Traditionally, the field consists of 33 starters, aligned in a starting grid of eleven rows of three cars apiece. The event is contested by "Indy cars", a formula of professional-level, single-seat, open cockpit, open-wheel, purpose-built race cars. As of 2015, all entrants utilize 2.2 L V6, twin-turbocharged engines, tuned to produce a range of 550 -- 700 horsepower (410 -- 520 kW). Chevrolet and Honda are the current engine manufacturers involved in the sport. Firestone, which has a deep history in the sport, dating back to the first 500, is the exclusive tire provider.

Answer this question based on the passage: what kind of cars race in the indy 500? Let's have some stream of consciousness first.
This is the relevant information: The event is contested by "Indy cars", a formula of professional-level, single-seat, open cockpit, open-wheel, purpose-built race cars. So, the answer is '' Indy cars '' , a formula of professional - level , single - seat , open cockpit , open - wheel , purpose - built race cars.