Volocity stack design considerations??, for my mickey mouse ITB's Options

[Mueller]

Slightly warmed up /4 motor applications...I'm thinking just about any design better than none at all....I'd like to metal spin these myself, however if I can get a good price on 8 from a fellow non-club member, I'll go that route (IMG:http://www.914world.com/bbs2/html/emoticons/smash.gif)

[jsteele22]

I don't know anything about the design of velocity stacks, but that ain't gonna stop me from spouting off....

I think Twystd1 is right : the place where stack design matters most is for carbs, 'cause the detailed behavior of the air flow determines how much fuel you get from the venturi, and also how well the fuel/air get mixed. Since F/I handles these two issues very reliably, you mainly want to keep the flow from going turbulent : laminar flow (where air flows smoothly in layers parallel to the tube walls) will get you more air, so more power. My guess is that using *any* sane velocity stack profile will get you like 80% of the way there, and the only thing to get you to the ideal is gonna be trial and error, and/or massive amounts of computer modelling.

If there is no velocity stack, only an abrupt open tube end, then when a "vacuum pulse" reaches the top end of the pipe, it's gonna want to draw air in from every direction. Air directly above the pipe will flow right in, but air that is directly to the side of the pipe is gonna have to make an infinitely sharp 90 degree turn if it's gonna follow the tube wall. Since it can't, it's gonna understeer and separate from the wall, giving rise to turbulence.

Adding a bell shape does (at least) two things. It gives the air thats out to the sides of the stack a nice gentle curve to follow as it goes from flowing sideways to flowing downward. This in itself reduces the likelihood of turbulence. But the area of the tube opening is also made quite a bit larger. Twice the area means that the air is traveling half as fast as it enters. Then add to this the fact that air travelling along the tube walls is gonna travel much more slowly, due to friction, than the air flowing down the center. So the outer part of the flare is like a landing pad/slow lane for getting the most problematic air flowing in a nice oderly way side by side with the bulk of the air which is coming from the more vertical directions. As for the rounded lip issue, yeah its got to help some, in theory, but I think that by the time you get out to the lip the air velocity along the bell surface is pretty small, so it's really gonna be a hair-splitting kind of issue. If you were going for massive HP (i.e. massive air flow) then it might be worth doing. But for this project, I can't see it showing up on the radar.

All of the above (besides being pure speculation on my part) only describes the steady-state behavior. In the intake, you're really looking more at pulses of negative/positive pressure superimposed on top of this. I've heard, and it makes some sense, that another thing velocity stacks accomplish is to broaden the torque peak.