Author Topic: Venturi Area Conversion Factor (Read 1123 times)

Tom Muggleton · « **on:** December 18, 2023, 08:17:03 PM »

I thought I might pass along some findings to compare venturis with a 4mm spray bar in the airflow (restrictor) and true, unrestricted "sprinkler" type.
I was asked to look at a Tiger .60 and find a true venturi diameter size to start with. Most of us are pretty familiar with our engine venturi sizes that
is the restrictor type but how can you convert that good running size to a true venturi size? Don Repp said you can get area conversions from a link in Stunt Hanger
and I looked it up and found it to be way too complicated for me. So, I fired up my laptop and launched my GibbsCam CNC software and used the CAD portion to
create geometry showing the area in square inches for the two venturi types. The software calculates the areas on either side of the spraybar and then I draw in a
separate window a circle that can be modified by enlarging or reducing with diameter that equals the same area. It is easy to zero in a diameter within a .001 of an inch.
I plugged in some sizes just to see the results. My ST.60 likes a .308" dia. restrictor type and that area equates to .190" dia. true type. Then I added some more samples.
Restrictor True Area in square inches

.320" .205" .0326
.310" .195" .0292
.290" .175" .023
.275" .160
Thats all very fine but calculating all this whenever you need it is sure a pain in the kneecap! Looking at the numbers between the two venturi area types
shows a difference of .115". So, subtracting .115 from a restrictor venturi diameter will give you the true venturi diameter and then adding .115 to a true
venturi diameter will give you the restrictor diameter. So, .115 is the factor to use. This factor works just fine for comparing areas only but doesn't seem
to correlate exactly what the engine ideally needs. I think that a true venturi for its size actually draws fuel much better by the velocity of the air induction.
For example, a Tiger G21-.46 with a stock 4mm sprinkler venturi guzzles 5-6 oz. This factor at least can be used as a ballpark starting point.
I hope this is helpful.
Tom

Tom Muggleton · « **Reply #1 on:** December 30, 2023, 12:24:19 PM »

I am replying to my own post as a retraction to my assertion of a so called "factor" or a "constant" to simplify venturi area conversions.
Evidently, Don Repp spent some time and checked out my post and politely approached me yesterday at the flying field and explained to me
that there cannot be a fixed factor to convert and that my restrictor diameters are not consistent because of this. Last night I fired up
the laptop and did find a "factor" difference when calculating various sizes. So, Don was right! Thank-you, Don. I hope this will end
the confusion I created and is not as helpful as first thought. Case closed.

Moving on. Also, Don gave me a printout of ten examples of conversions ranging from .320" to .25" restrictor diameters with corresponding
true diameters and some differed as much as .011". Not a biggie with the restrictor type but more of a biggie with the true type. .010"
makes a difference in flow with a true type. Checking the computer calc with the pi x r squared did match but Don's restrictor sizes did
not. I also accidently found Bill Lee's post showing the NCRLA's link to a beautiful conversion that is within .001-.002" of the computer
results, so I didn't ask what method Don used. Perhaps typos? The programming software I have used for this venturi stuff was used for the Del Monte
R+D pet food pilot plant at Terminal Island, CA to make experimental Chewlotta dog food bone molds. The design engineers wanted to know
the volume and parting line areas. The software calculates any closed shape geometry and volume accurately.

I also saw Randy Smith's recommended listing of venturi sizes showing his real world experience with stunt engines. Super stuff!
For me I think I will use the NCRLS's link for just standalone area to area conversions for starting point sizes after all. It's a lot easier
than using the computer.
Did you find my posts somewhat amusing? I thought so!
Tom

BillLee · « **Reply #2 on:** December 30, 2023, 04:12:42 PM »

The NCLRA link is https://www.nclra.org/Programs/Venturi.php

Picture below an example for your base venturi .308" by .1575"

Istvan Travnik · « **Reply #3 on:** December 30, 2023, 05:13:20 PM »

This is a good/typical example from geometry, from our childhood, between 15 and 17 years old ages.
That time I took a millimeter-grid sheet paper, and calculated manually some points, using different spraybar diameters as constant parameters.
I connected continously the points and got 4 or 5 nice, parabolic - like lines. The actual areas could be read very precisely from the millimeter-grid. It was some 50-55 years before, and I am sure the sheet is laying still somewhere deep inside of old plans' wardrobe...
Istvan

Paul Smith · « **Reply #4 on:** December 31, 2023, 07:50:21 AM »

I rung out Bill's link and it works very well.

But I tried to use 0" for the spray bar to get a result for an open sprinkler type, but that didn't work. So it's just compute the area of circle for that.

It does accept .00001" for the spray bar and give very nearly the area of a open circle.

Author Topic: Venturi Area Conversion Factor (Read 1123 times)

Tom Muggleton

Venturi Area Conversion Factor

Tom Muggleton

Re: Venturi Area Conversion Factor

BillLee

Re: Venturi Area Conversion Factor

Istvan Travnik

Re: Venturi Area Conversion Factor

Paul Smith

Re: Venturi Area Conversion Factor