At the full circuit length fast Corvettes/Camaros are faster than GT4 - and we've chose GT4/Spyder knowing that.
But will see how PDK versions will do - so we will know whether manual gear ratios the reason for handicap or it's going beyond this - in which case it would be quite difficult to understand since the car drives so good.

 

Bringing up tires again, GM’s tire specs are much more aggressively optimized for dry grip above all else, and are much closer to the Cup 2 R than the Cup 2 spec used by Porsche. For example, in Car and Driver’s grip tests:

• 991.2 GT3: 1.11G on Cup 2, 70-0 in 146 ft
• 718 GT4: 1.10G on Cup 2, 70-0 in 148 ft
• C7 Z06: 1.19G on (chevy) Cup 2, 70-0 in 128 ft
• Camaro ZL1 1LE: 1.18G on (chevy) Eagle F1, 70-0 in 141 ft
• 991.2 GT3 RS: 1.24G on Cup 2 R, 70-0 in 128 ft

I’m curious how a 718 GT4 PDK coupled with Michelin Sport Cup 2 R tires (more comparable to chevy tires) would fare. While there are no 295 width Cup 2 R options, there is a 305 width that would work.My guess is that a Chevy would still be quicker because of higher power even with equal grip. No magazine will ever do this test with a regular 718 GT4, but a GT4 RS should be exactly this plus some aero that doesn’t matter much on slower tracks, and some negligible weight savings. A GT4 RS should equalize power to weight with a regular C8, and its Cup 2 R tires should be equivalent to Corvette spec Cup 2s, so a C8 on Chevy-spec Cup 2s would be a fair fight against a GT4 RS.

On a similar note, compare C&D’s lap times of an AMG GT S to an AMG GT R. Both were on Cup 2, but one was on a Porsche-like mild spec, and the other was with Corvette-spec like tires. The power to weight difference between the cars and rear wheel steering can’t explain the massive disparity in lap times, but using Chevy-spec tires can.

Anyway, I don’t judge a car’s handling by who has the stickiest tires for maximum raw grip. For me, good handling is about how precisely a car can be maneuvered at speed, how it responds to transitions and direction changes, how it reacts to breaking loose at the limit (on track or autocross), and how the steering relays information about the car’s state through force feedback. A car’s suspension, suspension geometry, and weight also play a role in dry grip, but it’s only fair to judge these with equal tires, and tire differences far outweigh these factors on modern performance cars - these aren’t 70s mustangs.

 

You're saying C7's Cup2 is better than Porsche's Nx Cup2?

 

Yes, most definitely by a big margin when it comes to dry grip. Porsche spec is better in cool and wet conditions, as well as tread life.

 

Skid pad numbers also tend to favor Chevys due to the wider front tires and more weight up front (Camaro and <C7 Corvettes). The double wishbone on Corvettes also help too.

There are many reasons why one car does better than another in a test that wouldn’t necessarily translate into better real world performance. Take skidpad, figure 8 and other similar test results with a grain of salt.