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Vacuum related

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  • Vacuum related

    Dummy Question of the day!
    Cam grinds will effect the amount of vacuum an engine produces with the throttle closed.
    I'm running the 270-62h cams and am seeing 12-14 vac. at idle(850-900rpm)
    What explanation is there for an engine that produces more vacuum with a slight tip of the throttle than what it does at closed throttle?(vac will increase to 17-20 or so at 1500-1800rpm)

    *Another issue that goes right along with this is that when light throttle is used in nuetral to bring the rpms up to 2000, the engine will drop back to 1500 and then back up to 2000 in a cycle of rev/drop/rev/drop. Nice and smooth, rev/drop, rev/drop.

    **When pressure checking the intake to the VG30, I can rolll the engine over to where all the valves are closed and pressurize the upper plenum/intake with @ 20psi of air and hear leakage. Soapy water shows me that the electrical end of the air regulator has the tiniest little seep, I can also hear the egr valve seep, but there's a fair amount of leakeage that can be felt with a damp hand down byside the throttle body(above cyclinder#6 spark plug). I can only get the intake to hold air for about 5 seconds before it all bleeds off. I know that it'll prolly loose some psi past the valves...but is the thing supposed to hold pressure for quite some time? What is acceptable?...If you've done this, post what it did for you.
    ~I'm puzzled!
    I am here to help...

  • #2
    Could the large leak be the intake manifold gasket?

    I might remove it and replace it just based on the position of the leak.
    It may not be a Z, but it's still got a turbo...


    • #3
      Back apart it comes.
      I am here to help...


      • #4
        Dummy answer of the day.
        When pressure checking in this manner, be sure to use more soap and less water.
        I found the mystery leak.
        Lower intake manifold gasket, drivers side.
        I checked the intake manifold nuts and bolts, found they were no longer torqued to spec.(gasket settled?...Settling from heat/cool cycle of a fresh rebuild/assembly).
        I re-assembled everything and re-pressure checked it.
        *The air presure will bleed off, but much, much slower and it'll settle to the point where there's 3-4 psi.(I figure the majority of my loss is from the air regulator feed hardline that I cap off, but still seeps)
        I am here to help...


        • #5
          You can usually hear air getting past the valves via exhaust pipe or valve cover breathers

          Terrible idea putting those wheels on...


          • #6
            Yes and no.
            With a gutted plenum, anything going in is now amplified. With-out those passages in there, it becomes an "echo chamber" and makes hearing anything like you mention, near impossible to track down to one thing. You'll see someday when or if you get to that mod...the differences in the sounds especially when the engine is running, makes you initailly think that something isn't right....but it is normal. The valve train sounds pretty sweet when the engine is sitting there idling.

            *The reasoning why I believe the majority of my loss is from that Air regulator feed from the hardline, is because first off..I can feel it seeping air, secondly, the engine didn't atempt to roll over at all, which it would with air pressure filling the combustion chamber and the valves closed.
            I am here to help...


            • #7
              What was the reason for the increased pressure when the throttle is cracked? Did that occurance go away with the other fix?
              Just stand back and throw money.
              Performance costs money.
              Reliable performance costs more.


              • #8
                The vac pulled at idle is mostly a function of valve overlap.

                Vac will change at different RPM and load, including holding the car at a higher idle. With 900rpm idle my car pulls ~14in with that same cam grind.


                • #9
                  OR-Zman wrote: What was the reason for the increased pressure when the throttle is cracked? Did that occurance go away with the other fix?
                  *Well, The car seems to be pulling "roughly" the same vac. as it was before...however, now it is slightly richer than it was before. This verifies that there was a smallish vac. leak occurring.
                  **With these cams, my vac. seems to do the same thing as I stated, more vac. if the throttle is tipped open slightly and rpms brought up.
                  ~Based on what Jason is saying....I don't think I need to worry anymore about it.
                  **Most importantly! Anyone ever experiences that situation I described where the throttle is opened slightly and rpms climb to just before 2000rpm and drop, then rise in a cycle.......
                  ~The answer to that is a simple one. The TVS or TPS was adjusted slightly on the high side, meaning that I went through and reset it so that the smallest possible movement broke continuity. Previously, I had it set such that the throttle stop moved about 1/16" of an inch to break continuity. The ECU was confused because I was opening the throttle, but the ECU didn't know it.
                  I am here to help...


                  • #10

                    I have a pair of Tempestas custom turbo grind 270/270 .480"/.480"
                    in my NA with a vac reading of 19in at 800 rpm.

                    1986 300zx GLL NA modified