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Technical Hemi blower manifold question

Discussion in 'The Hokey Ass Message Board' started by Never2old, Feb 4, 2019.

  1. Never2old
    Joined: Oct 14, 2010
    Posts: 718

    Never2old
    Member
    from so cal

    I've searched for a while but failed to find an answer.
    My intake manifold is one of those antique Cragars with two pop off valves on it's bottom side.
    At what manifold pressure should I set the valves to open?
     
    chryslerfan55 likes this.
  2. moparjimd60
    Joined: Jan 22, 2018
    Posts: 71

    moparjimd60

    I’m not sure on the old cragar , I have a hot heads manifold with the front mounted plate. It torques at 25 lbs. call hot heads or don Hampton blowers they will both know.


    Sent from my iPad using H.A.M.B.
     
    Deuces and chryslerfan55 like this.
  3. Mike VV
    Joined: Sep 28, 2010
    Posts: 2,389

    Mike VV
    Member
    from SoCal

    A couple of pounds higher than the pressure you will be running..!
    Seems pretty simple..!

    25lbs. seems awful high for a street engine, unless the blower is putting out 23psi.

    Mike
     
  4. Black Panther
    Joined: Jan 6, 2010
    Posts: 1,762

    Black Panther
    Member
    from SoCal

    I didnt think they were adjustable....dont think it's intended to bleed excessive boost, they are there to keep stuff from breaking into bits if there is a backfire.
     

  5. Black panther is correct. A blown engine has alot of fuel running through the manifold. If there was a back fire all that fuel would explode possibly breaking the manifold. It's more common on nitro cars. The valve acts as a pressure release. Alot of those style manifolds are found broken and rewelded on the bottom. If it's not repaired they bring good money from restorers. The down side to them is the manifold has to come off the car if the gasket is damaged after a back fire. The manifolds hot heads sells have the pop off plate in the front. It's not required to remove the manifold to service the gasket. You might want to consider selling it. You could either spend the whole day changing a gasket or five minutes. They never blow in your garage. It's always far from home. When the gasket goes the engine runs lean with the rpm sometimes very high. Especially if a piece of the gasket sticks the valve open or the bolts get bent.
     
  6. DDDenny
    Joined: Feb 6, 2015
    Posts: 15,042

    DDDenny
    Member
    from oregon

  7. Never2old
    Joined: Oct 14, 2010
    Posts: 718

    Never2old
    Member
    from so cal

    Panther...The pop off valves are adjustable.
    I'm pretty sure it would take a whole hell of a lot of pressure to lift the blower.
    My dilemma is what do I set them at.
     
  8. Never2old
    Joined: Oct 14, 2010
    Posts: 718

    Never2old
    Member
    from so cal

    DDDenny….That guy is having too much fun.
     
  9. DDDenny
    Joined: Feb 6, 2015
    Posts: 15,042

    DDDenny
    Member
    from oregon

    Yeh, that was my take also!
     
  10. Bandit Billy
    Joined: Sep 16, 2014
    Posts: 7,769

    Bandit Billy
    ALLIANCE MEMBER

    I have 2 on the nararro intake under the 471. 15 pounds if remember correctly. They have worked during startup judging from the crap they blew all over the intake.

    My understanding is that they are not necessary if you are running v belts, since the belts will slide in case of the backfire where a notched belt will not. ???
     
  11. Never2old
    Joined: Oct 14, 2010
    Posts: 718

    Never2old
    Member
    from so cal

    Set them to begin to open at 25 psi. and putting it together.
    I offed the original 4 barrel manifold and resold a dual 4 barrel manifold, so I'm going to run it on the stand with two 500 cfm Edelbrocks on an "home made" adaptor plate to the Cragar manifold at first.(pics to follow)
    When everything checks out I'll continue with all the details of the blower drive system.
    I keep telling myself I'm having fun.
     
  12. mgtstumpy
    Joined: Jul 20, 2006
    Posts: 8,838

    mgtstumpy
    Member

    [​IMG]
    [​IMG]
    Pulley size determines boost, swapping top and bottom pulleys alters boost. Smaller pulley on blower (overdrive) increases boost whilst a larger pulley (underdrive) reduces boost. Pop off valve is different to a burst panel. Short of someone providing specifics I'm sure that the following specialists will be able to advise on installed height specific for your application.
    From an old HAMB post, determine the amount of boost you want to run and add 25%, then calculate the area of the pop off valve. e.g. 1.5" diameter with 8lbs of boost + 25% = 10lbs x 1.5 = 15. You will need at least 15lbs of spring pressure to seal the valve under full boost.
    As a preliminary test before fitting intake to engine, test the valve pressure by making block off plates to seal all orifices (Ports). Allow for a fitting on top plate so that you can deliver a metered air supply (Pressure gauge) allowing you to test the 'blow (pop) off valve'.
    If you want to run 15lb boost allow an additional 25% safety margin and pump about 18-19psi into the manifold plenum. This should allow you to fine tune the spring seat pressure by simply winding or unwinding the spring retaining bolts. When the valve starts to release air at the desired pressure, you're done.
     
  13. dreracecar
    Joined: Aug 27, 2009
    Posts: 3,174

    dreracecar
    Member
    from so-cal

    My guess you could use a spring tester and get a poundage at installed height and go from there if you want, but in answer to you question I have never heard of a setting for the pop-offs, most of the time the racers of that era just cranked the bolt down til coil bind. Those are only for pressure spikes and not to regulate boost
     
  14. RichFox
    Joined: Dec 3, 2006
    Posts: 10,020

    RichFox
    Member

    SFI burst panels give up at 225-250 psi I'm told. The pop-offs are not pressure regulators. they are to release a sudden spike in pressure caused by a back fire.
     
    Last edited: Mar 7, 2019
  15. Never2old
    Joined: Oct 14, 2010
    Posts: 718

    Never2old
    Member
    from so cal

    mgt , great info on the charts.
    I measured the spring rates and the valve diameter so I feel confident with what I'm doing now.
    Here is the adaptor plate I finished today.
    adapt.JPG
     
    mgtstumpy and Deuces like this.

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