Cat offered the 500 fan motor as an option starting 1978 model year. You could still get the four port FA for 78. The FA got dropped for 1979. You lost about 7 hp with the Fan but you gained mileage (single carb) and itís less finicky. It outsold the FA first year, is what I heard. So Cat went where the money led them and dropped the FA. Iíve ridden both and from zero to fifty thereís not a huge difference. The FA pulls harder on the top end . If a guy wanted a 500 FA they just sold him an El Tigre 5000 for 79/80/81. From experience I think that weighed less than the Pantera and it had the better six port top end so it put out 62hp verses 55 for the four port. The fan was 48 to 50 depending on which book you look it up in.