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Thread: Compression

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    Compression

    I finally got around to running a compression test on my TR with the "Lumpy" idle.
    It's got 10:1 compression all the way across. This backs up our theory that the car has been modified and may have a high performance cam.

    On separate note, has anyone been successful at buying anything from TRF lately. I've been waiting over a month for a carburetor rebuild kit. When I call they finally find my order and say it will be sent soon.

    They seem to be in disarray since the founder passed away.

    S

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    Obi Wan
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    Re: Compression

    The news letter that was sent out last week indicated that they were working hard to get the orders out. It must have been a big upset for everyone there when Charles passed suddenly.

    I have not needed to order anything in the last month.

    David
    TR3A TS75524L

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    Yoda CJD's Avatar
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    Re: Compression

    Curious...how do you do a compression test to 10:1? When I run a compression test it comes out a PSI. I.e. 186psi or such. When I calculate compression based on stroke, bore and chamber volume, then I get something like 10:1. The former is a "test", while the latter is a straight mathematic calculation.
    John

    1955 TR2

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    Re: Compression

    The compression in psi divided by atmospheric pressure is the ratio. 14.69 psi is one atmosphere.
    Wow if you are getting 186 psi compression you must need 106 octane fuel.

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    Re: Compression

    Interesting...I have never heard that relationship.

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

    https://www.theturboforums.com/threa...-ratio.295006/

    But it sounds like it is rampant on the web?!?
    John

    1955 TR2

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    Re: Compression

    I have been sitting here trying to figure out how to verbalize this. It boils down to the fact that some cars have low speed cams with very little valve overlap. Then there are performance cams with a whole lotta valve overlap. In general, the higher the actual compression ration of the engine, the more overlap it's cam will have. So, I have had engines with 12.5:1 compression that had 45 degrees of overlap...giving a compression check of about 175psi. Then, I have had engines with 9:1 compression and an "RV" style cam with almost no overlap...giving me 220psi on the compression check. My TR2 has an 8.7:1 compression and reads about 155psi during a compression check.

    So, while I see where you're coming from...there is not necessarily any repeatable correlation between compression "test" and Compression "ratio".
    John

    1955 TR2

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    Re: Compression

    I think you are right that conditions could cause you to score low on compression even though you have a high compression ratio but I don't think it is possible to score higher than the number "Comp. Ratio time atmospheric." I'm also learning that it's not a perfect straight line correlation, but will get you close. The best I can find is that the pressure should be between 15 and 20 time the compression ratio.

    That all said, maybe I do have a 9.1:1 compression ratio.

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    Re: Compression

    Quote Originally Posted by STeve 1958 View Post
    The compression in psi divided by atmospheric pressure is the ratio. 14.69 psi is one atmosphere.
    Not true. The problem is that compressing the air also heats it, and the heat also increases the pressure. The equation you give works for what is called "isothermal" compression, meaning the result is cooled back to the same temperature it started with. The more general equation is PV = nRT where PV is pressure times volume, n is the number of moles of gas (a constant), R is a magic number (also a constant), and T is the temperature. Unfortunately, that doesn't cover how much the temperature rises.

    But what happens when taking a compression test is somewhere between isothermal and adiabatic (which means the result is not cooled at all). The rules for adiabatic are a lot more complicated; you can read more about them at
    https://en.wikipedia.org/wiki/Adiaba...sible_process)

    Then of course, there is the issue that John mentioned; the valves do not open and close precisely at the beginning and end of the compression stroke. The intake valve in particular stays open for some time after BDC, at cranking speeds some of the air escapes through the intake. So a performance camshaft (which keeps the valve open longer for better cylinder filling at high rpm) will actually lower the measured compression pressure. You can even see the change from changing the valve lash.

    Compression ratio is defined strictly in terms of cylinder volume at BDC, divided by the volume at TDC. It does not take into account any of these other effects.
    Last edited by TR3driver; 01-23-2019 at 04:57 PM.
    Randall
    56 TR3 TS13571L once and future daily driver
    71 Stag LE1473L waiting engine rebuild
    71-72-73 Stag LE2013LBW waiting OD gearbox rebuild

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    Re: Compression

    I honestly wish it were that simple...and I had to think for a while to be sure it isn’t. Otherwise I have wasted many months of my life behind head plates and burrets calculating compression ratios. If it were correct, then I never would have been able to run a 9:1 ratio and get 225psi...and I am positive I did!
    John

    1955 TR2

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