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Triple ZS carb, balance tube and V8 technical questions

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  • Triple ZS carb, balance tube and V8 technical questions

    I have a question for the 6-pack community. Because I love the idea of increasing my chances of a stuck needle valve by 50%, I am adding a triple ZS carb set up onto my car. And as I typically do, I have gone down the rabbit hole about carbs, intakes, induction and the overall how an engine works. This leads me to this conundrum that is keeping me awake at night.

    I know that our cars need a balance tube on the intake to even out the intake charges coming from the carbs to each cylinder. Something about inline 6 cylinders and different cylinders fighting each other for the intake charges. Apparently without the balance tube some cylinders would be running rich and some would be lean if I understand it. I don't fully understand it but I can live with it.

    So why don't V8 engines with 3 dual carbs on them need balance tubes? Or even dual 4 barrel carb set ups?
    1972 Sapphire TR6 #CC84,something

    1959 Red TR3 (Wife's)

  • #2
    A V8 with three carbs runs on only the center carb most of the time. When the outer two are called for, they feed into the same set of runners the center one was using, so they are effectively interconnected.
    The same is true of a dual four barrel setup. On an inline six, each pair of cylinders are connected to one carburetor, so the three carbs need balance tubes.
    I hope this simplistic answer helps!
    Dennis

    Comment


    • #3
      Originally posted by Dens6 View Post
      A V8 with three carbs runs on only the center carb most of the time. When the outer two are called for, they feed into the same set of runners the center one was using, so they are effectively interconnected.
      The same is true of a dual four barrel setup. On an inline six, each pair of cylinders are connected to one carburetor, so the three carbs need balance tubes.
      I hope this simplistic answer helps!
      Dennis
      Hey thanks fort he reply.

      The V8 part I understand, but the inline 6 not so much.

      If one carb feeds two cylinders and those cylinders are not sucking at the same time, and they are isolated from the other two carbs........What needs balanced? I guess that is the question.
      1972 Sapphire TR6 #CC84,something

      1959 Red TR3 (Wife's)

      Comment


      • #4
        Balance tubes are there for a few reasons - without getting deeply into the theory, when a valve opens in a cylinder, the fuel-air mixture is forced (by atmospheric pressure, in our engines) down the intake manifold, past the open intake valve, into the cylinder. When the valve closes, there is still a pressure wave of air/fuel heading down the runner, but now it meets a dead end. Hence the charge tends to bounce back up the manifold (known as reversion). This effect can also be compounded especially with long-duration cams, when due to early opening of the intake valve, cylinder pressure momentarily is higher than atmospheric, causing exhaust gases to escape out of the intake valve back into the manifold

        A balance tube tends to dampen the effects, by giving the pressure waves somewhere to go. That's one reason. As previously stated, most V8s have a common plenum chamber with runners leading off it, so it handles the reversion without requirement for a tube.

        Another important function of the balance tube on the TR6 is to aggregate the vacuum from all 6 cylinders - thereby providing enough vacuum for the brake booster. As a practical example of this - some manifolds (such as the TVM design used for Weber DCOE carburetors) have no balance tube, and only has a vacuum port on one of the intake runners, meaning that brake booster vacuum is sourced from a single cylinder. They typically don't generate enough vacuum for satisfactory operation of the brake booster.
        Keith, Huntsville AL, 1971 CC66559U
        10.0:1 CR gasflowed head | Weber DCOEs | CP "150hp" Cam | Distibutor by Advanced | Lightened flywheel | Phoenix SS Exhaust System | HVDA 5-Speed | Good Parts suspension and anti-roll bars | Konig Rewinds | Boyd 15 gal tank | Miata Seats and Mr Mikes covers | Carl Visser dash | Mohair hood | Gas-strut bonnet and boot lift kits

        Comment


        • #5
          Do we already have a balance tube-- where is it?? Do we/ should we add them??

          Comment


          • #6
            Aaron,

            Have you done anything to the lump to increase the need for CFM (more gas/air) that 2 ZS’ can handle? Otheriwse it’s just for looks and the aggravation of keeping 3 synced and running well.
            I72 Pimento w/overdrive

            Comment


            • #7
              GP2 and 9.5:1 compression, Lou.
              1972 Sapphire TR6 #CC84,something

              1959 Red TR3 (Wife's)

              Comment


              • #8
                Keith, your explanation is perfect. Thank you.

                But if we had one carb, we would not have the problem, correct? Or I should say, if we had one carb, we would have a plenum and that would take care of the problem. And if we had a plenum with multicarbs, we would not need the tube. Correct?
                Last edited by skootch13; 02-09-2019, 05:59 PM.
                1972 Sapphire TR6 #CC84,something

                1959 Red TR3 (Wife's)

                Comment


                • #9
                  Originally posted by Greenmudshark View Post
                  Do we already have a balance tube-- where is it?? Do we/ should we add them??
                  Yes, the tube running perpendicular to the intake runners.
                  1972 Sapphire TR6 #CC84,something

                  1959 Red TR3 (Wife's)

                  Comment


                  • #10
                    Originally posted by skootch13 View Post
                    GP2 and 9.5:1 compression, Lou.
                    What are you hoping that third ZS is going to do? The two ZS provide 480 CFM to an engine and the stock lump needs 140, so say you’ve increased it to 180 with the cam and compression, 2 is still more than double the CFM needed.


                    I72 Pimento w/overdrive

                    Comment


                    • #11
                      lotta bling
                      1972 Sapphire TR6 #CC84,something

                      1959 Red TR3 (Wife's)

                      Comment


                      • #12
                        Originally posted by skootch13 View Post
                        lotta bling
                        An honest answer . As they said in the IMF... “good luck Mr. Phelps....”
                        Last edited by lfmTR4; 02-10-2019, 01:41 PM.
                        I72 Pimento w/overdrive

                        Comment


                        • #13
                          Years of watching this forum ( the UK forum scarcely ever sees triple Zs discussed, and dissed is more apt ) the jury is still out for me whether a 3rd Stromberg does any good. If I had to guess I'd say " Maybe a little..."

                          The GP2 and 9.5:1 c/r with triple DCOEs, OTOH, will bring it into CP territory performance wise ( that's early P.I. for the uninitiated ). I got roped into one tuning exercise with that cam and got it going to the satisfaction of the owner ( who later got $30K for the car ) and it didn't even have a c/r increase.

                          Tom

                          Comment


                          • #14
                            On early TR6's (69-71) which have the stubby intake manifold (replaced by a far better design on 72+ models), I will tell you from personal experience, the triple ZS kit makes a noticeable improvement - not so much from the 3rd carburetor, but from the triple manifold design. Cylinders 2 and 5 on the older manifold were known to be somewhat restricted, especially as the revs increase. The triple manifold apparently improves that situation, so those cylinders pull their weight. I doubt there is much if any performance improvement on cars with the later intake manifold.

                            As a bonus, I experienced a small improvement in mpg following their fitment. This has been reported by others as well.


                            Keith, Huntsville AL, 1971 CC66559U
                            10.0:1 CR gasflowed head | Weber DCOEs | CP "150hp" Cam | Distibutor by Advanced | Lightened flywheel | Phoenix SS Exhaust System | HVDA 5-Speed | Good Parts suspension and anti-roll bars | Konig Rewinds | Boyd 15 gal tank | Miata Seats and Mr Mikes covers | Carl Visser dash | Mohair hood | Gas-strut bonnet and boot lift kits

                            Comment


                            • #15
                              Originally posted by lfmTR4 View Post

                              An honest answer . As they said in the IMF... “good luck Mr. Phelps....”
                              IMF?

                              At TRials last year I really liked the look of the Triple Zs. I'm putting on a GoodParts header while I'm in there. And still while I'm in there, I'm going to yank the drive shaft and get it balanced.

                              In the words of Jeremy Clarkson, "MORE POWER!"
                              1972 Sapphire TR6 #CC84,something

                              1959 Red TR3 (Wife's)

                              Comment

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