mavisky said:

I havent seen a 100% confirmation on what's changed other than what's externally visible such as the extra canards.

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• More Power: Hardware updates and aggressive tuning push Mustang GTD Competition’s supercharged 5.2-liter V8 to a targeted engine output beyond the original Mustang GTD’s 815 horsepower
  • I know there were images of GTD's at the ring with a Whipple installed instead of the stock blower to 900hp is easily achievable
    
• Advanced Aerodynamics: Building on the existing Drag Reduction System (DRS), the Mustang GTD Competition adds rear wing modifications, secondary front dive planes, and rear carbon fiber aero discs to increase total downforce without sacrificing efficiency or aero balance
  • Looks like they may have attached a gurney to the rear wing's 2nd element.
  • 
• Increased Grip: New high-performance tires
  • All the images of testing I have seen show Michelin street tires so likely some sort of Cup 2 R designed for the massive heft of this car.
    
• Weight Reduction: New magnesium wheels, new carbon bucket seats, lighter damper system and additional actions help reduce weight
  • Wheels and carbon seats are awesome, but wondering if the "lighter damper system" involves the removal of the car's dual ride height feature that they pulled over from the GT.

Junkyard Dog said:

Slicks do not qualify for production prototype times. Also, Michelin slick sidewalls do not look like that. But, man, I can't see any tread . . .

I bet the DRS contributed a huge amount to the time, given the high speed stretches at Nurburgring. Full downforce when you need it, low drag on the straights.

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