I think the 5 is for a 5 port? Some of the car motors are even seven, or sometimes three. I had been thinking of the .28 for a piped B speed control line plane for a record attempt. I need to find a place to fly from pavement first. I think the record here is around 180 mph, If I could do that I would be more than happy, If I could break 200 mph with some cheating (nitro) I would be even happier. It has been done in the 1980's, so I think it is possible with the old line sizes and high nitro.