Piston compression psi help

Discussion in 'Dyna Models' started by rush, Aug 10, 2014.

  1. rush

    rush Member

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    Hey I need some help finding the correct PSI reading when doing a compression test on a 2012 wide glide with a stock unmodified 103 engine. Anybody know what it is? All the searching I've done here and other places has only given me PSI readings on modified engines. Not a stock 103.

    Thanks in advance for any help
     
  2. 03HD883R

    03HD883R Active Member

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    Your engine is 9.6-1 ratio doing the math it brings it out to 141 psi.

    The math: 1-1 = 14.7 psi... now the magic 2-1 = 29.4 psi... that is 2*14.7 = 29.4 psi.

    :)

    General rule of thumb 135 psi or better is good for gas engines.
     
  3. Jack Klarich

    Jack Klarich Expert Member

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    Welcome to The Forum
    With a good gauge no leak down and throttle full open you should see 180 lbs both cyls +or - 5 to 10 lbs If you have ACR be sure they are working properly before you do a test
     
  4. rush

    rush Member

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    Thanks for the reply's, and I do appreciate the help, but the first reply is 135 psi the other 180 psi. So I'm still stuck on what I should be seeing for a reading.
     
  5. Breeze3at

    Breeze3at Well-Known Member

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    My Twin Cam service manual does not have compression values, but both my Evo and Shovelhead manuals say compression should be over 90 psi. @ operating temp., with less than 10% variance between front & rear.
    The 180 psi Jack mentioned is what I normally think of as good. This is for stock or moderate cams. Cams with a lot of overlap, can significantly change the numbers.
     
  6. rush

    rush Member

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    Ok I last heard 125 psi from someone who said they had a service manual. But that sounds low for a 103 engine. The psi for a sportster I rode for a little while was 220 to 225.
     
  7. Breeze3at

    Breeze3at Well-Known Member

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    Sportster, Fatboy, 1200 cc, or 400 cubic inch, pretty much the same. It's psi, not total pressure.
     
  8. BUBBIE

    BUBBIE Well-Known Member

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    Stock 103" using the stock 30* intake cam and .045 head gasket...

    .....static compression= 9.87:1
    corrected compression= 9.43:1

    Sea-level should Pump 196.7 PSI

    signed....BUBBIE
     
  9. Jack Klarich

    Jack Klarich Expert Member

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    125 would be the lowest reading before HD would be concerned with engine problems, the 96 motor calls out for 100 being the low number
     
  10. Breeze3at

    Breeze3at Well-Known Member

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    Bad choice of last word above. Pressure should be volume.