry before having a shop do a full corner balance and four-wheel alignment at a later date.
On the street, the suspension felt and behaved like stock, but with a minor increase in stiffness when approaching bumps head on. The vehicle would also rock side to side on offset bumps because of the stiffer sway bars.
One rear corner with all components installed
Rear-end with all links, bushings and sway bar fitted
String alignment allowed preliminary set-up to give us baseline settings
At the track, the GT3 showed far more poise in the corners, making the rear-end feel truly planted. The solid rear links gave more consistent feedback, while the sway bars provided flatter, more neutral cornering.
The goal when you make handling upgrades is always to ensure the contact patch of the outer tire remains as flat as possible to the road surface under load in a corner. We achieved this by keeping the car flatter (with the sway bars), using a more aggressive alignment and eliminating the rear-wheel steer (with solid link joints).
Once we were comfortable with the new set up, our GPS-timed laps demonstrated a reduction of 1.5-2sec per lap at our two-mile track. Finally, we had gone from low 1:25 laps to the low 1:23s, which is a great result for these relatively affordable parts.
||Rear Bump and Toe Steer kit (2)
||Front Bump and Toe Steer kit (2)
||Dogbone Links (4)
||Thrust Bushings (4)
||Sway Bars set
GMG on track
Towards the end of each year, GMG Racing rents Willow Springs International Raceway so that customers and sponsors can exercise their machines in the presence of GMG's Cup cars. The company also provides the track briefing, instructors for private coaching, GPS data logging and a great lunch too.
Nothing compares to being on track as a 911 GT3 Cup racer approaches and passes you at high speed. It's a wonderful experience and we'd like to thank GMG for allowing us to bring our T-Rex GT3.