Slant Six Forum :: experimental venturi fabbing

The wide mouth version might not be as bad as first appears to common sense...as this is known as an undercamber airfoil and can have extemely good efficiencies but typically at lower velocities...velocity numbers are critical in your choice...

A: would be easier to make (round off the corners of where you change direction)

Have you thought of having a ring inside a ring where the exit of the smaller ring is at the narrow point of the next larger ring??

The discharge goes about .030" downstream of the minor diameter.

I don't know much about aero, thermo or fluid dynamics, but I would think there is a way to model for your desired effects. That said, I don't think any design with squared edges in the shaft would be beneficial.

Popular Hot Rodding just did a big article on booster venturis. Very nicely written by Dave Vizzard. Would be worth checking out. They were saying that something like your C design on the first post gave the biggest amplification for fuel draw (biggest "signal").
You want to minimize restriction (edges tapered all the way to the outside of the booster body at the entrance, and straight outside walls).
Lou

Can't find any pics.......... my old 4bbl mazda rotary engine carb did it on the primary barrells........I believe it helps with breaking up the fuel droplets.

With your last set of pics, pretend B2 is half the diameter shown, then pretend you're using both B2 & B3 at the same time stacked so the the outlet of B2 is in the throat of B3...

A thicker 'wing' section will increase the flow thru the venturi.... good for low rpm, but restrictive at high. The longer venturi would allow a more gradual change, and you can get away with a smaller throat......

Also I think you should round off the leading edge a bit (even with what is effectively a flat bottom), and have the trailing edge be a knife edge

Now here's another idea........... You don't have to the venturis indentical.... have one mainly for low speed idle/cruise and the other for high speed.............. (of course a primary/secondary setup would be best for this, but it might work on a 2bbl setup where both barrels are in operation.)

Plus one more idea.......... I wonder if inducing a swirl on the trailing edge would help with the mixture..................

I just typed in http://popularhotrodding.com/ and they are there...
I would send them an email and ask about getting a copy of the article. Look in "this month" table of contents.
Lou

Author:	argentina-slantsixer [ Sun Oct 02, 2005 6:18 pm ]
Post subject:	experimental venturi fabbing
Hey there. I'm starting to play with annular venturis fabrication. (since they never used them down here and I can't get them I have to make my own) So far I bored a few (a few people, I bored to death. ) and when I was starting to line the inner insert... I started to think well, something like thinking The inner shape of the venturi affects velocity, ain't that true? I'm considering this profiles: also I think that the hole placing has to make some diffrence... (since I'm fabbing them anyway) would you use the black placing or the dark grey one? (in this diagram) The outter shell is original venturi, I grab it in the lathe chuck and drill it, then line the inner walls, then I plan to line the inner insert in delrin.

Author:	emsvitil [ Sun Oct 02, 2005 7:19 pm ]
Post subject:
Take this as my 'hunch'........... I'll call the venturis top to bottom as (named on major shape) A: straight sided B: wing shaped C: rocket nozzle D: velocity stack E: exit point.. Ok, here goes E: exit point should be at the narrowest point in the venturi (black point). The velocity is highest here, and thus the pressure differential would be greatest. D: I think your wide mouth would hinder air flow to the outside of the venturi, and act as a limit to air flow thru the carb. You'd also get turbulent flow on the outside as air splits to the inside and outside of the mouth, which would hinder airflow even more C: You'd get a major air flow seperation where the 'rocket nozzle' shape starts with turbulent flow. The turbulent flow would cut down on air flow thru the carb. C.1: As a primary barrel in a primary/secondary setup, this turbulent flow might actually help mix the air/fuel mixture up in low speed conditions. A & B: Either of these should work. A: would be easier to make (round off the corners of where you change direction) and the narrowest section should be where the fuel exits. B: the closer the profile is to an actual wing section, the better off you are. (narrowest section applies here to) With A & B turbulence flow will start to form somewhere on the divergence. You might want to check wing profiles (model airplanes, NACA,kit planes) for airfoil shapes (look into laminar flow and shapes that work good in rain (fuel droplets)). Then base your profile off one or a combo of those. But as a further hunch with the small scale involved, the shape might just come out to a version of A:........... Have you thought of having a ring inside a ring where the exit of the smaller ring is at the narrow point of the next larger ring??????

Author:	panic [ Sun Oct 02, 2005 7:44 pm ]
Post subject:
The discharge goes about .030" downstream of the minor diameter.

Author:	gearhead [ Sun Oct 02, 2005 8:12 pm ]
Post subject:
I don't have much time since I need sleep and only just caught this post...but you need to start researching airplane airfoils as Panic has eluded...I did not read his post fully cause I really must quit and sleep. That said...I have designed many succesfull model airplanes, and specifically sailplanes is where you will find info on airfoils. Most powered planes use symetrical airfoils that generate zero lift and use angle of attack for flying because the engines have so much power. But sailplane guys, get serious in the science of airfoil design by neccesity, and I am sure a quick search of the net will reveal interesting tidbits on the subject. Basically, you want to understand the center of pressure since that is where you will create the differential between atmospheric pressure in the float bowl, and that which resides at your discharge port at the venturi design. The Qjet uses turbulance from the third boost ring to atomize fuel and boost from the second ring to increase signal. But how much I could only very roughly guess. Obviously...much time and money was spent over the years by various companies to discover what works best. I have experimented with the secondaries on a Qjet to achieve better atomization...and achieved failure. Went back to stock...not to say it couldn't be improved, but my great ideas didn't work at all and created misfires that promptly vanished on removal of mods. So I am none to smart this area... The wide mouth version might not be as bad as first appears to common sense...as this is known as an undercamber airfoil and can have extemely good efficiencies but typically at lower velocities...velocity numbers are critical in your choice... that is all for now Must sleep Karl aka Gearhead

Author:	argentina-slantsixer [ Sun Oct 02, 2005 8:44 pm ]
Post subject:
Hey guys Good thread! we're talking serious $#!+ here... I do have experience in sailplanes. That said I came up with this 2 alternate wich we can call B2 and B3 Quote: The wide mouth version might not be as bad as first appears to common sense...as this is known as an undercamber airfoil and can have extemely good efficiencies but typically at lower velocities...velocity numbers are critical in your choice... I thought so... I think that's a good way to go on a enlargened diffusor (top side specifically) since the velocity will drop... huh? And turbulence may help a little there. If mixture richens due to restricted airflow I can always lean it out... Quote: A: would be easier to make (round off the corners of where you change direction) yep, that would be the easy one. corners would be rounded off since I plan to wet sand (600 then 1200) then highly polish the inside of the venturi. Quote: Have you thought of having a ring inside a ring where the exit of the smaller ring is at the narrow point of the next larger ring?? mmmmhh... not sure I follow... pics or a drawing? Quote: The discharge goes about .030" downstream of the minor diameter. Ok... what about the shapes? let's get it going boys...!!! I'll document all the fabrication process and I'll take a snaps of some trick tools I've been doing for venturi removal and some screw-in holding pieces I've made in the past and worked great. ed and karl, since you guys brought up the plane comparission, what do you think about increasing the "wing section" (like in the B2 and B3 shapes) ? You may want to consider that this would be going on a carb that has been exhaustively polished inside the barrels and the diffusor has been enlargened some... I even deburred, cleaned and polished the venturis that I installed on my present carb.

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Author:	emsvitil [ Mon Oct 03, 2005 12:55 am ]
Post subject:
Can't find any pics.......... my old 4bbl mazda rotary engine carb did it on the primary barrells........ I believe it helps with breaking up the fuel droplets. With your last set of pics, pretend B2 is half the diameter shown, then pretend you're using both B2 & B3 at the same time stacked so the the outlet of B2 is in the throat of B3... A thicker 'wing' section will increase the flow thru the venturi.... good for low rpm, but restrictive at high. The longer venturi would allow a more gradual change, and you can get away with a smaller throat...... Also I think you should round off the leading edge a bit (even with what is effectively a flat bottom), and have the trailing edge be a knife edge Now here's another idea........... You don't have to the venturis indentical.... have one mainly for low speed idle/cruise and the other for high speed.............. (of course a primary/secondary setup would be best for this, but it might work on a 2bbl setup where both barrels are in operation.) Plus one more idea.......... I wonder if inducing a swirl on the trailing edge would help with the mixture..................

Author:	Ice Titan [ Mon Oct 03, 2005 3:40 am ]
Post subject:
I don't know much about aero, thermo or fluid dynamics, but I would think there is a way to model for your desired effects. That said, I don't think any design with squared edges in the shaft would be beneficial.

Author:	Dart270 [ Mon Oct 03, 2005 5:17 am ]
Post subject:
Popular Hot Rodding just did a big article on booster venturis. Very nicely written by Dave Vizzard. Would be worth checking out. They were saying that something like your C design on the first post gave the biggest amplification for fuel draw (biggest "signal"). You want to minimize restriction (edges tapered all the way to the outside of the booster body at the entrance, and straight outside walls). Lou

Author:	argentina-slantsixer [ Mon Oct 03, 2005 7:04 am ]
Post subject:
Quote: I don't know much about aero, thermo or fluid dynamics, but I would think there is a way to model for your desired effects. That said, I don't think any design with squared edges in the shaft would be beneficial. Square edges? it's a view of the thing like if you would have cutted it in half along the longitudinal axis... and the drawings are kinda adlibed on the moment.... don't spank me about edges man

Author:	Dart270 [ Mon Oct 03, 2005 7:22 am ]
Post subject:
I just typed in http://popularhotrodding.com/ and they are there... I would send them an email and ask about getting a copy of the article. Look in "this month" table of contents. Lou

Author:	emsvitil [ Mon Oct 03, 2005 11:00 am ]
Post subject:
Like your last one.............. Here's another variable for you: The height of your venturi in relationship to the existing throat......... While you're fabricating, come up with a way to adjust the height...

Author:	Dart270 [ Mon Oct 03, 2005 11:15 am ]
Post subject:
That was critical, in the PHR article. For max booster effect, you want the top opening of the booster venturi to sit in the center (narrowest point) of the main carb venturi. Lou

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