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Visiting car guy, got a quick question about volume and compression

  1. Jan 10, 2010 #1
    Hi everyone. Hopefully this should be a relatively simple question here.

    I'm running a compression test on my car and I am using a Bentley Manual (very good mechanical Bibles car people use that depict how to take apart the entire car) as a reference for the PSI I should be shooting for.

    A compression test basically works by plugging in a special gauge into each cylinder spark plug hole, while cranking the engine in order to get a pressure reading. This pressure reading is used to assess the condition of the engine internals.

    Assume that temperature and altitude are constant, while rubber hose expansion is nonexistent.

    My concern is that the Bentley Manual doesn't state the length/diameter of the extension hose that they used, and I know that this affects the readings. Obviously the longer the hose, the more air, the lower the readings.

    I was debating between two extensions, a 28 cm and 15 cm. After doing some math though, it seems that these differences should be quite insignificant. Here is my math, with a brief explanation below:

    http://img64.imageshack.us/img64/4543/ecui.png [Broken]

    Basically I calculated the volume of each of the two extensions, and then calculated that as a percentage of the total cylinder volume. The delta between these turned out to be 0.18%, a very small amount. Then I just used a value of a 150 PSI compression reading and found I should be expecting a difference of only ~0.3 PSI due to the length differences.

    Does this sound about right? Any help is greatly appreciated. Thanks.
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Jan 10, 2010 #2
    Sounds about right. Only thing comes to mind is, the gauge has a check valve (such that the gauge 'pumps up' over several cycles, and keeps the reading until released. Is the check valve downstream of the hose, or is it at the base of the hose?

    Either way, the conclusion is, the reading (typically 125 to 150 psig) is insensitive to the length of the hose (I mean, you can't read the pressure to an accuracy of tenths of a psi anyway). Finally, a compression test is usually looking for anomalous readings between cylinders, and in that case the 'hose effect' applies equally too each cylinder.
  4. Jan 10, 2010 #3
    At the base of the hose, right below the gauge. I imagine such a small deviation is within the mechanical margin of error anyway, so this is good news.

    Also, true the point of a compression test is to compare relative cylinders of a motor, but there are "service limits." Check out this page from the Bentley:

    http://img683.imageshack.us/img683/9390/img093d.jpg [Broken]

    Last edited by a moderator: May 4, 2017
  5. Jan 11, 2010 #4

    Ranger Mike

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    Gold Member

    ref shorter is better in this case..so go with the shortest hose...but not really a point since the data for each length is meaningless when compared to each other...
    your assumption is right about increased error due to length..theoretically not hose should be used.

    compression test is a diagnostic tool and not a finite test of CR...to do CR. accurately you have to CC the cylinder head combustion chamber, measure the thickness of the head gasket, measure the deck height to tdc piston, in some cases CC the piston if it is not totally flat. only then ca the Cr. be calculated..the compression test checks for leakage due to rings or valves leaking or worse..cracked piston. You will not get a very accurate reading due to checking when the engine is off. max ring sealing is when engine is warmed to optimum temp

    btw the diameter of the gage will impact the measurement too..we use a 4 inch diameter leak down gage when checking the engine...has more precision in that the dial face has more tick marks between the numerials.
    Last edited: Jan 11, 2010
  6. Jan 11, 2010 #5
    And the internal diameter of the short hose is rather small, so its volume compartively inconsequential compared to the volume of the cylinder.
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