Depends on your engine and year. My 65 289 would take the 18 inch one. Here is some reading I dug up for you.

>>>A better street performance and at many times street/strip fan is using a mechanical fan with a fan clutch. The fan clutch is designed to drive the engine fan when air movement is needed to cool the engine. A thermostatically controlled fan clutch employs a bi-metal spring to adjust fan speed in response to operating temperature. As engine temperature rises and the radiator heats up, the air passing the radiator into the fan heats the spring coil and a silicone fluid in the clutch that enters the chamber increasing tension in the clutch and drives the fan. As coolant temperatures reduce, the fan clutch is allowed to slip, offering increased fuel efficiency and quieter operation. On the "non-thermal" fan clutch a silicone fluid with a very high shearing capability is used to drive the fan and cool the engine at lower engine speeds. As RPM increases the drive fluid allows the fan clutch to slip, increasing engine efficiency when less fan-assisted air movement is needed due to higher vehicle speeds.<<<

 

thank you so much i cant wait tell i get all my stuff...
i will post pics soon i need to get them off my camera...

 

OK, I heard that. We will be expecting pictures.