The more extreme GT3 is the RS. Porsche says 85 percent of RS buyers take their cars to the track regularly. And now Porsche is topping that with the GT3 RS 4.0, which is lighter and more powerful and handles even better. That 4.0 in the car’s title refers to the displacement of its engine. Whereas other GT3s use a 3.8-liter flat-six, the 4.0 borrows the upsize engine from the track-only GT3 R.