One of the most popular FAQs ( = frequently asked questions ) is the question for carburettor diameter. This doesn’t come as a surprise - most engines are sold in an RC version, and for this purpose the manufacturer has taken care of this problem. Very often we have to buy or make our own carburettor. But even if we acquire a purpose designed control line engine we have to decide on a venturi diameter. We have learned that we can have quite some control about the power output of our engines and the way they behave during a flight.
This question cannot be answered by simply giving a certain number in Millimeters or Inches. It depends on the type of carburettor we use. There are different versions, and these are explained in a different thread on this site. The basic difference is whether we have a ( almost ) true venturi, or a venturi with the spray bar running right across the open area. There may be different reasons for preferring one way or the other ; they are not discussed here. However the type of carburettor is a deciding factor on what diameter size we are going to use.
I think I need not tell you the formula for calculating the area of a circle, which by the way is the “open cross section” of a carburettor ( radius x radius x 3.14 ). For a true venturi this would suffice. However if we have the spray bar running right through the venturi, we have to detract the “area” of this spray bar from the venturi cross section. The thickness of available spraybars can differ slightly. But since dimensions of our carburettors are already small, the spray bar thickness can have a tremendous influence on the final “real” venturi opening. Let’s call this the “actual cross section area”.
Some people have the skills and the possibility to turn their own carburettor. If not, some “cottage industry” manufacturers may be able to help. However the needle valve assembly is usually bought. So it makes sense to look for the spray bar first, and then look for the venturi. The Super Tiger spray bar is a very popular item. It’s still available, and the thickness is 4 mm. For some OS engines in the 40 to 46 size a control line version was made. They had a spray bar with 3,5 mm thickness. As for me I’m using needle valves from old Taifun engines ( I’ve bought the rest of the Graupner stock ). These have a very fine thread on both sides, thus they can be centered precisely. The thickness is about 3,2 mm. For smaller engines there are N/V assemblies still available; for instance from Webra with a 3 mm thickness. There are many other spray bars available, too, and you’ll have to find out about availability and dimensions.
Now that we’ve got the thickness of the spray bar we can easily calculate the area which “disturbs” the airflow in our carburettor. Thickness times venturi diameter , and subtracted from the venturi cross section area - and we have the “actual” area, or cross section.
This final number is what we are actually looking for. There’s a lot of information about venturi diameter for given engines. Usually these numbers are accepted since the spray bar thickness is taken as a fixed value ( spray bar as supplied by the manufacturer ). If you ever use another item, general recommendations are worthless. I’ve tried to calculate the “actual” venturi open area , depending on different spray bar thickness. The range includes numbers for thicknesses 4, 3,5, 3,2, and 3 mm. Since N/V assemblies are hard to come by these days it might be helpful to look for this item first and then to produce/ order the venturi.

 

venturi- diameter

0pen

cross section

actual cross section in sq mm for

spraybar thickness in mm

mm sq mm 3 3,2 3,5 4
7,2 40,7 19,1 17.7 15,5 11,9
7 38,5 17.5 16,1 14 10,5
6,8 36,2 15,8 14,5 12,4 9
6,7 35,2 15,1 13,8 11,8 8,4
6,6 34,2 14,4 13,1 11,1 7,8
6,5 33 13,5 12,2 10,3 7
6,3 31,2 12,3 11,1 9,2 6
6,2 30 11,4 10,2 8,3  
6 28 10 8,8 7  
5,8 26,4 9 7,9 6,7  
5,7 25,5 8,4 7,3 5,6  
5,6 24,6 7,8 6,7 5  
5,5 23,7 7,2 6,1    
5,4 22,8 6,6 5,5    
5,3 22        
5,2 21,2        
5,1 20,4        
5 19,5        
4,5 15,8        
4 12,5        
3,5 9,6        
   
In the first column the table will show you the diameter of the venturi in millimeter. The second column shows the “open area” in square millimeter. In the next four columns the actual cross section areas are shown, depending on spray bar thickness, again in square millimeter.
´The picture at left shows several versions of needle valve assemblies which can be used for our needs. At far left is a carburettor made by Brian Eather with an ST spraybar. Next is the popular Super Tigre needle valve assembly at 4 mm thickness. The Graupner "Taifun" spraybar ( no longer available ) with threads at both ends has about 3,5 mm thickness. Threads are very fine. At far right is a Webra spraybar , originally used on small diesel engines. Thickness is 3 mm. It's very short, so it can be used for small engines only.
I’ve also included another table which tries to give some very “basic” recommendations as to what venturi openings can be used and what range is available. The information about the numbers in this table were mainly found and measured on commercially available engines or based on personal experience. No guarantee can be given on these values. Some fliers largely deviate from these figures. It’s just a general information about where to start if you have equipment out of the rut. Now I have to point your attention to one important aspect.
 
engine capacity opening in sq mm
60 12 - 16
45 12 - 15
40 11 - 13
35 10 - 12
25 9 -11
15 8 - 10
09 5
 
 
  All the numbers given in the table were calculated the easy way. That means : the “spray bar area” was simply calculated by using spray bar thickness and venturi diameter. ( in sketch 1 this is shown as Dv and Ds ). Of course this is not 100% correct. Since this “spraybar area” is partly reduced by the circular shape of the venturi opening, the actual ”disturbing” area is somewhat smaller than what is given in the table ( see position C in the sketch ). This may have some significence for the smaller venturi diameters and thicker spray bars. I’ll have to leave this for the highly talented mathematics to figure out the correct numbers ( sorry, I can‘t do this; but maybe the difference is negligible). Anyway:
I wanted to start us thinking about what we have and what we want to achieve. Just relying on simple numbers might not be the way to success. We have to understand the basic principles and work accordingly. Maybe this way we may find the correct diameter for this black hole in our carburettor world.