Results 1 to 6 of 6
  1. #1
    Join Date
    Sep 2010
    Location
    Nova Scotia
    Posts
    891

    Using a Compression Gauge on a Small Engine

    A Compression Gauge is a useful tool, but sometimes it can prove misleading.

    It’s great if you take a reference of when an engine’s in good shape for comparison purposes later in the engine’s life.

    It’s also great for seeing if there’s a significant difference in cylinders in a multi-cylinder engine.

    However, it isn’t necessarily great when you’re trying to determine the condition of an engine you’ve never tested before, especially a single cylinder engine, and in this particular case, a “small” single cylinder engine.

    I’m going to use my Honda Z50 engine as an example. It turns out the adapter for the compression tester is a problem. In the picture below, you can see the compression gauge with the Schrader valve at the end. It’s a one-way valve, just like on a bicycle tube.

    The adapter is used to screw into the spark-plug hole; I think a 10mm diameter thread. You can see the large air cavity in it.



    This large cavity affects the chamber volume as shown in the picture below:



    A little math now. Based on the swept volume of 49cc and the known compression ratio of 8.8, the chamber volume works out to 6.28 cc.

    CR = (swept volume + chamber volume) / chamber volume)

    Based on dimensions with the adapter screwed into the compression gauge hose, the adapter volume works out to 2.86 cc.

    Using Boyle’s Law concerning pressure and volume (P1 * V1 = P2 * V2), one can work out what the gauge pressure should be knowing what was measured. In my test I got a reading of 90 psi, which I considered low. In fact, so low I thought it might not allow the engine to start, or maybe start, but run with low power, but that was not the case. It ran fine.

    So, the pressure I should be seeing is:

    P = 90 * (( 6.28 + 2.86) / 6.28 ) = 131 psi

    That’s more like it. In fact, the example in my Clymer Overhaul manual says 130 psi is for a new engine, with the lowest reading being 95 psi for an engine with 8000 miles on it. So, it looks like everything is OK, but as I suspected, an automotive compression gauge that uses adapters will not give an accurate reading for a small engine.
    John

    “A prudent person profits from personal experience, a wise one from the experience of others.”

  2. #2
    Join Date
    Sep 2007
    Location
    Peak District
    Posts
    261
    interesting stuff.

  3. #3
    Join Date
    Feb 2011
    Location
    Northampton
    Age
    41
    Posts
    4,205
    John, is there no end to your talents? Your posts are always informative, with supporting pictures, diagrams etc.. you didn't, by any chance, write technical/training materials in a past life, did you? If not, you probably missed your calling!
    My DeDion build diary..
    Hon Sec of the Digidash branch of the Unpopular Kit Car Design club

  4. #4
    Join Date
    May 2016
    Location
    Hockley ,Essex
    Age
    57
    Posts
    136
    With compression gauge it will be more accurate with the least amount of adaptors used,
    It's a bit like when using a torque wrench, torque readings become inaccurate with extensions and the thickness of the extension used.
    Also there are different ways to check compression,
    1) dry test with cold engine.
    2) dry test with hot engine.
    3) wet test with cold engine.
    4) wet test with hot engine .
    The above checks are usually carried out with wide open throttle.
    Chris.

  5. #5
    Join Date
    May 2016
    Location
    Hockley ,Essex
    Age
    57
    Posts
    136
    You could carry out a "leak down check" ,
    This is in some times considered a better check than the traditional compression check.
    Chris.

  6. #6
    Join Date
    Jun 2004
    Location
    Northampton
    Posts
    1,698
    I didn’t think I was going to be home schooled! Thanks John for reminding me about Boyle’s Law. It’s been a while!
    Dek

Tags for this Thread

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •