The factory radiator cap is rated at 1.1 bar (approximately 16psi) and is very suitable for stock cars. However, when you begin to boost-up your Supra, it is always a good idea to take care of the factory cooling system as well.
At SupraStore.com we recommend the GReddy 1.3 bar (19 psi) radiator cap.
Question: What does an extra 3psi of pressure accomplish and will it harm my car?
Answer: Good questions and we have the appropriate technical answer in laymens terms so you know what is happening. The radiator cap is the final portion of the equation on any cooling system. The removable radiator cap is there to "seal" the cooling system and allow it to build pressure. When you raise the pressure in any fluid control system, you are raising the point at which boiling can take place and you also help force water to every cooling surface in the motor. Raising the pressure an additional 3 psi allows the system boiling point to be raised and helps create a more robust cooling environment to help remove heat from the engine and get it to the radiator.
Since this is a factory GReddy part designed and fabricated for the 2JZGTE Supra, you know it has the quality and correct operating characteristics for your car.