Some of the top competitors in our circles give high priority to the performance and running characteristics of our engines as a basic requirement to successful stunt flying. One important part of the equation is the carburettor, in our case almost always a venturi. It mainly decides how the engine breathes: how it takes the air and how well it can mix it with fuel. It’ not just the cross section we have to deal with. The basic system, the dimensions, and the shape will have a definite influence on engine performance. Quite some information for this story I’ve got from Eric Janssen, Netherlands, so a big part of credit should go to him.
 
     
Let’s look at theory first to understand how a venturi works. Long ago Bernoulli discovered the physical laws which control the behaviour of fluids. He found out that a fluid changes its velocity depending on the cross section of the passageway it’s going to pass through ( you can easily see this on the course of any little brook or big river) , and the pressure of the fluid depends on the velocity. The smaller the area ( in our case a venturi hole ) the higher the velocity - and the lower the pressure. For us low pressure means better suction. The way our fluid - the air - flows through the venturi is also influenced by the venturi system, the shape and the surface of the inner venturi walls.
A straight hole drilled through the carburettor body cannot be the best solution. For economical reasons very often a simple tapered opening is used, but the ideal shape is the parabola, looking pretty much like a trumpet . Smallest radius is at the entry, gradually enlarging towards the point of smallest diameter. So the cross section is reduced gradually which increases airflow speed. The fuel jets ( whatever type ) should be located here since this is the point of highest velocity, hence lowest pressure = best suction. This also helps to better pulverize the fuel, which makes for a better fuel-air mixture which in turn improves combustion. Downstream, the diameter should gradually increase again. The diverging part should mate with the cutout of the crankshaft opening as good as possible. This venturi shape can provide a smooth air flow without vortices, flow separation, resulting drag and speed loss, loss of suction, and thus loss of power.
Since there’s a wide variety of venturi shapes and spray bar versions, there’s a big number of venturi systems. Now let’s examine the peculiarities of the different types as shown in the first sketch. Mounting methods are not considered here with the exception of those mentioned.

Probably the most popular venturi type is the one as shown in drawing A . It’s used on most smaller capacity or control line only engines. It has an opening close to the ideal parabola shape with the spray bar “straight through”. This brings the fuel jet to the centre of the venturi bore where the pressure is supposed to be the lowest . However the spray bar is a bluff body, destroying a clean airflow and reducing the “open cross section” ( more on this later ).  
If the venturi body is held by the spray bar itself, the spray bar runs right through the crankcase boss thus fixing the venturi to the crankcase. In this case the spray bar is located much closer to the crankshaft . Depending on engine design this may have an influence on running characteristics in a certain RPM range.
Figure B shows the “half spray bar” type which ends in the middle of the venturi with the needle entering from the opposite side. This version somewhat reduces the area of the “disturbing” spray bar and is often seen on racing engines who like to breath as much air as possible.
One step further is shown in drawing C. Here we already have a true venturi. Photo (3) shows a version with needle and fuel inlet on the same side. The needle valve assembly was taken from an RC throttle and mounted into a home made venturi ( a special tab was needed to cut the fine thread ). A similar version is shown in drawing D with a different way of mounting a standard needle valve.

In drawing E the “sprinkler” type is shown. Controlled by the spray bar the fuel is fed into a groove running around the venturi, and enters the venturi by one or more tiny holes. This type is usually found on Cox engines and has proven to be quite satisfactorily. A similar version was used on Super Tigre 46 engines with multiple holes and an inner diameter of only slightly more than 3 mm ( drawing F ). Their venturis had a pronounced sharp edged step shortly above the feed holes ( nobody knows what it was meant for ).
 
For ease of production some people have built venturis with simple shapes. The American Bob Baron made his venturis with a simple conical entry and exit as seen in drawing G. He recommended the cone angles to not be wider than 10 degrees. The American Scott Bair allows for a “downstream angle” of up to 14 degrees ( sketch 5 ). Now these numbers call for a relatively long venturi. Maybe that’s not a mistake. It is generally accepted that a long intake will improve suction. It seems obvious that on a very short intake opening the air is still quite turbulent when hitting the spray bar. With a long intake the air has more time to form a smooth and constant flow, allowing for easier needle setting and maybe better suction. The same is true for the downstream part of the venturi. Here Eric Janssen gives another reason to make this part long.
Maybe you’ll have noticed that sometimes the engine likes to spit fuel out of the carburettor. Because of the extreme port timing of modern Schnuerle engines a high pressure wave can reach the sprabar when the shaft intake port is opened. This is exactly what we do not want. With a long distance between crankshaft opening and spray bar we can reduce this effect. Eric recommends a distance of about 20 mm.
Finally in drawing H you’ll see the venturi design of Jaco de Ridder, Netherlands. Eric Janssen as well as Henk deJong have used it quite successfully for many years. I’m told that the parabola shape was computer designed. The step downstream of the multiple jets is supposed to create a very good mixture of the fuel with the air because of the induced vortices. Only a thoroughly mixed gas provides a uniform combustion , hence smooth and reliable running, high performance, and a long engine life.

   
 
  If we have the spray bar running right through the venturi we should make sure what type it is. There are spray bars with one suction hole and those with two holes. If we have two holes, of course these should be arranged exactly horizontally ( means exactly at 90 degrees to the venturi axis ). With a “one hole” spray bar this should be mounted that the hole is located slightly downward from horizontal ( say at “ 4 o’clock” ) . Tests have shown that at this position the airflow around the spray bar reaches its lowest pressure, thus best suction ( see sketch 6 ).
Spraybar location has yet another effect. Sometimes we want to vary the vertical tank position according to requirements of a symmetrical engine run ( upright and inverted ). With a spray bar location too close to the engine bearers we might run into problems when it comes to shifting the tank level - the engine bearers might be in the way. While the old rule “ tank centre line = spray bar centre line” is no longer supported we should still spend some thoughts on spray bar location.
It’s quite obvious that there’s more to venturi design than meets the eye. Many of us still prefer the typical “four stroke/ two stroke” running mode. For this characteristic it’s presupposed that we have a smooth and reliable engine run at a rich mixture setting. The venturi is responsible for that. Additionally it controls power, fuel consumption, 2/4 break , and the tendency of modern Schnuerle engines to “run away” ( = means the reduced ability to return to four stroke after the break into two stroke ). Just one example: if you increase the venturi diameter on a given engine, you’ll have to set the needle slightly more rich to get the same lap time. At the same time this will shift the 2/4 break point to another RPM level, thus to another point in the manoeuvre. This is only one example out of a few hundred; it’s quite obvious that it’s impossible to cover that whole topic here. In most cases the choice of venturi system is already given by the engine manufacturer. But we can use our knowledge to adapt what we have to what we want. In photo (2) several types of venturis can be seen, showing different designs and materials. In photo (4) there’s a homemade venturi with ST needle valve (left ) and an original Merco 60 venturi ( right ).

   
When making venturis some consideration should be given to materials. Venturis can be made from aluminium or plastic. There’s no information available whether temperature ( transformed from the crankcase ) would make any difference; probably it would be neglectable. Plastic is easy to work with, but it is too soft to cut threads into it should that be required . If the fixing method with a clamp bolt a la Super Tigre is used, the plastic stuff should be pretty hard. Apart from this, plastic has one big drawback : it just doesn’t gleam as bright as polished metal !