BJ,

Both of the tailpipes I built used the John Wright thrust augmenting pipe design. They were used for a Jetcat Titan and a Merlin 160.

They were about 86mm @ the inlet, tapering up to 96mm (at about 180mm from the inlet) and then 96mm all the way to the exit. On a test stand I measured about a 5# increase in thrust (over no tailpipe at all) with the Titan but never tried the Merlin on the stand.

I'm probably going to build a new tailpie for my JMP T-33, but I won't be able to build a thrust augmenting pipe due to the scale exhaust opening being too small. It would be nice if someone published a chart with tailpipe sizes for various engine sizes.

Paul, the spot welder shwn is the one I used to weld my pipes. Harbour Freight sells longer electrodes for that welder.