Compression #'s Yamaha V-6 ????

Discussion in 'Outboard Maintenance' started by TomFL, Aug 14, 2009.

  1. TomFL

    TomFL Well-Known Member

    Looking at an older boat rigged with a Yamaha Pro-V 175. Hull and motor are '96 but have only a little over 400 hours.

    Does anyone know what the compression #'s should be for a motor like this? I am buying this boat from a reputable source and they've emailed me compression #'s at 110lbs and even.

    Naturally I will check them myself on Monday when I get there, and normally I see #'s in the 110-130lb range.

    So the #110 has me a bit concerned, and I know there can be some difference from one gauge to another but 20lbs???

    Is this a normal compression figure for a motor like this?

  2. Brett

    Brett > PRO STAFF <

    No two gauges read the same in the same cylinder.
    The same gauge will give a different number the next day.
    110 is not a bad number as long as all cylinders test the same more or less

    Lets see ...

    1 atmosphere of pressure is about 14.7 psi
    You'll need to know the compression ratio for that engine to calculate the theoretical max pressure.

    But a basic calc would be something like this...

    given a 10 to 1 compression ratio per cylinder

    10 x 14.7 = 147 psi

    subtract 14.7 that being the existing air pressure outside the cylinder

    147 - 14.7 = 132.3 psi difference between TDC and BDC

    Which is a totally bogus answer! 4:49 pm
    There is not a direct conversion from compression ratio to the psi measured with the gauge.
    Got to thinking about it as it's a rainy day, and I realized that there are a lot more variables involved which I left out.
    Like the pressure inside the 2 stroke crankcase that moves the air/fuel charge into the firing chamber.
    And it's a dynamic system, with pressure changes taking place depending
    on where the piston is in relation to the intake and exhaust ports.

    What else can you do on a rainy day but read through mechanics and physics websites?   :D

  3. TomFL

    TomFL Well-Known Member

    I usually check each cylinder with a different gauge and hope they all come out even. ;)

    But seriously it could have been done cold, or with throttle closed, etc. I guess.

    I just called Yamaha about the motor and it came back perfect, no warranty issues no recalls, etc.

    Tech said 110 is no problem at all, naturally as always watch the spread rather than the number.

    For some reason I just like to see #'s a little higher but maybe when I toss my gauge on it Monday it will read higher as I'm used to.

  4. TomFL

    TomFL Well-Known Member

    Brett, buddy, am I missing something in your math or are you doing this with bagel/stern lift math?

  5. Tom_C

    Tom_C Well-Known Member

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