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Thread: High compression vs low compression

  1. 18th July 2013, 12:52 PM #11
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    101RRS
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    What Steve said - atmospheric pressure is 14.7 lbs per square inch. If you had an engine of compression ratio of 10:1 then the engine is compressing the air 10 times so 14.7 would give a theortical compression psi of 147 on you compression meter but as the links say they are reasons it may be a lit less.

    So for your 8.13:1 standard low compression rover engine at tdc on the compression stroke you will have 8.13x14.7= 119.5 psi on your compression gauge - it will be a bit less on a perfect engine and a lot less if the engine is old and worn.

    Maybe it has been rebuilt or had a over heating episode at some stage. Does your engine number have the dot matrix style of engine number or one that has been stamped on?? I guess 80s engines were all stamped and maybe not the did not stamp the CR on it. Not sure about very early v8s.

    Cheers

    Garry
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  2. 18th July 2013, 04:10 PM #12
    snowbound
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    Wow! so this old bugger is on a learning curve! The engine number is stamped on, and thge area where it is stamped is very shiny, it is not the original donk so goodness knows. I am just trying to ascertain if the PSI reading I was given as "poor" is actually poor or whether someone was trying to get workout of me! Call me a cynic!
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  3. 18th July 2013, 04:18 PM #13
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    Chances are its had a new block, the number is stamped on an angled flange so is not removed during deck machining.
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  4. 18th July 2013, 04:31 PM #14
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    Quote Originally Posted by loanrangie View Post
    Chances are its had a new block, the number is stamped on an angled flange so is not removed during deck machining.
    Certainly was on the 4.0 block I have.
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  5. 18th July 2013, 04:42 PM #15
    TheTree
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    Quote Originally Posted by snowbound View Post
    Wow! so this old bugger is on a learning curve! The engine number is stamped on, and thge area where it is stamped is very shiny, it is not the original donk so goodness knows. I am just trying to ascertain if the PSI reading I was given as "poor" is actually poor or whether someone was trying to get workout of me! Call me a cynic!
    Hi

    The motor can still run with lower PSI than when new, whats more important is that there is no more than about 5PSI difference between the cylinders

    You may wish to read this thread, it suggests 135 PSI

    V8 compression test?

    Steve
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  6. 19th July 2013, 11:32 AM #16
    snowbound
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    Quote Originally Posted by SteveFarmer View Post
    Hi

    The motor can still run with lower PSI than when new, whats more important is that there is no more than about 5PSI difference between the cylinders

    You may wish to read this thread, it suggests 135 PSI

    V8 compression test?

    Steve
    Awesome Thanks Steve. It seems that my PSI is down but apparently down all around and within the 5 psi range, so that's not so bad... I hope! Actually it is going pretty good on petrol, just need to sort the gas situation. Next job!
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  7. 19th July 2013, 11:35 AM #17
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    135psi would suggest a high compression engine (135/14.7= 9.18:1).

    Garry
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  8. 19th July 2013, 04:05 PM #18
    TheTree
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    Quote Originally Posted by garrycol View Post
    135psi would suggest a high compression engine (135/14.7= 9.18:1).

    Garry
    Garry

    I reckon you are on the money there !

    Steve
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  9. 19th July 2013, 04:06 PM #19
    TheTree
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    Quote Originally Posted by snowbound View Post
    Awesome Thanks Steve. It seems that my PSI is down but apparently down all around and within the 5 psi range, so that's not so bad... I hope! Actually it is going pretty good on petrol, just need to sort the gas situation. Next job!
    Hi,

    In an ideal world they would all be the same, but especially on an old motor, there will be variances

    As Garry noted, 135PSI is a high compression engine, what sort of number were you getting?

    Steve
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  10. 19th July 2013, 04:22 PM #20
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    Lightbulb

    Quote Originally Posted by snowbound View Post
    Staying on topic, I am hijacking! Sorry intheozone. My 3.5 has a psi reading of 120 -125 on all cylinders. I'm told this is low? I went looking for the number on the block but couldn't find it. I will look again.

    If it is an 8.13:1 engine those numbers would be as good as you can get!

    More likely though is that you have a slightly worn hi-comp engine. However I wouldn't be rebuilding it just yet unless it had lots of blowby or was using oil, etc...

    The most important thing is all cylinders are close by the sound of it.
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