After the first attempt of a batchbox crumbled I redesigned and built another. This one is also a 4 inch made of local clay, wood ash, sand, and straw. Rather than off set the riser to create a single cyclone this time I centered the riser and used a twin cyclone idea to try to maximize the "ram horn" effect. The idea is to create more turbulence in the riser than my first design where I tried to speed the gasses up the riser. Today I got it put together and fired it up.
I think my cobbing is getting better as this one turned out smoother than the first one (which you can see the crumbled remains to the left) and I didn't get the mix anywhere near as wet this time. You can see the port is just a few holes drilled in the inner mold so it wont come up to full power until it burns out.
As you can see it immediately came up to speed and burned smokeless, oh wait.....
By the way I used the same heat riser from the first version just cobbed up some cracks and sealed it up to the new core with cob.
Most of the form was burned out by this time and the first "ram horns" appeared. The whole thing is still pretty soft and wet and I haven't put the P-channel in yet so maybe I can get some performance on the next burn.