Manifold Pressure Equalization Tube

Discussion in 'Volkswagen' started by N804RV, Jul 10, 2019.

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  1. Jul 10, 2019 #1

    N804RV

    N804RV

    N804RV

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    Anyone here running an equalization tube between the intake manifolds on a dual-port VW engines. The guy that built my engine originally had it installed in his Quickie. He says this made his engine run as smooth as a sewing maching and improved his fuel economy.

    As you can see in the bottom photo, my cowling is quite a bit more snug over the top of the engine that the cowling on the Quickie (top photo). I'm thinking about using flexible tubing for this. Wondering if any of you have something similar.

    [​IMG]

    [​IMG]
     
    Last edited: Jul 10, 2019
  2. Jul 11, 2019 #2

    BBerson

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    The Limbach (VW based) has a 1/2 " black rubber hose.
     
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  3. Jul 11, 2019 #3

    TFF

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    Within reason, bigger the better. I would say 1/3 of the intake tube diameter would be great.
     
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  4. Jul 11, 2019 #4

    Pops

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    I have thought about trying it but never did. After I installed the hot oil box the engine couldn't run any smoother and also had more power.

    I usually relocate the oil dip stick to the rear of the engine case. Lot easier to use instead through the nose bowl air opening.
     
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  5. Jul 11, 2019 #5

    Tuneturkey

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    Has anyone tried an equalization tube on the intakes for a 1/2VW "better Half" engine, with dual Mikuni carbs?
    Balancing the two carbs (not at that point yet), one on each cylinder is difficult, so I've heard.
    What is the down side of the equalization tube?
    Johnc
     
  6. Jul 11, 2019 #6

    TFF

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    With two carbs on a 1/2, it will not do anything. The balance tube works because a cylinder can steal mixture and flow from the opposite bank, it’s like an accumulator. If you have a carb per cylinder, the cylinders can feed as much as they want, where they can’t if they have to share. Nothing looks prettier that a long bank of Webers standing tall in a row.
     
  7. Jul 11, 2019 #7

    Tuneturkey

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    TFF,
    Sorry, but I'm not following you.
    Problem I am facing is I have 32mm VC carbs which should be 28's, per Leonard Milholland. Current advise is that they are too large and draw too much air and not enough gas when the throttle is advanced too fast. Since the engine is a 4 cylcle, and the intake valves are not open at the same time, IMHO, with the balance tube, fuel mixture could come from both carbs, the main plus some through the balance tube.
    Am I all wet?
     
  8. Jul 11, 2019 #8

    fly2kads

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    N804RV, I think that is the first time I have seen a balance tube on a VW aside from the Limbachs that BBerson mentioned. It's not commonly used on these engines, that's for sure! If the prior owner says it helps, keep it, I suppose. No reason you can't use a flexible hose like the Limbachs do.


    Tuneturkey, the typical problem with carbs that are too large is that the airflow through them is too slow to provide a strong enough vacuum signal for the carb to operate properly. The carb needs a certain level of vacuum in its internal ports in order to meter the fuel properly. When the carb is too large, the engine pulls its air through the carb at a lower velocity, which results in a low vacuum through the venturi. The carb therefore can't provide enough fuel to match the air mass. That's not something you can solve with a balance tube. The balance tube would draw the same lean mixture from the other side, not provide the needed additional fuel. A small balance tube *might* be able to make it a little easier to synchronize the carbs, but it won't fix a mixture that is too lean to begin with.
     
  9. Jul 11, 2019 #9

    TFF

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    A balance tube is not going to do anything for you. The big carbs are just going to have to be tuned immaculately. A balance tube would probably hurt.
     
  10. Jul 11, 2019 #10

    Hot Wings

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    VW used a balance tube on the dual carb TP III (square/fastbacks. This helped with the 'charge robbing' due to the unequal intake pattern. When they adopted the dual port heads and EFI this was no longer needed.

    I'm not sure why it seems to have worked on this particular application but it could be because of the curved intake manifolds used. It has been reported that the mixture distribution on aircraft with dual port heads is more even with the "T" shaped intake manifolds when used with carburetors.

    [​IMG]

    It could be that this tube, which appears to link the leading intake port on both sides, may help with preventing charge drop out, which is different than charge robbing, the same as the "T" shaped intake.
     
  11. Jul 12, 2019 at 1:30 AM #11

    Dan Thomas

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    FWIW, the Continental 470, 520 and 550 engines have a balance tube between the two manifolds, whether the engine is carbed (single carb) or fuel injected. It's a big tube, seen at the lower front of the engine:

    [​IMG]

    It's only used in the bottom-manifold engines, where the air inlet is at the back, making for long induction runs and some uneven air delivery due to drag. The models that use top-induction don't have it:

    [​IMG]
     
  12. Jul 12, 2019 at 4:24 AM #12

    Rockiedog2

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    Leonard knows
    32 is too big 28 is the right size. In my experience on a Casler 45. And that’s the Mikuni carbs, only ones I know anything about.
     
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  13. Jul 12, 2019 at 10:55 PM #13

    Tuneturkey

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    Ok! You convinced me. 28's it it!
     
  14. Jul 12, 2019 at 11:51 PM #14

    Pops

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    To large is worse than to small.
     
  15. Jul 13, 2019 at 12:58 AM #15

    Rockiedog2

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    Mikuni has a chart MM throat/engine size. Believe it said the 45 was right on the line between 28 and 26.
    Pops knows this for sure one way or the other...if the carb is too big the motor doesn't have enuf suction to use all the throat and the last some part of the travel doesn't do anything. So not optimum. That right Pops?
    There are 2 different carbs...one is aluminum or like that and the other is heavier pot metal or something like that. The light ones are obviously the ones most folks want

    edit: I see fly2kids has already said what I did but more scientific. Go with his
     
    Last edited: Jul 13, 2019 at 1:11 AM
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  16. Jul 13, 2019 at 1:42 AM #16

    Pops

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    Great Plains list Carb Model 1617 for a 1600cc to 1700 cc engine. Also they list Carb Model 1819 for a 1835 to 1915 cc engine.
    I used a model 1617 for my 1835 engine with single post heads for a higher velocity charge in the intakes for max packing of the charge in the cylinders for max power.
    There is a down side. That creates a lower pressure in the carb throat that lowers the temp for easier carb ice. Also lowers the temp of the intake charge and the charge was condensing into droplets in the long run of the intake tubes before getting to the carb with the carb running lean and then rich when all the droplets went into the cylinders at the same time. Cure for that was the carb heat box that also lowered the oil temp between inlet and outlet 20 degs. Before the carb heat box the engine ran rough and after the heat box it ran smooth and with more power and stopped the carb ice problem I was having.
    In the end I believe that the engine puts out more smooth power than if I was using the Model 1819 Carb.
    Used 1 1/4" dia intake for the single port heads. For a mini-supercharger effect.
    Most people use 1 1/2" dia. intake tubes with dual port heads with a slower intake charge velocity and less torque on a straight drive engine below 3K rpm.
     
    Last edited: Jul 13, 2019 at 1:49 AM
  17. Jul 13, 2019 at 1:46 AM #17

    Pops

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    Right.
     
  18. Jul 13, 2019 at 7:29 PM #18

    rv7charlie

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    RE the Continental crossover tube: If failing memory serves, there's good odds they did it to keep the *aft* cylinders from being starved, with those log style manifolds. The 'dead end' end of the T extending past the front port on the VW 2 port head pictured earlier is for the same thing. If a tapped tube 'dead ends' at the last tap, the last tap gets most of the air. If you look at the tapped air duct of most home central HVAC systems, you'll see the same look: the 'tube' extends past the last vent tap by at least the duct diameter, rather than turning directly into the last vent.

    The 2nd Continental pic with the top mounted manifold has a plenum feeding equal length runners, so (hoepfully) no flow imbalance.
     
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  19. Jul 13, 2019 at 11:37 PM #19

    Pops

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    I extended past the tap the dia of the tubing. I have the front cylinder very slightly richer that the rears by about 20 degrees and a very slight difference color in the spark plugs. Not enough to do anything about.
    You need the inside of the tube a smooth as possible with no large welding humps to disturb the flow of the charge. Used a dremel tool to make everything as smooth as possible on the inside welds and purged the inside of the tube with argon doing the SS welds.
     

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