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Thread: Compression 3A90, anybody know?

  1. #11
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    Quote Originally Posted by nickels View Post
    Fine.

    11.5X14.7 is 170psi.
    You say: 11.5 (the compression ratio of the 3A90) times 14.7 ???
    Please explain the equation.



    (I know 205psi = 14.14bar is normal for a new 3A92, but this one is a 3A90)


    Last edited by foama; 09-12-2019 at 01:23 PM.

  2. #12
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    I too look forward to an explanation of the equation and it's values. I haven't heard of this equation for calculating compression.

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    Quote Originally Posted by Eggman View Post
    I too look forward to an explanation of the equation and it's values. I haven't heard of this equation for calculating compression.

    This covers it pretty in-depth.

    https://itstillruns.com/calculate-co...e-6453303.html

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    Quote Originally Posted by AtomicPunk View Post
    Too many unknown values, and would have to make too many assumptions doing it that way. The result would be off, and thus be of little value.


    On the other hand, if I wanted to settle for a compression calculation based on assumptions, I would try to make as few assumptions as possible.
    For example one could take the official compression values for the 3A92, divide them by 10.5 (compression ratio 3A92) , and multiply the result by 11.5 (ratio 3A90). The result would not be terribly far off, but still it would not reflect the official figures.
    Last edited by foama; 09-13-2019 at 07:57 AM.

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    Quote Originally Posted by AtomicPunk View Post
    I'm not so sure about the 'in-depth' claim. It is a brief (as in short) article that starts out saying there are many factors to consider, much more than the formula posted by nickles:

    Calculating (actual) compression pressure in automotive engines is not a straightforward process of comparing the cylinder's maximum and minimum dimensional volumes as the piston travels through its full cycle. That comparison merely defines mechanical compression ratio. Most engines never achieve a final compression pressure that is purely a result of compression ratio. Other factors such as valve timing, ambient conditions, throttle position and engine load can affect actual compression pressure significantly, and a usable calculation becomes an educated estimate that must consider the other terms.
    Wouldn't the shape of the combustion chamber influence this? Also, nickles' formula says nothing about RPMs or valve timing.

    In any case, I think it is easy to understand why anyone would question information found on the internet.


    All that aside, if I understand what foama is asking he is looking for low-speed compression numbers, right? I'm not so sure a generic formula is what foama is after, but rather Mitsubishi's posted figures. I haven't seen them, and it may come down to owners sharing their results here on the forum.
    Last edited by Eggman; 09-13-2019 at 09:02 AM.

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    Clearly it's more in depth than 11.5 x 14.7...

    And the shape of the combustion chamber would automatically factor into the cylinders dimensional volumes, would it not?. We will stay away from variable compression engines, as this is not widespread (yet), and certainly not in the Mirage.

    Why doesn't someone just ask Mirage.moises93 to get the numbers? He stated he works in a Mazda/Mitsubishi dealership; last I checked they should be able to find the compression pressure range. Emphasis on "should"...

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    Quote Originally Posted by AtomicPunk View Post
    Clearly it's more in depth than 11.5 x 14.7...

    And the shape of the combustion chamber would automatically factor into the cylinders dimensional volumes, would it not?. We will stay away from variable compression engines, as this is not widespread (yet), and certainly not in the Mirage.

    Why doesn't someone just ask Mirage.moises93 to get the numbers? He stated he works in a Mazda/Mitsubishi dealership; last I checked they should be able to find the compression pressure range. Emphasis on "should"...
    I'd be curious to know how far off I am.


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        click to view fuel log View my fuel log 2018 Mirage ES 1.2 manual: 43.1 mpg (US) ... 18.3 km/L ... 5.5 L/100 km ... 51.7 mpg (Imp)


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