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Thread: Vacuum guage

  1. #1

    Join Date
    Aug 2011
    Location
    Fort Vermilion Alberta
    Posts
    146

    Vacuum guage

    I installed a vacuum gauge, it is teed into #4 cylinder but it pulses with that cylinder.
    My O320 has a carb but the hole is taken up by a carb temp gauge.

    Any suggestions on where to install the line to the main intake (to dampen out the one cylinder pulses?

  2. #2

    Join Date
    Nov 2012
    Posts
    1,519
    On an auto or V style engine. You take a vacuum reading and with this reading you get from this vacuum gauge you can tell the health of your engine. The gauge should hold steady and not be bouncing or fluctuating back and forth. If it is indeed sweeping back and forth or fluctuating this indicates a valves system problem. At different throttle setting and loads these reading tell a lot. Now on your engines with opposed cylinders as your aviation engines, I am not sure if this applies.

    My 1/2 vw has a manifold gauge as does my full vw. The full vw the gauge sweeps back and forth to the point you cn not see it. I was told this is normal using this style of gauge. In my or on my 1/2 this gauge is rock steady. I have no idea what style of gauge is in the 1/2 vw. I purchased that used off e-bay. The one in the full VW was purchased I believe with out looking at the paper work from GP.

    If no one would have said anything to me or if I was not told this is normal. I would be taking this full vw engine apart and looking for the cause of this fluctuation. This winter new heads and some other work will happen to this Full vw. We will see what happens with this MP gauge after this work is complete.

    Now you may say, wait a MP gauge and a Vacuum gauge are two different gauges. That would be correct. But they work off the same function. Vacuum. They just give different interpretations of this vacuum.

    Tony

  3. #3

    Join Date
    Nov 2012
    Posts
    1,519
    I should also mention. Both these engine I am speaking of have the MP gauge hooked into the same spot. This would be on the intake runners and where I would install a gauge such as this, be it a vacuum or MP gauge. You want the gauge in the intake system or intake manifold. Airplane engines use runners and not manifolds. Put this gauge in one of the runners. I would think on an GA or aviation certified engine there would be a bung already in one of these runners for one of these gauges. If not one will need to be installed.

    Tony

  4. #4
    Quote Originally Posted by raytoews View Post
    I installed a vacuum gauge, it is teed into #4 cylinder but it pulses with that cylinder.
    My O320 has a carb but the hole is taken up by a carb temp gauge.

    Any suggestions on where to install the line to the main intake (to dampen out the one cylinder pulses?
    I had this same problem many years ago on an IO-540. I think the problem is not a matter of where you tap in the line. The manifold pressure is actually pulsing as each cylinder draws in air. This shows mostly at very low RPM and very low manifold pressure. Some MP gauges have a screw orifice inside the inlet fitting to help damp this out. But I found that it was hard to adjust this orifice to get the right amount of damping, it was too sensitive to minor adjustments. I ended up putting a fixed size home made orifice closer to the engine to put a larger airspace on the gauge side of the orifice. It was just a small plug forced into the aluminum line with a small hole drilled into the center. ( I may have also used some Loctite.) That fixed the problem.

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