In the past decade, dual-clutch transmissions have come to dominate the performance car market. They're used in everything from hot hatches to the latest, greatest hypercars. They provide the best of both worlds, providing smooth automatic operation in normal driving, and lightning-fast gear changes while going flat-out. But when it comes to actual racing, they're still not the gearbox of choice.
Nate Vincent of put together a great video comparing his GTI TCR race car's old DSG dual-clutch transmission to a new sequential transmission that his team will use for the 2019 season. The biggest difference is weight. Including the flywheel and clutch, the sequential weighs nearly 100 pounds less than the DSG, which is a huge amount for a race car. The sequential also has fewer gear shafts to spin—just two versus three for the dual-clutch. Paired with a lighter flywheel, that means less rotating mass for the engine to spin.
Unlike a dual-clutch, which uses helical-style gears, a sequential has straight-cut gears, meaning less power loss traveling through the transmission into the axles. It also uses dog teeth connectors instead of synchronizers to get from one gear to another, forcing the gears together instead of smoothly transitioning from one to another. The downside is that it's a lot less smooth, extremely loud, and requires the transmission to be rebuilt after just a few thousand miles. That's why you don't see many sequential gearboxes used in street cars.
The full video goes into much more detail, so if you've got a few minutes to kill, it's certainly worth a watch.