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Thread: What are Velocity Stacks?

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    Altia ER34 GTT Domestic Engineer JetspeedCamry's Avatar
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    Default What are Velocity Stacks?

    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.
    Kind Regards,
    Kurt.

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    Junior Member Backyard Mechanic Dimitri's Avatar
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    Default Re: What are Velocity Stacks?

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

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    Estranged Member Chief Engine Builder mullett's Avatar
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    Default Re: What are Velocity Stacks?

    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.

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    Altia ER34 GTT Domestic Engineer JetspeedCamry's Avatar
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    Default Re: What are Velocity Stacks?

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



    ^ From a Toyota 20R Intake Plenum.
    Kind Regards,
    Kurt.

    1998 ER34 ニッサン スカイラインGT- T
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    Junior Member Too Much Toyota oldcorollas's Avatar
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    Default Re: What are Velocity Stacks?

    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).
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    Junior Member Too Much Toyota oldcorollas's Avatar
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    Default Re: What are Velocity Stacks?

    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/
    Last edited by oldcorollas; 18-03-2006 at 07:46 PM.
    "I'm a Teaspoon, not a mechanic"
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    Junior Member Too Much Toyota oldcorollas's Avatar
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    Default Re: What are Velocity Stacks?

    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)
    Last edited by oldcorollas; 17-03-2006 at 10:09 PM.
    "I'm a Teaspoon, not a mechanic"
    "There is hardly anything in the world that a man can not make a little worse and sell a little cheaper" - John Ruskin (1819 - 1900)

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    Junior Member Domestic Engineer urantia's Avatar
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    Default Re: What are Velocity Stacks?

    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.
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    Altia ER34 GTT Domestic Engineer JetspeedCamry's Avatar
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    Default Re: What are Velocity Stacks?

    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"?
    Kind Regards,
    Kurt.

    1998 ER34 ニッサン スカイラインGT- T
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    Junior Member Too Much Toyota oldcorollas's Avatar
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    Default Re: What are Velocity Stacks?

    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?
    "I'm a Teaspoon, not a mechanic"
    "There is hardly anything in the world that a man can not make a little worse and sell a little cheaper" - John Ruskin (1819 - 1900)

    AU$TRALIA... come and stay and PAY and PAY!!! The moral high horse of the world!

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    Altia ER34 GTT Domestic Engineer JetspeedCamry's Avatar
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    Default Re: What are Velocity Stacks?

    Perfect. Thanks mate.
    Kind Regards,
    Kurt.

    1998 ER34 ニッサン スカイラインGT- T
    RB25DET 5 Speed Manual | Blitz SE Return Flow FMIC | Greddy Profec II Spec B BC | Apexi N1 Turbo Back Exhaust

  12. #12
    Unbiased Grease Monkey Earlyrolla's Avatar
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    Default Re: What are Velocity Stacks?

    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).

  13. #13
    Altia ER34 GTT Domestic Engineer JetspeedCamry's Avatar
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    Default Re: What are Velocity Stacks?

    ^ Good info there. Thanks man. How does this 0.15 constant come about?
    Kind Regards,
    Kurt.

    1998 ER34 ニッサン スカイラインGT- T
    RB25DET 5 Speed Manual | Blitz SE Return Flow FMIC | Greddy Profec II Spec B BC | Apexi N1 Turbo Back Exhaust

  14. #14
    Unbiased Grease Monkey Earlyrolla's Avatar
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    Default Re: What are Velocity Stacks?

    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'.

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    Altia ER34 GTT Domestic Engineer JetspeedCamry's Avatar
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    Default Re: What are Velocity Stacks?

    Ok dude. Will do. Thanks for your help.
    Kind Regards,
    Kurt.

    1998 ER34 ニッサン スカイラインGT- T
    RB25DET 5 Speed Manual | Blitz SE Return Flow FMIC | Greddy Profec II Spec B BC | Apexi N1 Turbo Back Exhaust

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