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Discussion Starter · #1 ·
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?

-T
 

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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
 

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Discussion Starter · #3 ·
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.

-T
 

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Discussion Starter · #4 ·
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 = 142 psi difference between TDC and BDC
Brett, buddy, am I missing something in your math or are you doing this with bagel/stern lift math?

-T
 

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From  iboats Boating Forums > Repair and Maintenance > Johnson & Evinrude Outboards

http://forums.iboats.com/showthread.php?t=313884

JB;2099500 said:
All psi readings should be within a 10-15% span.

* Don't get psi hysteria. Remember this is a 2 stroke engine and you are an amatuer at compression testing.
* Rope start engines will usually give lower psi readings than your know-it-all neighbor claims are required. Same for big JohnnyRude V6s
 
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