formula for rear stack box size ?

[smokeyamber]

Hi, I am building an arch and remember seeing on here a formula for the size for the stack size, particularly the rectangular opening that would taper up to the round stack. Arch will be approx a 2'x5", I was planning on 8" stack pipe. I don't want to make this too small or too big, mainly too small as I can adjust to opening smaller easier than bigger.

Any rule of thumb or formulas would be appreciated ! :)

[RileySugarbush]

8" diameter is 50 sq. inches. Should be fine on a 2' wide arch. The rectangular base should have at least that much area, so if your base is 16 or 18 inches across, 3-4 inches side will be fine. 5 to 6 ok too to make it stable.

[smokeyamber]

Just what I needed !!! Now I can start cutting, bending, welding up the new arch !!! Thanks ! :lol: