Jerry,
An effective chuff pipe is one of the things steamers have been experimenting with for a long time with only limited success. At Diamondhead this year, I saw and heard a new design that I think is the most effective. It's a pot belly configuration that replaces the current exhaust pipe. It was installed in a K-27 and is louder than any I’ve heard before. I was told that they are experimenting with two diameters for different size smoke boxes. I was told they are about to make several and release them for sale, so keep your eye out.

 

I too have been experimenting with chuff pipes and after several iterations, I’ve settled on a rectangular one, 3/16 x 5/16 K&S stock. I used a vise to crimp and seal the end, then drilled a #70 hole in it. Its length is so that it goes to the bottom of the smoke box with the crimped top 1 cm below the top of the stack. In my case, the length of the chuff pipe is 12.5 cm. Three cms down the side of the pipe, I filed a slot with sharp edges. The slot is 3 mm deep and 1 cm long. The chuff pipe just slips right over the current exhaust pipe.

 

My rectangular pipe works well at slow speeds and under load, but is not near as loud as the pot belly one I saw at Diamondhead.