Maximum Venturi Diameter?

Hi Leonard,

What would you say is the maximum venturi diameter I could get away and still have a steady run on suction with a TT .25GP (sprinkler type venturi)? Same question for an OS .25LA (thru the center needle). I know the width of the needle body makes a difference on thru the center, so what venturi dia with what needle width do you think on the LA? This is for a local club racing event being developed as an expansion of Fox racing to include plain-bearing schneurle .25s.

If anyone else has info on this subject, I would appreciate it also.

Thanks!
Kim Mortimore
Alameda Aer-O-Nuts

If you are wanting to go racing, then I would think the Thunder tiger is going to be the better engine over the LA. Here you are going for maximum rpm and pitch with just the minimum amount of diameter to provide adequate thrust for the desired speed.

But when it comes to the proper venturi for racing on suction, I do not have data on this. We know the 20 FP will run on a .257 venturi so the 25, although supplied with the same venturi, should be able to run on a considerably larger size. I would suggest that you just do some experimenting to see what you can get away with.

The choke area for a .257 venturi is .017. That would equate to about a .147 for the TT. But we know we can go larger than that. Ironically the stock venturi on the TT 25, which is way too big for stunt, is .173 That is probably getting closer to maximum for good needle run, etc. That would be an approximate equivilent of .281 for the LA.

Maybe start there and then try something a little bigger just to see what happens. With every engine you are going to reach a point where there aren't enough cubic inches to draw the fuel properly.

Do some experimenting and report back.

Paul Smith

Max venturi size is the essence of speed and racing. Use trial and error. Increase venturi diameter and/or shave spray bar until it won't draw gas, then back off.

Hint: if you are required to use suction,keep fuel as far inboard as you can. Also, design airplane with shortest possible fuel line.