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  1. #1
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    What do compression gauge numbers mean?

    One thing that always surprised me was the high readings from my compression gauge. Normally one doesn’t care about the actual gauge reading when testing an engine, just that all readings are close to being the same.

    With a 10:1 compression ratio I get 205 psi when performing a compression test. I normally expect to see something in the 140 to 160 range on stock engines, but mine is modified and uses a performance camshaft, although it’s a mild one as I wanted to get max performance, but retain a smooth idle.

    I wanted to know if there is a correlation between compression test pressure and static compression ratio, and this took a while to research.

    Basically, there is, but one needs to find the dynamic compression ratio first. Unable to sort out how to actually compute the DCR, I used an on-line calculator as per this link:

    http://www.wallaceracing.com/dynamic-cr.php

    I used what I knew about my engine and camshaft and calculated the DCR as 6.81:1



    I thought the next step was using Boyle’s Law of gasses, but I was wrong. Apparently for an engine cylinder, one needs to use the adiabatic expansion formula. Don‘t ask me to explain it, but apparently a pressure change is not proportional to volume change if there is no exchange of heat. I don‘t remember much from my course in thermodynamics back in 1979, but I do remember an old stationary engineer who gave me his definition of thermodynamics: “The thermo is when you drop your thermos on the floor, and the dynamics is when you bend over to pick it up!”

    I digress… so back to the adiabatic expansion formula:

    P2/P1 = (V1/V2)^1.41

    Substituting V1/V2 with 6.81, I get 14.95 and;

    (Comp Pressure + 14.7) / 14.7 = 14.95

    The Comp Pressure works out to 205.11 psi which compares nicely to my measured pressure of 205 psi.

    I’m not sure I’m satisfied with all this until I can understand it more, yet I’m pleased to find that (it seems) SCR can be mathematically massaged, along with engine and cam specs, to find the DCR and therefore predict a compression gauge maximum reading.

    Anybody know enough about thermodynamics to confirm or deny all this?
    John

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

  2. #2
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    I'm not sure what the camshaft has to do with the compression ratio unless actual pressure is being measured for a given weight of charge. CR is the swept volume plus clearance volume over clearance volume. SW+CV/CV. That would be for static atmosperic pressure. I guess the camshaft lobe height and duration comes into play when the engine is running because that would determine the weight, or mass of charge entering the cylinder. The peak pressure of the charge would therefore be different for an engine that is running compared to the mathematical formula of reduced volume for a given mass of air. i.e, halve the volume, double the pressure.
    I suppose that in your equations the temperature of the air is taken into account due to compression. Bicycle pump theory. Pump up a tyre and the end of the pump gets warm.

    Thats my take on it.

    David.
    David.

    Life is driving the Cobra. The rest of the time is just waiting.

    Wessex region caretaker rep.

  3. #3
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    The compression ratio we are most familiar with, often called the Static Compression Ratio, is independent of the camshaft. The ill named Dynamic Compression Ratio is different in that it is dependant on the camshaft chosen. The DCR is based on the geometry of the engine and camshaft and not dependant on piston speed. Spin the engine fast enough and volumetric efficiency would come into play, but I suspect it's insignificant at cranking speeds.

    Essentially, the DCR uses a smaller stroke to find the swept volume and it starts when the Intake Valve closes and ends with the piston at TDC. Basically, there is no compression as the piston moves up until the intake valve closes.

    I think the stroke for my engine is 2.57 inches based on an intake closing angle of 82degATDC. The full stroke is 3.98". Since I wrote the original post, I have been able to figure out the stroke and DCR from basic geometry. It's a bit more complicated than it first appears.

    The other part of this process is using the adiabatic compression formula vs. regular gas laws. Apparently adiabatic means "insulated" and although the air does heat up as the piston rises, it has no where to go. No transfer of heat occurs, so the regular gas laws aren't used. The 1.4 figure, or gamma, is peculiar to air and gamma has other numbers for different gases.

    I was pleased that I was able to correlate compression gauge readings to compression ratio, so I thought I'd post it for others who may have wondered about it.
    John

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

  4. #4
    Join Date
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    Quote Originally Posted by Eggbert View Post
    The compression ratio we are most familiar with, often called the Static Compression Ratio, is independent of the camshaft. The ill named Dynamic Compression Ratio is different in that it is dependant on the camshaft chosen. The DCR is based on the geometry of the engine and camshaft and not dependant on piston speed. Spin the engine fast enough and volumetric efficiency would come into play, but I suspect it's insignificant at cranking speeds.

    Essentially, the DCR uses a smaller stroke to find the swept volume and it starts when the Intake Valve closes and ends with the piston at TDC. Basically, there is no compression as the piston moves up until the intake valve closes.

    I think the stroke for my engine is 2.57 inches based on an intake closing angle of 82degATDC. The full stroke is 3.98". Since I wrote the original post, I have been able to figure out the stroke and DCR from basic geometry. It's a bit more complicated than it first appears.

    The other part of this process is using the adiabatic compression formula vs. regular gas laws. Apparently adiabatic means "insulated" and although the air does heat up as the piston rises, it has no where to go. No transfer of heat occurs, so the regular gas laws aren't used. The 1.4 figure, or gamma, is peculiar to air and gamma has other numbers for different gases.

    I was pleased that I was able to correlate compression gauge readings to compression ratio, so I thought I'd post it for others who may have wondered about it.
    Must admit that's something I've always thought about but never got around to working out. Thanks for posting.


    Where does the 14.7 come from BTW?

  5. #5
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    14.7 psi is atmospheric pressure. It is added to gauge pressure to find the actual pressure.
    John

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

  6. #6
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    Quote Originally Posted by Eggbert View Post
    14.7 psi is atmospheric pressure. It is added to gauge pressure to find the actual pressure.
    Of course it is, I must have been having a blond moment!

  7. #7
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    I have to disagree that the camshaft has anything to do with the compression ratio. The CR is fixed for a given engine. The camshaft dictates how much, or little charge enters the combustion chamber. Once those valves are closed, the engine does its job by compressing the charge available subject to the design of said engine. 7:1. 8.5:1. 10:1. for example. The resultant peak pressure is dependant on what camshaft is used on a naturally aspirated engine.
    In essence, the engine has a CR built into it. The performance depends on how much weight, or mass of charge you can get into the cylinder for a given moment.

    Don’t forget that on a naturally aspirated engine, the charge trapped in the cylinder PRIOR to the compression stroke will always be short of atmospheric which under ISA is given at 14.7 psi absolute pressure. The charge can not move fast enough to enter the cylinder and equalise with atmospheric. So camshaft selection is important here when choosing power vs torque as this dictates the weight and velocity of the charge entering the cylinder.

    As a side note, days of high atmospheric pressure and low temperatures are perfect for naturally aspirated engines. Quite a few horse powers are gained under these conditions as cold air is more dense, thus has more mass and high atmospheric pressure helps push this cold heavy charge into the cylinders.
    Last edited by Darnpistonbroke; 26-01-21 at 09:28 PM.
    David.

    Life is driving the Cobra. The rest of the time is just waiting.

    Wessex region caretaker rep.

  8. #8
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    At this point, I think it best to give links to the two articles I relied on the most as a basis to my posts:

    https://mgaguru.com/mgtech/power/pp105.htm

    https://www.enginebasics.com/Advance...20Dynamic.html
    John

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

  9. #9
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    Aug 2007
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    They are good threads and explain things much better than I can, but the essence of what is written is what I was trying to explain in my last reply. The CR is fixed. Its the resultant peak pressure that can and will differ between the same engine design but equipped with different camshafts.
    David.

    Life is driving the Cobra. The rest of the time is just waiting.

    Wessex region caretaker rep.

  10. #10
    Join Date
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    Hockley ,Essex
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    Quote Originally Posted by Darnpistonbroke View Post
    I have to disagree that the camshaft has anything to do with the compression ratio. The CR is fixed for a given engine. The camshaft dictates how much, or little charge enters the combustion chamber. Once those valves are closed, the engine does its job by compressing the charge available subject to the design of said engine. 7:1. 8.5:1. 10:1. for example. The resultant peak pressure is dependant on what camshaft is used on a naturally aspirated engine.
    In essence, the engine has a CR built into it. The performance depends on how much weight, or mass of charge you can get into the cylinder for a given moment.

    Don’t forget that on a naturally aspirated engine, the charge trapped in the cylinder PRIOR to the compression stroke will always be short of atmospheric which under ISA is given at 14.7 psi absolute pressure. The charge can not move fast enough to enter the cylinder and equalise with atmospheric. So camshaft selection is important here when choosing power vs torque as this dictates the weight and velocity of the charge entering the cylinder.

    As a side note, days of high atmospheric pressure and low temperatures are perfect for naturally aspirated engines. Quite a few horse powers are gained under these conditions as cold air is more dense, thus has more mass and high atmospheric pressure helps push this cold heavy charge into the cylinders.
    The side note you quoted is what some of us used to call the "turbo effect".
    It was great wasn't it, free horses !
    Chris.

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