If I follow the 1:1.5:3 (Wisner's) or 1:2:4 J tube ratio suggested by Glenn Hebert the barrel seems too close to the feed tube.
This is my revised mock up after realizing (with help) that my burn tunnel was too long.
Check out my main build post here for additional info.
I am not finished building the heat riser. This is the internal pipe only.
If I center the barrel it seems to close to the feed tube.
Do I give the back side(move barrel to the left in above image) more space so the exhaust exit has more room and leaving more space for the feeder?
And if I can move by how much can I move it?
The external tube is 15" (not cut yet).
What are you using as "1" in the ratios? Peter van den Berg in I think the donkey32 forum said that it could probably be between 0.7 and 1.3 times nominal system diameter. From your pictures, it looks like you have an 8" system, and the burn tunnel roof is the same as the feed tube opening, around 7.5". The E & E standard build uses four firebricks for the burn tunnel roof, giving 10" here, which is sufficient for easy clearances with the barrel centered. You can offset the barrel as long as you leave at least 1 1/2" to 2" gap. You will get more airflow and more heat on the side with the wider gap, which can be used intentionally to tune the output characteristics.
A 7 x 7 burn tunnel is for an 8" diameter system. You could make the dimensions 6:12:24 or 8:16:36 or 10:20:40 or 12:24:48... as long as they are in the right proportions. Larger lengths will give more power to the rocket effect.