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Intake Runner Air Velocity Question

  1. Jan 3, 2012 #1
    looking at the 2 intake ports in head and
    i'm looking at 2 bbl intake runners.
    i would say if i blew foam into runner and
    took it out the cross section would be
    smaller than pair of ports at the cylinder head.
    questions: does this speed up the air?
    if i keep the runner 'full size' up to the
    carb base plate will it be a 'dog'?
    do i want a smaller runner?

    have a nice day!
  2. jcsd
  3. Jan 3, 2012 #2
    Tough question without seeing the ports; do you have casting numbers of the intake and heads or at least the application that they came from? What's your planned usage?

    Typically 2 bbl intakes are about economy of production and little else, so I wouldn't use what you see as any indication. That being said, a smaller runner will give a higher velocity for a given volume of air passing through.

    Whether that engine will benefit more from higher velocity than an increase in flow from a larger cross section is the question. As usual it's a package deal.
  4. Jan 3, 2012 #3
    that's a good take on the situation
    'Whether that engine will benefit more from higher velocity than an increase in flow from a larger cross section is the question.'
    their small valve heads and rest of engine stock.
    'a torque engine'. so full size runners would probably work against
    the way it was designed.
    i think save full size runners for another project.
    thanks for smacking me in the face.:smile:
  5. Jan 3, 2012 #4
    Any time.:smile:

    If you want to practice your porting skills you can do a quick bowl clean-up and blend any obvious casting issues; the more air going through the motor at any given rpm, the more torque it will make so your time won't be wasted.
  6. Jan 3, 2012 #5
    does that statement point back to 'full size' intake runners.

    'the more air going through the motor at any given rpm, the more torque it will make'

    intuitively i say yes and yes to 'stacks' exhaust ( no restriction at all )
    on the intake stroke each cylinder is a 'pump' and
    i think there is plenty of 'draw' there.

    Have A Nice Day!
  7. Jan 3, 2012 #6
    Generally speaking an engine will respond better to a small efficient port than a larger "normal" port with the same flow, so smoothing and blending without hogging is the best path to power. Usually the runner sizes are not too far off what would be appropriate but the execution can leave a lot to be desired, especially where the runners join the plenum.

    http://www.castheads.com/view.php [Broken]

    On the exhaust, a header or manifold that has long tubes of equal length that flow smoothly into a collector before having to push against atmospheric pressure works better than stacks.

    There are "restrictor" racing classes that require 2 bbl intakes and carbs, and the engines can develop a surprising amount of power; what are your plans for this engine?
    Last edited by a moderator: May 5, 2017
  8. Jan 4, 2012 #7
    cfm is in minutes
    rpm is in minutes
    it takes 2 rpms to fill each cylinder
    on intake stroke
    5000/2 rpm = 2500 rpm
    292/4 = 73 cu in
    want to use 350 cfm carbs
    probably one per pair of cylinders/intake ports

    now i'm stuck:smile:

    got unstuck a little.how many cubic inches in a cubic foot? 144? no that would be square inches.
    1 cubic foot = 1728 cubic inches-wikipedia
    Last edited: Jan 4, 2012
  9. Jan 4, 2012 #8
    It'd be a lot easier if you just tell me what you have for an engine and what you're wanting to do with it; chances are pretty good that I've built something close to it already and can give you specific recommendations.:smile:
  10. Jan 4, 2012 #9
    'the easy road gets harder and the hard road gets easier':smile:
    the formula you see around is:
    CFM = (RPM x CID) / 3456.
    they just leave out the 2 revolutions to complete
    four cycles.
    answer is: 5000 x 73 /3456 = 105.61342 CFM
    so a 350 CFM carb on each pair of intake ports
    is a little too much carb.
    more like one 350 CFM carb per pair of pairs.:smile:
    you know the displacement here's another hint
    intake ports are 'stacked' one over the other.

    Have A Nice Day!
  11. Jan 4, 2012 #10
    Like I said, chances are good that I've done one, even an old brute like a Y-block Ford 292 (no I didn't peek). I might still have the dyno results from the 312 I built for a guy's '57 T-Bird about a decade ago.
  12. Jan 5, 2012 #11
    my contribution to the little following of this engine
    is to come up with intake setup.
    the clean way to build this is four independent manifolds.
    MAYBE build each side onto a bolt down plate later.
    left head & right head intake ports are not straight across from each other.
    anyhow i'm going to build one manifold for pair of ports and
    that's it for me.
    i'll let the enthusiast's cast up set's of four somewhere.
    i sure hope there's 1 bbl 150-200 CFM out there that's reasonable.

    Have A Nice Day!
  13. Jan 5, 2012 #12
    Individual runner manifolds (IR intakes) have a different carb requirement than a plenum style intake.

    With a plenum style manifold (virtually all domestic intakes) the carb has a more or less continual draw into the plenum, which acts as a buffer or reservoir of air/fuel mixture. With IR, the carb has to be able to supply a cylinder worth of air during the intake stroke without unduly restricting the flow. That means that the usual rule of thumb for carb sizing is way too small.

    In short, four 350 cfm carbs will actually be pretty close to what the engine will need for your planned intake arrangement. I'll do some checking and comparisons to the IR equipped engines I've built. In the meantime, have a look at the cfm ratings of side-draft webers on IR systems for 2 to 2.5 litre engines; that'll be pretty close to what you need.
  14. Jan 5, 2012 #13
    good.did find Autolite 1100 in smaller CFM.
    have 2100 2 BBL already so that's a real good thing
    for mocking up this intake.
    the situation is pair of intakes are basically 3" x 3" and
    the carb pad is 4" x 6" .
    i'm up for just putting rectangular box on top
    of 3 x 3 runner.
    i do want to keep runner 3 x 3 all the way.
    NO bologna slicing it because the cross section area will
    get all messed up.
    so it will need to have a CURVED runner.

    Have A Nice Day!
  15. Jan 5, 2012 #14


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    Gold Member

    Do you want max intake velocity? Then do not polish the intakes. You need a rough enough surface to provide a small layer of microturbulence so that the mass of the flow zips through as easily as possible. Taught to me by a very successful drag-racer over 25 years ago. You can hog out the intakes and enlarge them, but resist the temptation to make the interiors really smooth. That will result in laminar flow at the air/fuel-intake boundary and cause drag. Just a thought.
  16. Jan 7, 2012 #15
    thank you for thinking about airflow in cylinder heads.
    having a problem with bend in manifold.
    the heads are at generally a 45 degree angle.
    the carb obviously sit level.
    if i keep the cross section equal the length of the manifold.
    then put a 45 degree sweep in it.
    i think the air/fuel mixture on inside of the curve is
    getting there first.it has less distance to travel.
    any solution to this?

    Have A Nice Day!
  17. Jan 8, 2012 #16
    No solution needed; for what you're doing there won't be a problem.

    If you make a small plenum under the carb that blends into the intake runners you won't need to have a curved runner if I'm remembering how the top of the intakes look. For carbs I'd check the Rochester 2G or even the Weber 32/36 (primary and secondary throats for better street use) unless you need to use a particular carb. If you want to do a Stromberg look-a-like but with EFI you could use single throat throttle bodies from the Chrysler Intrepid and hide the injectors. Just a thought.

    Turbo, hogging/enlarging the ports is what drops the intake velocity; surface finish will have a very minor effect on flow and the rough surface is mostly to get the raw fuel off the walls and back into suspension.
    Last edited: Jan 8, 2012
  18. Jan 9, 2012 #17
    1 i mean like an oval track field.you know they put the
    start and finish at different places for each lane.
    keeping the runner 3 x 3 and curving it.the inside of
    the curve is less distance than the outside.
    but i'll pass that up for now.
    2 i just had an evil thought.:smile:
    i've been paying attention to Autolite 2100 bolt pattern
    and carb pad which i thought would be @4 x 6.
    i'm going to go measure the two VENTURI'S .
    maybe they'll fit right on 3 x 3 runner.
    3 you said runners, you know this is individual manifold for
    each pair of intakes, right?

    Have A Nice Day!
  19. Jan 9, 2012 #18
    So it sounds like you're having a mini-plenum serving each pair of intake ports, right? That reduces the size of carb needed and more or less eliminates the need for a curved runner; just make sure there are no abrupt turns or edges in the air path.
  20. Jan 9, 2012 #19
    here's the venturi size/CFM chart for Autolite 2100
    which CFM size for each manifold?
    formula for 2 cylinders 73 cubic inches says 105 CFM.
    then Holley says go up 50%.
    but holleys talking about 4 bbl on top of regular manifold.

    Have A Nice Day!
  21. Jan 9, 2012 #20
    So it sounds like you're having a mini-plenum serving each pair of intake ports, right?

    If so, pick the cheapest carb that flows between 245 and 351.

    Also, there is quite a difference in how a 2 bbl and a 4 bbl are rated; 2 bbl carbs at measured at a higher pressure differential so their numbers are quite a bit higher for the same size venturi. Translation: you can't size a 2 bbl carb using the 4 bbl formula.
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