Hi Kelly,
John and I tested the J600R in the Starfire II yesterday and I used 1/4 to 3/8" according to Dean. It was apparently pretty good. I imagine there was some thrust loss in the 27" tube, but that is to be expected with a tube that long. However; there was no heat escaping with the thermal probe half way between the tail cone and the pipe mouth all the way around the mouth. In fact it was cooler a tad with the airflow being sucked down the pipe. The supreme test as far as I am concerned is when you put the hatch on. It should not suck down hard, which means there is not enough incoming air and you will suck some heat back, nor should it ever blow out as that means there is already heat escaping from the tail pipe into the fuselage. The thermal probe was cold even at the corrugations around the tail of the pipe showing that cool air is being sucked through with the rapid speed of the exhausting airflow from the inner tube of the pipe. Of course the fuselage was cool to the touch too.

Anyway Dean's range of 0.25 to 0.375" (6.5 to 10 mm) seems to be a pretty good starting position on this plane.

Len