What are Velocity Stacks?

[JetspeedCamry]

I'm currently working on creating a custom intake plenum for a V6. Researching this through Google and what not, has come up with standard intake plenums (i saw one from a 20R Toyota engine) with internal "velocity stacks". What do these do exactly, and how do they do it? From the name, i'm gathering it speeds up airflow through the plenum, but also looks like a restriction..?, Just wanted some expert knowledge.

Thanks.

[Dimitri]

inlet trumpets. as found on the end of multiple throttle bodies.

[mullett]

hard one, try and google some pics. As far as I understand it, they increase airspeed and swirl the air, by reducing the diam of the inlet (like a venturi), and the bellmouth swirls the air. Just a hint, I don't know much about them.

RM.

[JetspeedCamry]

Hmm. Well this is the picture i got from Google searching.



^ From a Toyota 20R Intake Plenum.

[oldcorollas]

i believe the term came about a long time ago, when long inlet trumpets were used on carbs to give air time to increase in speed before reaching the carb, as opposed to having to start from a stop at the carb entrance..
i remember the term being usually used with ITB style carb setups....

it's basically a stack (liek smoke stack ) that increases air velocity (by giving the air a longer path, and often decreasing in diameter).

[oldcorollas]

from 1964 ATS 2.5L V8...
the shiny bits on top are called velocity stacks, going into ITB type arrangement of carbs, and then into what was often called a "tunnel ram" manifold, ie a tunnel to ram air into engine

http://www.atspeedimages.com/pebble_...s_engine_3.jpg
more sexy pictures here
http://www.atspeedimages.com/pebble_..._ats_2500_gts/

[oldcorollas]

this is the kind of image it usually brings to mind for me



edit, my bad, missed the part where you said "inside plenum"
same reason, give the air a longer time to accelerate, which should give it faster port speed.
they are also used so you can keep the throttle plate closer to the engine, and yet still have a more ideal length for intake tuning (pulses etc)

[urantia]

Ah! i was wondering about them things! when i looked inside a beams inlet manifold, each cylinder had it's own velocity stack, which were all contained in the inlet plenum. Was quite impressed by looks, but didn't know the purpose behind it.

[JetspeedCamry]

Thanks oldcorollas.
It is what i figured then.

So, if the throttle plate was position away from main part of the plenum, like the stock toyota V6 plenums are, there is no need for these stacks because the air already has a long path to travel and "accelerate"?

[oldcorollas]

the stacks are basically just port extensions to allow you to modify the length of the port for intake pulse tuning stuff...
it also stands the actual intake a little off the base of the plenum, which some say is a good thing.

i think they work both ITB and conventional single throttle setups, as long as the total port/runner length is what the engine needs for the rpm you want

just think of it as a port/runner extension, and a convenient way to shape the transition between plenum and runner?

[JetspeedCamry]

Perfect. Thanks mate.

[Earlyrolla]

After doing a bit of research myself I have found some things out that could be helpful to you.

The radius of the bellmouth needs to be greater than or equal to 0.15 times the diameter of the intake runner at the end of the runner where the two join for the best flow co-efficient.

I.e. radius (bell mouth) >= diameter (runner) * 0.15

Depending on the book your read it is either 0.14 or 0.15 so sticking with 0.15 is better.

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

As stated by oldcorollas, the diameter and the length of the runner need to be calculated for tuning the runner. From what I have understood it is best to tune the runner for peak torque of the cam profile you are using.

The velocity stacks on carbs help to promote a laminar flow of air into the carby. A laminar flow has less losses than turbulent flow so in theory the engine should be able to produce more power.

Again from what I understand, individual stacks are only really good for wide-open-throttle as you don't get the same advantage as a Helmholtz resonator (runners to a common plenum).

[JetspeedCamry]

^ Good info there. Thanks man. How does this 0.15 constant come about?

[Earlyrolla]

I am not right into the fluid dynamics so I am definitely not the best person to try to explain the derivation.

Theoretically the 0.15 multiplier will give the flow into the runner a Cd (co-efficient of discharge) of 1. Which is perfect, but in reality things are never perfect

A tube butted up against the plate has a Cd of around 0.6.

The Cd is a way of accounting for the losses associated with the movement of the fluid through an orifice.

Best to do a quick Google on 'coefficient of discharge'.

[JetspeedCamry]

Ok dude. Will do. Thanks for your help.