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Technical 6x2 log balance tube ?’s

Discussion in 'The Hokey Ass Message Board' started by 51box, May 14, 2019.

  1. 51box
    Joined: Aug 31, 2005
    Posts: 734

    51box
    Member
    from MA

    Looking for some info regarding the balance tubes on my edelbrock 6x2 log setup. Its on a 425ci Nailhead with 97’s straight linkage. What exactly is the theory behind the balance tube sizing? This intake has 3 large I think they are 2” balance tubes where something like a Offy only had 2 small ones.

    I’m being told that reducing the opening size from log to log will help tremendously with low end torque which is all these Nailhead are about. I currently have the intake off to add a PCV and gauge port so it wouldn’t be hard to restrict them at this point in time.

    The engine runs good but if this will help get more torque out of it I would give it a try.
    Thanks for any info, Joe
     
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  2. The theory is that you need to maximize the air/fuel mixture's velocity inside the intake (especially at the ports) at low RPMs to maximize torque (and horsepower). The high velocity promotes good mixing, swirl and maximizes the amount of air drawn into each cylinder. This can be done by reducing the internal volume of the intake by making the logs smaller.

    However, like everything in life there is a trade off. There is now less volume to pull from and performance on the top end at higher RPMs suffers as the engine is unable to efficiently fill the cylinders.
     
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  3. 51box
    Joined: Aug 31, 2005
    Posts: 734

    51box
    Member
    from MA

    @FrozenMerc Thanks for the info, so where this engine is maxed out around 5,500rpm anyway would restricting them give me a significant gain in torque like 20+ ft lbs or are we talking a negligible gain?
     
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  4. I can't begin to say. The only way you would know for sure is to run it back to back on a dyno and compare curves. Lot's of other factors to consider.
     
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  5. Rusty O'Toole
    Joined: Sep 17, 2006
    Posts: 8,947

    Rusty O'Toole
    Member

    The main reason for balance tubes is to smooth out the idle by equalizing the vacuum on the carburetors. I don't think the size of the balance tubes is critical.

    Back in the day 6 2's was considered a race only manifold, with an idle speed over 1000 RPM, max full throttle HP and who cares in between. A few guys tried to run them on the street but getting decent driveability took some real dedication and a lot of experiments and tuning.
     
  6. bchctybob
    Joined: Sep 18, 2011
    Posts: 1,555

    bchctybob
    ALLIANCE MEMBER

    51box, Care to share your set up with us? Pictures? How usable is it on the street in regards to idle, traffic driving, highway cruising, etc? I'm asking because I have a Weiand Drag Star 6x2 set up that I would love to use on my 345" Olds but I'm not sure it's worth the hassle.
     
  7. dana barlow
    Joined: May 30, 2006
    Posts: 3,387

    dana barlow
    Member
    from Miami Fla.

    For the street,we see now a lot of "good for drag race only set ups",just for a sexy look*=but you pay for it,by bad idle an stalling or too high idle. 3x2,4x2,6x2 an 8x2 are cool race stuff,even I love the look and have played with them. Some for racing an a few for street,on buddy's cars in the case of 6x2 an 8x2,an 3x2 n 4x2 on street for my own cars. As for idle on the street,3x2 n 4x2 canbe tuned for street idle with work,even 6x2 if engine is big enough,an about 1in. ID crossovers-2 is enough,3 or bigger is for high end racing {never totally close any carb,or you get bog on opening} Jetting is a game all by it's self.read plus!. If your going to play on the street an use 6x2 or 8x2,it makes life better too fake it some by blocking off some carbs,too get idle steady,unblock at the drags an use idle around 1200 +. I drive my own 4x2 log set up street with idle of 7- 800// but I built my own log 4x2,an the runners off the heads up to the log have divider between ports up an into log about 1/2 in.. Power valves are blocked off. Those dividers help idle. Most alum cast intakes are open between ports,like many steel logs as well. 001 (4).JPG
     
    Last edited: May 16, 2019
  8. 51box
    Joined: Aug 31, 2005
    Posts: 734

    51box
    Member
    from MA

    The car idles nice at 700rpm and drives very well. Doesn’t stall or belch smoke etc. It’s a 425 Nailhead .020 over with a basic rebuild and mild .470 lift cam vertex mag 12* initial and 32* total by 2,500rpm. 6 original 97’s straight linkage stock jetting & power valves. 3spd top loader in a model A coupe. It’s definitely not a drive ability issue just wondering if restricting them down will make a big difference in low end grunt.
     
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  9. bchctybob
    Joined: Sep 18, 2011
    Posts: 1,555

    bchctybob
    ALLIANCE MEMBER

    Do all the carbs have functioning idle mixture screws or does it idle on two and the rest are shut tight. Where did you start with the jet sizes?
     
  10. gimpyshotrods
    Joined: May 20, 2009
    Posts: 15,944

    gimpyshotrods
    Member

    Think of it this way:

    Buick 425 engines have a firing order 1-2-7-8-4-5-6-3. All of the odd numbered cylinders are on one bank, and the even numbered ones on the other.

    Imagine that you have no balance tubes, and two separate logs.

    The intake pulses go odd-even-odd-even-odd-odd-even-odd. Looks pretty balanced, right?

    It's not. Let's arbitrarily shift the firing order by one, so it starts with #2. Just a different point-of-observation.

    So:

    2-7-8-4-5-6-3-1<-- You might notice that is actually even-odd-even-odd-odd-even-odd-odd.

    Irrespective of your point of observation, the engine sees two back-to-back intake pulses, out of the same log, the odd-side log. This does not happen on the even side.

    That is why a you should have a balance tube.

    Now, having the largest balance tubes that you can possibly have is a maximum performance, at redline thing. Giant balance tubes give intake ports access to part or all of all six carburetors. There is no way that you need all of this for the street.

    All you need to is cover the 3-1 paired pulse. I suspect that the Offy one had two balance tubes, so they could have one casting. Just the one adjacent to 3-1 probably would have been fine, but one offset tube would look weird (and required two separate castings).

    If this were mine, I would do this large change, to see if there is a noticeable difference:

    Block off the center balance tube completely.
    Restrict the size of the front and rear balance tubes to 1", each.

    See what that does, and report back.

    Oh, and it is a 425, in a Model A. How much grunt do you want?
     
    Last edited: May 16, 2019
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  11. gimpyshotrods
    Joined: May 20, 2009
    Posts: 15,944

    gimpyshotrods
    Member

    I think that, short of modifying the manifold, you are not going to see this gain, from a straight linkage.

    Bringing in the outboard 4 carbs after the first two, will increase the velocity through the first two, and push fuel metering through them to the maximum possible efficiency.

    A progressive linkage would need to be tuned to get the transition correct.

    This setup really is a "light switch style" fuel delivery system, which is meant for idle-to-max RPM, with no though of in-between. If you want that, you will need to work for it.
     
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  12. gimpyshotrods
    Joined: May 20, 2009
    Posts: 15,944

    gimpyshotrods
    Member

    For reference, the last 425 that I worked on got up, and off of the line in a hurry, with two 500cfm Edelbrock carburetors. Pretty good, considering that it was a ~2-ton Riviera, or likely twice the mass of your A.

    1000cfm might sound like a lot, but you need to remember, those are vacuum secondary units. Yours are not, and you are at 990cfm*.

    To further complicate matters, and to add weight to the progressive linkage idea, those Edelbrocks do not have identical barrels for the primaries and secondaries. The primaries are just under 45% of the flow, with the secondaries bringing in the rest. Rolling off the line, that Riviera was operating on introducing 450cfm of primaries, first, with the secondaries coming in, only as required by vacuum.

    You are tipping in the potential of all 990cfm, in one shot. That's a lot. That will wreck your vacuum signal.

    Ever wonder why a Chevy C60 dump truck only has a two-barrel? Intake velocity = torque, and low-end torque, at-that.

    *There are a few issues that make comparing a modern Edelbrock, to an original Stromberg, but you get the idea.
     
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  13. 51box
    Joined: Aug 31, 2005
    Posts: 734

    51box
    Member
    from MA

    Yes they are all set at 1 turn out. They all function as one unit. I’m running .045 jets.
     
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  14. 51box
    Joined: Aug 31, 2005
    Posts: 734

    51box
    Member
    from MA

    @gimpyshotrods
    Thanks for your input, your reasoning definitely makes sense. I keep hearing how much of a nightmare these are on the street so where this one drives really well so far I would only want to make a change if it made a big difference in acceleration.
     
    Last edited: May 16, 2019
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  15. 51box
    Joined: Aug 31, 2005
    Posts: 734

    51box
    Member
    from MA

    Well my gaskets arrived today so it was sh*t or get off the pot. I’ll do some experimenting and keep this thread updated with how it works.... or doesn’t work. Started by putting 2 7/64” freeze plugs with 1/2” holes drilled in all 3 located on the bottom to let the fuel pass if needed instead of puddling at a “wall”
    0C8300DA-165B-4494-9449-C5E4A1AE57B0.jpeg
     
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  16. Looking forward to this, thanks for the updates..
     
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  17. gimpyshotrods
    Joined: May 20, 2009
    Posts: 15,944

    gimpyshotrods
    Member

  18. Yes, inquiring minds need to be fed ;)
     
  19. 51box
    Joined: Aug 31, 2005
    Posts: 734

    51box
    Member
    from MA

    I have put about 100 miles on it since adding the restrictors and really feel no difference. I wish I had dyno numbers or some track times before/after to see if there was any gains. Based on the old butt dyno I’d say it’s not a substantial improvement by any means.

    Also to note this engine is around 430ci so maybe on something like a 322ci or 364ci it would make a bigger difference necking down the balance tubes ?
     
  20. There is a TON of engineering that goes into Intake manifolds. OEM's and engine companies have been hiring extremely smart and well paid engineers and scientists to work on the problem for over a 100 years now, and they still are trying new things all the time. There are a ton of other factors that are influencing your engines performance besides the balance tubes. I am not surprised that you could detect no appreciable difference. Seat-of-the-pants testing produces different results every time you change your underwear. On a dyno (especially one with an emissions bench), I am guessing the change would have been noticeable, with one factor or the other.

    That being said, don't stop and keep working at. Worst case scenario is you learn something new.
     
  21. bchctybob
    Joined: Sep 18, 2011
    Posts: 1,555

    bchctybob
    ALLIANCE MEMBER

    Thanks for the info, that's a nice set up. Are those 97s real old versions or the re-issues currently available? They look like old ones nicely detailed.
    My Weiand Drag Star only has two 1" balance tubes. Interesting how different the Edelbrock and Weiand are. This thread is really making me want to try the 6 x 2 route on my Olds.
     
  22. Appreciate your time trying this and posting results !!
     
  23. 51box
    Joined: Aug 31, 2005
    Posts: 734

    51box
    Member
    from MA

    @bchctybob They are original 97’s. I sent the bases out to Max at 97 express to refurbish and then did the rest of the rebuilds at home. They are set up completely stock with 1 turn out on the adjusters currently.
     

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