What are Velocity Stacks?

[jeffro ra28]

ram tubes is also another name for them

[takai]

The best flow is from a flat plate blending into the runner (see link).
http://img135.imageshack.us/img135/2...erpics24oa.png

This is actually quite debatable. But more in real world application rather than modelling. When you have a plate blended into the end of the mouth you can get a higher flow than say if the radiused mouth was suspended above the plate like so:
http://www.billzilla.org/inman.jpg (thanks Bill for the pic)

However, in the real world, and especially on NA engines where the maximum amount of air needed by the engine doesnt max out the flow rate of the throttle/runner assembly the protruding style seems to be more effective as it promotes a higher airspeed through the center due to the reduction of the width of the eddies on the edge of the runner. This is quite important in NA engine design.
But for FI where you are effectively ramming more air into the throttle than it can theoretically handle, the absolute maximum flow rate does play a part and therefore having the stacks blended into the runners serves as an advantage.

Ill try and post more after i go out for my 9k walk this morning

[thomasbl]

Some designs are based on the concept of increasing laminar flow into the inlets as well as speeding up the flow. The spcific design, or tuning, of the velocity stack can aim to reduce turbulent air flow into the inlet. The basic concept of them is quite straight forward, however in practice, there are a lot of variables to consider which will cause the results to differ from that which is modelled.

[Toysrme]

Hey Jet.
When used inside a plenum chamber; it's generally the simplest, mostly-consistent way to keep an even mixture across the chamber. While there is some velocity increase from laminar flow. It's a very minute change. Does practically nothing. Sounds good, looks good, adds length so that the intake resonance can be tuned (Just another way of making it longer).
In general, velocity stacks fall in with your typical *working* ram-air intakes. Yes, there is some measure of positive effect, but it is a near un-measurable affect on the engine itself.

If you ever wonder why Toyota has such funky castings into the v6 intake runners. It's not because "That was the best, cheapest" way to do it. It's simply how they came up with repeatable flow across all the runners.



If your new intake manifold has a plenum chamber. Be sure the full chamber extends beyond the runner<s> farthest from the throttlebody. Also offset the throttlebody<s> at a 30-35* angle from the chamber, to facilitate a more even mixing.

[ed]

bellmouths also have better loss co-efficients to reduce turbulance and subsequently head losses on pipe interfaces improving flow efficiency

http://www.v-eight.com/tech_forum/viewtopic.php?t=23

[JetspeedCamry]

Wow! Toys did you cut your plenum in two!?? Your as daring as Tony!

Hmm..by looking at that 3VZ plenum, it seems the internals are a bit more complicated than just runners going into chamber. I might have to get myself a spare one and cut it open to see what it all looks like. The first thing that came to mind about creating one of this myself, is that if the intake chamber was a simple box, and 6 individual runners exited from it...what was to stop air flowing down No.1 intake runner more than No.6...there has to be some sort of guide for equal flow. Hence my idea about velocity stacks! haha.

[takai]

The first thing that came to mind about creating one of this myself, is that if the intake chamber was a simple box, and 6 individual runners exited from it...what was to stop air flowing down No.1 intake runner more than No.6...there has to be some sort of guide for equal flow. Hence my idea about velocity stacks! haha.

Assuming that you are forcing air into the engine (i.e. FI) then there is nothing that stops more air going into cyl1 than cyl6, as air, like other liquids, will follow the path of least resistance.
However, this is vaguely countered under NA conditions as rather than forcing air in you are sucking it down. So if all cylinders are sucking air in theoretically there woudl be no more air going into cyl1 than cyl6. This isnt always the case though, and has a lot to do with how large the plenum is. Which again commonly results in the "last" cylinder running slightly rich. i.e. cyl4 on a 4A or cyls1 and 6 on a 7M (central intake).
Theoretically the plenum should be at least two or three times the amount of air that the engine is able to take in on any one intake stroke. This will assist in equalising the flow to each cylinder. Also remember that you have to have an intake to the plenum large enough that it can "fill" the plenum again within one or two strokes of the engine.

For example i have an almost 4 litre plenum for the race car, and a 3" inlet to feed it, and it is being sucked through from 4x45mm throttles, with 70mm bellmouths. That is about right for a 4AGE, if not a bit big. Commonly on road cars they dont worry as much about the size of the plenum as simply packaging it nicely, as having an engine run rich under certain conditions is not as bad as having it run lean. For V engines they commonly use the runner length to influence the intake air speed and therefore how much air gets into the engine.

Anwyay im rambling again.

[JetspeedCamry]

I didn't think of it that way....with NA engines, the air is being sucked in by each piston on its intake stroke, and thus...theoretically each gets equal amounts of air.

[Toysrme]

Jetspeed what are your goals & how are you going to go about making them?
Be the first person to sling a turbo off that 2vz-fe front bank. Come on... You can do it. You'll never find an easier, or cheaper way to make 200-220bhp without a nice money investment in parts & management of various kinds.

You planning N/A, or boost?