CSF 15+ WRX/STi 2-Row Race-Spec Radiator 2015+ STi Fuel The main difference is that

The main difference is that an AOS-"R" system "returns" what is collected back into the oil pan so there is no servicing required

and inject that knowledge into creating performance systems that maximize horsepower

AOS Diameter: 3

and instead maintain the factory plumbing strategy

-Effectively raises the fuel octane and increases engine performance with cleaner inlet air

CSF 15+ WRX/STi 2-Row Race-Spec Radiator 2015+ STi Fuel The main difference is thatDesigned with industry first B Tube technology, these new units will keep the most sophisticated and demanding engines cool in the most extreme temperatures and driving conditions. Using their state of the art ISO 9002 engineering and testing facilities, these radiators have undergone industry leading; leak, thermal cycle, burst pressure, vibration, and wind tunnel testing. Each individual radiator is meticulously handcrafted by a master class welder