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Author Topic:   2006 Mercury 90-HP FOURSTROKE Cylinder Pressure Readings
jaymoussy posted 05-31-2015 07:35 AM ET (US)   Profile for jaymoussy   Send Email to jaymoussy  
I am new here and just about to sign on a used Montauk, but I am confused about [cylinder pressure] readings I got from brokerage. The 2006 Mercury 90 FOURSTROKE EFI was [apparently measured to have peak cylinder pressure readings in PSI (assumed) of] 235, 240, 240, and 240. Those are very close--good--but everything I have read seem to indicate these are high. I was surprised, and talked to the shop. The technician confirmed that [those figures are] "what they see all the time." Are these numbers commom? I understand there may be a slight differences in method and gauges. Should I be concerned about these values?
jimh posted 05-31-2015 08:31 AM ET (US)     Profile for jimh  Send Email to jimh     
When you insert a pressure gauge in place of a spark plug and move the piston up and down to obtain a peak pressure reading, you are not measuring "compression." You are measuring the peak pressure in the cylinder compared to the static air pressure. If we assume the air pressure was nominally 15-PSI, then a reading of peak pressure in the cylinder of 240-PSI suggests (assuming the gauge measures the presure above the static atmospheric pressure) the pressure ratio is on the order of 17:1, that is (240 + 15) / 15 = 17. Using a formula published in WIKIPEDIA at Compression_ratio#Compression_ratio_versus_overall_pressure_ratio

that converts the pressure ratio into a compression ratio , we see that a pressure ratio of 17 suggests a compression ratio of 7.6:1. That seems quite reasonable for a typical four-stroke-power-cycle engine that can run on 87-Octane gasoline.

The math is as follows:

PressureRatio = VolumeRatio^1.4

Solving for a volume ratio when the pressure ratio is 17:1 produces a volume ratio of 7.6:1. This is also known as the "compression ratio" of an engine.

On that basis, the report from the technician that readings of cylinder pressure in the range of 235 to 240-PSI are common seems to be quite a reasonable statement, and I do not see a basis to suspect the technician of trying to mislead you about the readings.

As you already alluded, to have the range of pressure readings among all the cylinders be limited to a small value is also a good indicator that there is nothing unusual about the condition of any one cylinder.

jaymoussy posted 05-31-2015 09:29 AM ET (US)     Profile for jaymoussy  Send Email to jaymoussy     
Thanks for this very thorough answer.

I wish this full and proper explanation was more common, easier to find...!

jimh posted 05-31-2015 12:50 PM ET (US)     Profile for jimh  Send Email to jimh     
You are most welcome.
jaymoussy posted 05-31-2015 11:51 PM ET (US)     Profile for jaymoussy  Send Email to jaymoussy     
Extra info: I believe the stock motor should have a compression ratio of 9.5:1 - compare to your above computed ratio of 7.6:1.

Spoke to a marine broker I met, and he feels the 240 psi numbers that I got are high, and he will check with his mechanics.

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