Power valve

Discussion in 'High Tech for Old Iron' started by jon2295, Feb 21, 2016.

  1. jon2295

    jon2295 Well-Known Member

    Ok so im trying to figure out if I have the correct power valve . 1971 skylark stock 350 with hx35 turbo. Demon 650 carb. Jets are 72 prim and 78 secondaries. Goes way lean at quarter throttle or so. The power valve is a 5. I am thinking I need a 10 or so. My vacum is at 19 psi on the boost guage. Not sure what that is in inches, which is how it is refered to in any info I found on the internet. Good news is that the turbo is spooling nice and early now that I went over all my welds again. I have read the sticky on tuning so thats why Im looking at the power valve first and not just upping the jets. Any help would be appreciated.
     
  2. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    A vacuum/boost gauge will read both vacuum and boost. Vacuum is read in inches/mercury, and boost in psi. It says that right on the gauge I use in my 98 Riviera.

    [​IMG]

    Sounds like your idle vacuum is 19" in Park, you need to take a reading at idle, in gear. Per Holley, you should half the reading in gear at idle, and use that to select a power valve. So if your vacuum in gear is 15", 15/2 = 7.5, and that is the valve you need. Now that is with a normally aspirated engine, with a turbocharger, you may need earlier enrichment, so you might need a higher number. The power valve you are using now is way too low I would think, but I am not versed in turbo charged engines, and a blow through carburetor.
     
  3. jon2295

    jon2295 Well-Known Member

    Wow, now I feel like a dummy, never noticed that on the gauge.Thanks Larry. The idle vacuum in drive is actually 19 so I think I will order a power valve around 9. Any turbo guys think thats correct?
     
  4. tinker14bs

    tinker14bs Well-Known Member

    I am running the 10.5 power valve. I still get a lean spike (16.5) until the vacuum hits that level and corrects (13). It is a fraction of a second. Would like something even higher if they have it. My idle vac is around 17-18 in-hg. I would get the 10.5. I am running the twin turbos at 6 psi. I think Jay maybe made his own power valve by adding a spring or something to help with this.
     
  5. jon2295

    jon2295 Well-Known Member

    Ok, thanks Brennan. I will get a 10.5 and see what that does. Another thing that I was considering might be a problem is my blow off valve. I put a bigger spring so that it only opens with vacuum and pressure from the turbo. So when driving normal it will be closed, could that mess things up for the air fuel ratio?
     
  6. tinker14bs

    tinker14bs Well-Known Member

    I don't think that would matter. It shouldn't make a difference for the air above the throttle blade. The blow off valve is more to release excess pressure that can cause damage to the turbo impeller when the throttle blades close.
     
  7. jon2295

    jon2295 Well-Known Member

    Ok, thats what I thought, thanks.
     
  8. jon2295

    jon2295 Well-Known Member

    I put the 10.5 power valve in and it is still not right . I watched the vacuum gauge and im not seeing a big change in air/fuel ratio at 10". I just ordered a jet kit and I am just going to go 90 jet on the primaries and see what that does. I think the power valve passages might not be working correctly. I have heard people have trouble with the demon carbs.
     
  9. tinker14bs

    tinker14bs Well-Known Member

    The power valve is only opening for the the power valve channel restrictions. You should try changing those (PVCR) to see the change in air/fuel. Power valve only opens the path. I bought a bunch of brass set screws and drilled them out using a 1-60 drill set. I can't recall exactly what I ended up at, but seem to remember around 0.050-0.055 thousands.
     
  10. Babeola

    Babeola Well-Known Member

    The Holley book recommended a power valve 2" above the level of vacuum in gear standing still holding the brake.

    Cheryl :)
     
  11. jon2295

    jon2295 Well-Known Member

    I made a little progress last night. I pulled the carb over the weekend and cleaned the heck out of it making sure all the passages were clear, got a new set of plug wires( first time doing the custom accell ,what a pain in the but getting the boots on) and then I retimed the engine. It is now running way too rich but im moving in the right direction. It has the 10.5 PV in now with 88 jets in the primaries and 90 in the secondaries. Hopefully if I get a chance tonight I will put 80 in the primaries and see what that does. It is so rich right now the a/f ratio stays at 10 which I think is as low as the meter goes. Hope to have good news to report tomorrow.
     
  12. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member


    What is the vacuum at idle, in gear? Does it fluctuate?
     
  13. jon2295

    jon2295 Well-Known Member

    Its right at 19 when in drive. pretty steady.
     
  14. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    You mean below? If the power valve opens at idle, the mixture gets excessively rich. I thought the book said half the idle vacuum in gear.
     
  15. Babeola

    Babeola Well-Known Member

    Yes 2" below, or so it needs less vacuum to hold it closed than is present. This was from a carburetor book Holley published long ago on the 4150 style carbs. You want it that close so that the power valve doesn't close at the top end of the track when vacuum is starting to return. The car would lay over in that case and stop pulling.

    Cheryl :)
     
  16. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    The way I understand it, if you have more than 1.5" of vacuum at WOT, that means the carburetor is undersized and acting as a restriction. As long as your foot is to the floor, the vacuum should be no more than 1.5" Hg, preferably less. No power valve would be closed at that point until you lift. Why would vacuum return at the highest load portion of the run with your foot to the floor?
     
  17. Babeola

    Babeola Well-Known Member

    I recall it had to do with higher rpms and those advancing slower at the top end. If I find the book, there is an explanation I will present.

    Cheryl :)
     
  18. tinker14bs

    tinker14bs Well-Known Member

    This is the current tune I am running in my carb (Quickfuel SS-650-BAN)...
    Primary jets: 64, 64
    Idle air bleeds: 63
    Secondary jets: 73, 73
    Power valve: 10.5
    PVRC: 0.050"
    Primary squirter: ?? Need to check
    Sedondary squirter: ?? Need to check

    I am running twin turbos with 6 psi of boost. I have a custom crower cam for the boost. I followed the tuning thread from Mark, but still ahve a few things to work out with mine as well. I get a momentary lean spike when I punch the gas, but then recovers quickly to about 13.0 and pulls hard. I am building boost within a second and at the 6psi within about 3 seconds. I started out real rich and had to jet down significantly as you can see above. I was worried it was too lean, but the engine likes it and stays in the 13.7 -14.0 at cruise and will drop to 12.5 at full throttle and part throttle at 13.0. I can't go WOT under 34 mph if I don't want to hang my ass out. It gets a bit scary fast. And to think I have a set of 11 psi springs and meth injection kit I want to add. The way the boost puts you in the seat is addicting.
     
  19. jon2295

    jon2295 Well-Known Member

    I did a little work in the past week or so. I need to order some jets for the air bleeds. I almost gave up since I could not get the a/f ratio correct. I figured I would try one last thing. I soldered the high speed air bleeds shut on the secondaries. That richened up the a/f ratio pretty good. I got a little video. Horrible cell phone video quality. I will get some Go pro footage soon. That will also help me figure out the next step. https://www.youtube.com/watch?v=9dkN80n2oB4
     

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