Compression Ratio Calculations ? 1289cc
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Thread: Compression Ratio Calculations ? 1289cc

  1. #1
    Fellow Frogger! R8philSA's Avatar
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    Default Compression Ratio Calculations ? 1289cc

    Is there a way of calculating your compression ratio working backwards from what the compression is ???

    Like: 180psi or that close to it in all 4 cylinders, and yes, the head was reduced in thickness before fitting.

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    Look forward to hearing from a physics PHD.

    Cheers
    Phil

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    1000+ Posts robmac's Avatar
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    Assuming atmospheric pressure in uncompressed state, 140 / atmospheric pressure should give an approximation.

    140/ 14.7 (approx at sea level )= 12:1 approx ? Does that seem correct ?

    I'm not a physics Phd, so I'll wait for someone who is to correct me
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    1000+ Posts Kim Luck's Avatar
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    I suspect that there may currently be a shortage of PhD wielding Froggers about today so I'll give you this:

    Engine Compression Ratio (CR) Calculator
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    I think the difficulty with approximating from compression readings and atmospheric is tjat the cylinders do not necessarily completely empty to atmospheric while the compression is being measured. Thus I think you would calculate an over estimate.

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    1000+ Posts robmac's Avatar
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    Quote Originally Posted by keith mcelroy View Post
    I think the difficulty with approximating from compression readings and atmospheric is tjat the cylinders do not necessarily completely empty to atmospheric while the compression is being measured. Thus I think you would calculate an over estimate.
    I pondered that too, but figured an inlet valve opening must give a starting pressure close to atmospheric.

    And I'm in furious agreement this method is only an approximation, the only accurate way is measure CC volumes.
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    1000+ Posts Frans's Avatar
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    My worth.

    You cannot do it. Years ago I built and patented an analogous variable cam engine and one of the things that limited my small duration/small overlap/hi torque/lowRPM profile on the lobe was compression ratio. The less/smaller the overlap the higher the compression.

    So what are your overlap and over and above that what is your valve clearance. I had to do an experiment for my Variable cam project and this is the figures that popped up on the engine that I used for the patent demonstration, a 1800 SOHC BMW 4 cyl engine.

    Rocker gap 0,2mm: Comp. 1150kpa: Intake vlv opens 24deg BTDC
    Rocker gap 0,3mm: Comp. 1270kpa: Intake vlv opens 12deg BTDC
    Rocker gap 0,4mm: Comp. 1350kpa: Intake vlv opens 5deg BTDC
    Rocker gap 0,5mm: Comp. 1350kpa: Intake vlv opens 2deg BTDC

    You can see how the pressure goes up on the same engine but with different rocker clearances.

    What I am trying to say is that I think it will be very wrong because you can alter your compression reading by increasing the rocker gap. There is a huge difference in static CR and running CR. You are trying to calculate the static CR, however you are trying to calculate it with running CR figures.

    Open for discussion on this one.

    Frans.
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    1000+ Posts geckoeng's Avatar
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    Frans you are right on the button here, with regards actual compression tests. The whole thing has too many variables.

    Actual volumetric compression ratio (CR) can only be worked out by measurement, as in Kim Luck's link.

    As for 180psi I would suggest Phil has his valve clearances a bit too loose.

    Ray
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    Ray geckoeng

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    Fellow Frogger! R8philSA's Avatar
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    Thank you all that replied, really given me something to contemplate.

    Phil



    Quote Originally Posted by R8philSA View Post
    Is there a way of calculating your compression ratio working backwards from what the compression is ???

    Like: 180psi or that close to it in all 4 cylinders, and yes, the head was reduced in thickness before fitting.

    Look forward to hearing from a physics PHD.

    Cheers
    Phil

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    A couple of thoughts that I had on the subject are that the compression pressure on cranking are directly related to the efficiency as a pump and subject to port speed , valve timing , cranking speed on the starter etc . The higher compression reading usually indicates good low torque and good ring sealing .
    By filling the combustion chamber with a known quantity of oil at bdc and rotating the crank to tdc , catching and measuring the expelled oil will give you what you are looking for [ assuming the spark plug hole is the highest point and there is no trapped air ]
    Have fun with that . David
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    Quote Originally Posted by Toolman View Post
    A couple of thoughts that I had on the subject are that the compression pressure on cranking are directly related to the efficiency as a pump and subject to port speed , valve timing , cranking speed on the starter etc . The higher compression reading usually indicates good low torque and good ring sealing .
    By filling the combustion chamber with a known quantity of oil at bdc and rotating the crank to tdc , catching and measuring the expelled oil will give you what you are looking for [ assuming the spark plug hole is the highest point and there is no trapped air ]
    Have fun with that . David
    Very clever. Logical when you think about it, but who else did?
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    1000+ Posts PeterT's Avatar
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    Always remember, the compression stroke doesn't start until the inlet valve closes.

    You can buy such a gadget as Toolman describes. Scrutineers use them to make sure race teams aren't cheating with compression ratios.

    '92 205 Mi16
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    If we have an engine, at BDC that has a swept volume of 900cc in the cylinder and a combustion chamber volume of 100cc, then this volume is reduced to the 100cc of the combustion chamber at TDC this would be a compression ratio of 1000:100, or reducing it fractionally, a compression ratio of 10:1. Courtesy Wikipedia.

    You already know the swept volume IE stroke by bore in cc's. Therefore given that the spark plug hole is at the top and no air trapped as Toolman says then surely all you have to do is set the piston at TDC on a firing stroke and measure the oil that it takes to fill just to the spark plug hole. (Allow a little for plug protrusion perhaps.) Isn't that the combustion chamber volume, and therefore the ratio can be calculated? Use a syringe to suck out the excess oil before trying to start the engine though. Or have I got this all wrong?
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    1000+ Posts Frans's Avatar
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    Remember that at BDC the intake valve is still open, and will remain open until about 30 degrees ABDC, so you'll have to catch the oil that runs into the intake port as well.

    Frans.
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