Venturi math?

Lakc · Jun 23, 2011

I am starting to work on carburetor design, and need to step turn the inside diameter into a venturi on the lathe. While I can draw a decent venturi, I would like to find out how to model one mathematically, so I can generate coordinates for step turning.

Of course, I could draw it out 10x desired size and measure everything, but that takes home shop time, and I prefer to get as much work done on my lunch hour, leaving home shop time to be more productive. :

gbritnell · Jun 23, 2011

Hi Jeff,
Why step turn the venturi? Why not just set your compound over and bore it at the desired angle? Or am I missing something?
gbritnell

Lakc · Jun 23, 2011

I had planned an airfoil shape for the restriction, which is a blend of several different radius's (radai?). If worse comes to worse I can approximate it, they dont take too much metal or time, and I have a half dozen different ideas I want to try anyway.

DICKEYBIRD · Jun 24, 2011

Internet pundits claim that a parabolic arc is the best shape for venturis. There is a formula for generating them but it's way over my head.

I've made quite a few for a friend who is an importer of Italian racing model aircraft engines. I draw a quarter ellipse shape in CAD to scale, print it out, glue it to a stick of HSS and grind a form tool. Works great.

kcmillin · Jun 24, 2011

FWIW I thought I would add how I cut the venturi on my last carb. I did not do any fancy curves just an angle (but the fancy curve is probably the way to go). I made a cutting tool, much like a valve seat cutter. It is easy to get the desired 'reversed' shape of the venturi on the lathe. I then used it like a reamer in the mill. I would imagine that this would work nicely with a curve shape as well.

Kel

Lakc · Jun 24, 2011

Thanks Kel, that certainly does look good. I have a whole lot of different ideas I want to try, so I will end up extrapolating for the moment, before I commit to any hard tooling, but thats definately the way to do it.