One of the most notable changes between the IR18 and the DW12 was the removal of the airbox over top of the driver’s head, returning the look of the IndyCar to that classic open air look prevalent up to the late 1990’s. That airbox, however, served an important purpose; supplying air to the engine’s combustion chambers. With the airbox gone, that means the openings in the side pods have been made larger to accommodate the addition of intake ducting alongside the car’s radiators.
The radiators in the IR18 work on the exact same principle as your car radiator. Coolant is pumped through the engine to keep the temperature within operating levels. The coolant is kept cool through the radiators, which are essentially giant heat exchangers. Hot coolant and cold air from the atmosphere are fed into the radiator, where the hot coolant transfers heat to the air. After losing heat to the air, the coolant is fed back to the engine in a substantially cooler state, ready to be cycled through the engine again. If an engine gets too hot, it will boil the lubricants facilitating its operation leading to catastrophic failure.
The addition of larger inlet openings to the side pods means a lot more drag, which hurts top speed. Where the setups come into play is IndyCar teams can select from a variety of inlet covers that essentially make those openings smaller. There are 5 settings to choose from in iRacing; open, 38% closed, 54% closed, 70% closed and 77% closed. Each opening setting retains the same amount of air flow to the engine’s intake but controls the amount of air that is supplied to the radiator, which cools the engine. The last 2 settings are only used for qualifying, while the other 3 are for race settings. Depending on how hard the engine works and how fast the car is going, different inlet covers will be used at different tracks throughout the season. Pick too small of an inlet opening and the engine will overheat. Pick too large of an inlet opening and the car will suffer from excess drag and be slower on the straights.