1. This site uses cookies. By continuing to use this site, you are agreeing to our use of cookies. Learn More.

how much compression in lbs.

Discussion in 'Norton Commando Motorcycles (Classic)' started by alan hodge, Feb 5, 2019.

  1. Bernhard


    Apr 20, 2011
  2. dynodave


    May 28, 2003
    A compression check and leak down tests do different things.

    Leak down ONLY test's for sealed surfaces, ie valves, rings, head gasket, cracked piston? You can do a leak down test with NO cam in the engine.
    The compression test is a larger scope dynamic test which includes the cam.
    Due to the amount of work involved usually the easy compression test is done first and the more difficult leak down follows if necessary for further diagnosis.
  3. MexicoMike


    Jan 31, 2010
    In my engine building days we considered a compression check to be 1/3 of an "engine test." A leak down was the other 2/3. As noted, a leak down takes more time/effort but tells you a lot more. I agree that most of the time, folks just do a compression check and if the numbers come out OK, they don't bother with the leakdown. "OK" means consistent numbers between the cylinders, not any specific PSI number. As far as specific PSI for an engine, anything from 120-150 is pretty common. The variation is the big thing. Building performance/competition engines we didn't allow more than a 5PSI variation between the high and low cylinder.

    Note that compression RATIO and compression PSI numbers often do not correlate.
    IOW, one would assume that an engine with 9:1 compression ratio would show a lower PSI reading that one with a 10:1 compression but the camshaft determines what the compression-check PSI numbers will be. We built engines with 13:1 compression that had lower compression PSI numbers than an engine with 10:1. That is because the 13:1 engines' camshafts usually had very large overlap specs so part of the "compression" at starter-motor speeds went out the exhaust! The amount of intake/exhaust valve overlap directly affects the compression PSI. So if you change a stock cam for a "high performance" cam and check the before/after compression, you will often find that the performance cam produces noticeably lower numbers in a compression test.

    When they finally get batteries with sufficient power to make electric cars/bikes useful range-wise or figure out a way to charge in 5 minutes, I'll be happy to see all this archaic combustion engine stuff disappear! ;)
    dynodave likes this.
  4. Triton Thrasher

    Triton Thrasher

    Feb 10, 2009
    MexicoMike- Long-duration cams reduce compression test pressure because the inlet cam does not close until the piston is well up the compression stroke.

    It’s not, strictly speaking, caused by overlap and the pressure is lost through the inlet tract and the carburettor, rather than the exhaust.
  5. Mike Dunlop

    Mike Dunlop

    Feb 10, 2019
    had 2 harbor freight guages both would only read 50 lbs while my snap on after fixing hose read 150 plus on same bike.

Share This Page