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Thread: Ahem! A question please ....

  1. #1
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    Ahem! A question please ....

    So, here's a technical question ....

    (Call me crazy or not but)
    Is there such a thing or way to find out compression numbers out of any or all cylinders without using the compression gauge/test/kit/equipment? Do you think this is possible or can be done? If so, how? Let's assume that it is possible then in what manner would you go about doing this? What things or parameters or data set would you look at or take into consideration?



  2. #2
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    I don't see how you could do that without using a pressure gauge.

    I mean sure there are ways to determine if it is developing pressure... (if you knew what I was thinking regarding that you'd laugh your butt off) but to actually get reliable numbers you could actually use to determine engine health you'll need the hardware for it.

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  3. #3
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    It would likely require considerably more expensive and more complex hardware and work then just using a compression gauge. Since there are usually no sensors in each individual exhaust header, it would be nearly impossible to even create a correlation between collected data off of the engine to the compression each piston is generating.

    If it is a compression gauge you need and you don't want to buy one, I suspect that some auto stores like Advance Auto or Auto Zone would rent them.
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  4. #4
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    Well, it's not about one to purchase a compression gauge test/equipment nor is it how to use them. It's about how to find out compression out of 'a' or 'one' cylinder without the need of using the actual hardware - alas the compression gauge test. Just wondering how can one go about doing this. For example, if you had an expensive scanner hooked up to the diagnostic port say like a Snap-On scanner that can not only read data, look at the graphs, but also do test runs like running the auxiliary electric fan for a short period of time as a system check. Performing a power balance test by switching off one injector while the engine is running and as such. So, if you were to hook up a software tool, what data set or parameters would you look at to determine cylinder compression. JimLev mentioned about the cranking current. The amount of current needed to overcome the compression stroke before encountering the next cylinder on compression stroke. He mentioned of when he had his Nissan ZX300 they (who, I don't know) could measure the compression by reading the cranking current while the engine was turning over. They had the fuel pump circuited disconnected to avoid start up. With a constant supply of voltage and a software hooked up looking at the current draw was one way to determine compression. He's not sure of how accurate this is or how possible it is. I remember one time in an automotive classroom where someone actually mentioned of how Ford can go about finding out by simply relying on their software hooked up. The guy didn't tell us how. Again, no typical compression gauge was utilized here at all. I think it was their Ford scanner that has the ability to measure it. So, this is where my question comes in. By assuming that this is possible, what sensors/data/info would you look at or take into consideration. Also, I firmly (or obviously) believe that everything is measured with respect to time. Not the actual engine timing but time measure in seconds to milliseconds. I think CKP and CPS would be the two priority sensors to collect data, to obviously know the actual engine timing with respect to time. The system should know the cranking speed at which the engine is turning over, the precise opening and closing of the intake and exhaust valves, to know what stroke the cylinder is, and then like Jim said the current draw I believe although of what purpose of this info would reflect I don't really know. I think knowing the intake manifold pressure or airflow even at cranking speeds is another vital info. Another thing that comes to my mind is the ion current measuring system in the ignition system as found in the BMW S85 and possibly S65 engine. It is a system to measure engine misfire as well as knocks simultaneously albeit more closely and for all cylinders. This system replaces the knock sensors entirely as it is not required. It sends a small or low voltage over the spark plugs immediately after the ignition. I think having these valuable information is probably one way of calculating or determining cylinder compression numbers that may be equal as when one compared by using the actual hardware. I was just curious about this topic as it just reminded me of this when the guy said there is a way. What do you think?



  5. #5
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    I am not sure exactly what the test is called but on the Ford diag program you can do a compression check. You crank the engine over until it tell you to stop. It shuts off fuel/spark to avoid a start up. I have no clue how you would do this without that exact function of the program. I do know the same type system is out there for other makes as well.
    Last edited by Houseboater; 12-21-2014 at 11:23 PM.
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  6. #6
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    Quote Originally Posted by Houseboater View Post
    I am not sure exactly what the test is called but on the Ford diag program you can do a compression check. You crank the engine over until it tell you to stop. It shuts off fuel/spark to avoid a start up. I have no clue how you would do this without that exact function of the program. I do know the same type system is out there for other makes as well.
    Ahah! So I was right then that there is a way of doing it but what exactly does it look at is what I'm trying to understand here.



  7. #7
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    Quote Originally Posted by auaq View Post
    Ahah! So I was right then that there is a way of doing it but what exactly does it look at is what I'm trying to understand here.
    Right. That, I don't have the answer for but would love to know. Surely someone will have more knowledge on it.

    Here we go. Called a relative compression test. http://scantoolresource.com/home/for...agnostic-tool/

    - - - Updated - - -

    More info http://www.tiepie-automotive.com/en/Measurement_examples/Miscellaneous/Relative_Compression_Test

    http://www.picoauto.com/video/relative-compression.html

    http://www.iatn.net/forums/12/78460/relative-compression-via-voltage
    2001 X5 4.4

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  8. #8
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    Excellent!



  9. #9
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    ....
    Last edited by Houseboater; 12-22-2014 at 12:18 AM. Reason: Meh
    2001 X5 4.4

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  10. #10
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    How about comparing the time it takes to complete one or more full engine revolutions? Assuming there is a battery charger connected and no issues with the starter, the time to complete a single revolution of a good engine could be known, and if the time is significantly less, then you'd know that one or more cylinders weren't creating compression. The problematic cylinder would be determined by where in the revolution a cranking-speed increase was noted.
    Quote Originally Posted by B4SH View Post
    If anyone knows, it's this guy who knows literally nothing about everything.
    Quote Originally Posted by fcvapor05 View Post
    Do you even OT? The mean 401k around here is probably about $6, what with all the shiny cars and $1,000 watches and donations to get other members' Volvos running.


  11. #11
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    I think just renting the compression gauge is easier than any of this.

    Any particular reason your trying to avoid doing it the "proper" way, auag?

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  12. #12
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    It sounds to me like an exercise in process thinking.
    Quote Originally Posted by B4SH View Post
    If anyone knows, it's this guy who knows literally nothing about everything.
    Quote Originally Posted by fcvapor05 View Post
    Do you even OT? The mean 401k around here is probably about $6, what with all the shiny cars and $1,000 watches and donations to get other members' Volvos running.


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