how to find cylinder compression PSI/BAR number from compression ratio?

[imcool]

hi,
I am trying to measure the compression ratio of cylinders and I have the factory repair manual and it says that compression ratio is 10.9:1 for my motorcycle.

I have no idea how to get the PSI from that ratio because the gauge is in PSI.

I googled and didnt  get correct information, perhaps someone has done it before can help me out.

I read answers from website below and got confused. how are we supposed to find the correct PSI number for our engine cylinders?
https://www.quora.com/How-do-you-convert-a-compression-ratio-to-PSI

This is the tool which is in PSI/bar


regards

[trophied]

I bet plenty of members here can give you some insight, but I would think having all the cylinders measure close to the same pressure would be an indicator of the health of the engine unless the pressures are really low.

[Coconut]

I had to do some research and believe the following to be correct - please let me know if not !

The "Compression Ratio" is an expression of the ratio between the uncompressed volume of the cylinder,
with the piston at it's lowest position ( "Bottom Dead Centre" ) and the fully compressed volume
of the cylinder when the piston is at it's highest position ( Top Dead Centre" ).

You cannot alter the compression ratio of the engine without some major engineering
such as changing Pistons and cylinder bores etc.

As trophied says, it is usual to compare the individual compression pressures of the 3 cylinders,
which should all be more or less the same.  If one ( or two ) is/are noticably lower than the other(s)
this should be investigated further as it may indicate a problem with the valves, piston rings,
cylinder bore(s), or gasket sealing of that/those cylinder(s).

To get an idea of the approximate pressure of a cylinder under compression,
we will use the compression ration of 10.9:1 that you quoted,
which means the compressed volume is 10.9 times smaller than the uncompressed volume.

NOTE : My Owners Handbook AND Service Manual both quote a compression ratio of 11:1 ! 

( If the volume of a single cylinder is 405cc ( 1215cc divided by 3 ),
then it's compressed volume at a Compression Ratio of 10.9:1 would be 37.156cc,
however these figures are not needed to calculate the compressed pressure ).

To calculate an estimate of what the compressed pressure should be,
obtain the Atmospheric pressure at the location of the test,
and multiply it by the Compression Ratio of the engine.

Local Atmospheric pressures can be obtained from Weather Apps, or here :

e.g.  Radar Live.com

Where I am right now the Atmospheric pressure is 1027mb

To convert mb ( millibars ) to psi ( Pounds per square inch ) multiply by 0.01450377

1027mb x 0.01450377 = 14.895375665 psi.

The estimated / approximate Pressure of the engine at maximum compression 
( assuming a "perfect" engine which returns to Atmospheric pressure at Bottom Dead Centre,
and has no leakage from valves, piston rings, cylinder head gasket etc. under compression ) would be :

Atmospheric Pressure ( 14.895375665 psi ) x Compression Ratio ( 10.9 ) = 162.359594 psi

Hope this helps !

Cheers 

[imcool]

 

[STJIM]

162 psi is not an unreasonable number based on numbers I have seem for many other car & MC engines.  What's important also is the variation between cylinders as previously mentioned.

Make sure you have all the spark plugs removed and the throttle plates of the throttle bodies wide open when you do the test or the readings will be low.

It may be a little "tricky" getting the adaptors screwed in to do the meaasurements  BTW, because of the depth of the spark plug holes.  Please let us know how that works out for you.

And just curious,  why are you doing the testing in the first palce ?   Also, doing a leak down test is a better test to do.

[PhilInAthens]

Originally Posted by STJIM

And just curious, why are you doing the testing in the first place ?   Also, doing a leak down test is a better test to do.

Thanks STJIM for asking this question. I am curious to know the answer myself! 

[STJIM]

If my bike was running good: Good idle and power and was getting good gas mileage and not using any oil,  I would feel it wasn't neccesary to check the compression especially since it's not that easy to do.  Maybe during a 20K mile maintenance check ........

I have an old 1976 Moto Guzzi Convert - the cylinder stick out at a 90 degree angle.  It takes about 20 minutes to do a compression test so that's another matter.  I might do a compression check just to satisfy muy curiosity.  When changing plugs it probably takes only an extra 8 minutes to do a compression test.  Gotta love those old bikes in some regards.