Venturi size for .40

Leonard:

What size venturi do you suggest for and O.S. FP .40 with a spraybar in the .155 range?

Second question - there was a chart of venturi size vs. spraybar and engine in the Pampa mag perhaps six months ago. Is it generally on the mark?

Thanks

That chart was done by Pat Johnston, and it is quite accurate for most CL stunt type engines. I work with a larger bore with my modified engines, but these are a different animal. George Aldrich did the same with his modified engines. In general, however, if you compute the choke area (area of the total opening on a "sprinkler" or "true" venturi, or area of the total opening less the needle valve choke on a restrictor" type venturi) and then divide by the cubic inches, you will want a result of .045 to .052. For a .40 size engine such as the OS FP or OS VF, that would compute to .260 to .270 with an OS needle (.135 diameter) and .280 to .290 with an ST needle (.155 diameter). Coincidentally, the current venturi supplied by OS for the .40 FP and .40 LA is .265, right in the middle of the range.

Say you want to check the choke area of a .285 venturi with a .155 ST needle. Compute it like this:

Radius (half the diameter of the opening) squared (.1425 x .1425 = .02030625) times Pi (3.1416) equals .06379396582196. This is the total area of the venturi opening.

Needle size (.155) times diameter (.285) equals .044175. This is the restrictor area of the needle.

Take the area of the venturi opening .06379396582196 and subtract the restrictor area of the needle .044175 to get the actual choke area = .01961896582196

Divide this number by the engine size (.40) and you get a number of .049, right in the middle of your target range.

I have a chart I have done up with all of the engines I have computed so far with target sizes ranges, and so on, so I can just go back and look it up. But this is a way to compute a "ball park" figure for most any engine. Just remember, for an engine with a side needle (most Super Tigers, Magnum, Thunder Tigers, and so on) you compute the choke area of the venturi only since the needle does not go through it. Some of these will look quite small, but they are very adequate.

Leonard:

Thanks so much for the information!

Leonard --

Looks pretty good, and I will build a spreadsheet based on your numbers. Slick! The ratio of displacement to venturi area has a mathematical flaw, not too critical for engines of about the same displacement. Can't wait to see how stock venturis measure up on a stock Fox .35!

The flaw is that displacement is cubic (power of 3) and venturi area is just area (power of 2), so it is like scaling up or down a popular stunt design. You have to do it cubically rather than arithmetically, which is perhaps one of the reasons you show a huge range for the ratio.

I haven't thought this through completely yet, but off the top of my head, seems like venturi area to the 3/2 power would probably be more linear to displacement than what you have now. Check it out on a .60 and a .19 and see if the range narrows.

>Leonard --

>The flaw is that displacement is
>cubic (power of 3) and
>venturi area is just area
>(power of 2), so it
>is like scaling up or
>down a popular stunt design.
>You have to do it
>cubically rather than arithmetically, which
>is perhaps one of the
>reasons you show a huge
>range for the ratio.
>
>Larry F

Larry, although rpm will affect this and timing will affect this, the truth is most of our engines have similar timing and run in similar rpm ranges. Even the pipe engines aren't running that much higher rpm.

The reason for the range mostly is to account for how we choose to run our engines. If you choose to run a smaller prop, you don't need as big of a venturi. If you choose to run a larger prop, you will need a larger venturi. Generally a smaller venturi gives less power and a sharper break. A larger venturi gives more power but a softer break. Increased compression translates to more power and sharper break. Lower compression will give less power and a milder break. So you can work these two together. With lower compression you can go to higher nitro and bring some of the power back. But now I am off the subject.

Now, you mentioned displacement as being cubic and venturi area as being only the power of two. Not quite correct. Since we are running our engines in similar rpm ranges with similar timing, the linear speed of the air flowing through the venturi remains relatively constant. That being so, varying the area of the venturi will vary the cubic volume of the air flowing through it. (Yes, a very tiny venturi in relation to a very large cubic inch displacement will result in a higher speed of air flow, but we are trying to keep these in a proper range. When we stay in this range the airflow will remain relatively consistent.) Thus the range holds true for most of our engines.

Matt's piped PA 61 was delivered by Randy with a number 12 (.189) venturi. Choke area divided by displacement = .046, the bottom of the range. This is quite adequate for the smaller props that some prefer, but would not get anywhere close to some of the larger three blade props. We ended up going to a number 9 venturi (.196) and this gave a CA/D number of .0495 which allowed the engine to turn the 13 inch 3-blade prop. Hot weather still requires 10% nitro, and that isn't a bad thing. We are planning on trying an 8 (.199) which would give a CA/D number of .051. Each change will affect it slightly, which is where you try things to see what works best.

How does this work with other engines? Well, about the largest venturi for a stock ST 46 on 10% nitro is maybe .173. That gives a CA/D ratio of .0511, right at the top range of the chart. It works with the ST 60. It works with the OS 20.

One more thing, The CL venturis supplied by some of the manufacturers on their engines are not properly sized for stunt. So, just because that is the venturi that came with it, doesn't mean it is the best to use. OS started out with a .287 on their 35 and 40 FP engines. They later went to a .282, a .277, even a few others, before settling on the present .265. The venturis supplied by Thunder Tiger on their engines were generally too large for stunt all the way around. The stock venturi on the 20 and 25 FP was too large. A new venturi can sometimes make a whale of a difference in how the engine will run. The numbers that were published by Pat Johnston (which I gave you above) are quite accurate for most of our applications.