Radiator Overflow

Radiator overflow is the excess coolant that escapes from a race car's cooling system when the engine gets hot and the coolant expands, creating too much pressure inside the radiator.

When a race car engine runs, it produces enormous amounts of heat. The cooling system's job is to keep the engine from overheating by circulating coolant (a special liquid) through the engine and radiator. As this coolant absorbs heat from the engine, it gets hotter and expands, just like water expands when it boils. This expansion causes pressure to build up inside the sealed cooling system.

The radiator cap acts as a safety valve. When the pressure inside the cooling system gets too high, the cap releases some of that pressure by allowing a small amount of coolant to escape. This expelled coolant is what we call radiator overflow. Without this pressure release mechanism, the radiator or cooling system hoses could burst from the excessive pressure.

In motorsport, radiator overflow presents a serious safety concern. If hot coolant spills onto the race track, it creates an extremely slippery surface that can cause cars to lose traction and crash. Even a small amount of coolant on the racing line can be dangerous for multiple cars. This is why racing regulations require special equipment to handle overflow.

An overflow tank (also called a catch can, puke tank, or expansion tank) is a separate container connected to the radiator by a hose. When the radiator releases excess coolant, it flows through this hose into the overflow tank instead of spilling onto the track. These tanks are mandatory equipment in most forms of motorsport.

There are two main types of overflow systems. A basic overflow tank simply catches and holds the expelled coolant. A recovery tank is more sophisticated—it can draw coolant back into the radiator as the engine cools down and the coolant contracts, making the system more efficient and reducing coolant waste.

Overflow tanks are typically made from either plastic or aluminum. Plastic tanks are lighter and cheaper, while aluminum tanks are more durable and can withstand higher temperatures. The size of the tank depends on the type of racing and how much coolant expansion is expected during competition.

Strategic placement of the overflow tank is crucial for safety. Many race teams mount the tank at the rear of the car, behind the back tires. This location ensures that if any coolant does leak from the tank, it won't drip onto the track where other cars will drive over it. Some series have specific rules about where overflow tanks must be mounted and how they must be secured to prevent them from coming loose during a crash.