Engine Compression at Altitude
Engine Compression at Altitude
(OP)
Greeting to you all. This is my first post on Eng-Tips.
Can anyone tell me how to calculate decreased cylinder compression of a naturally aspirated internal combustion engine at various altitudes? Actually, I will work the formula backwords:i.e. I have measured compression numbers and want to determine how much the decrease from OEM specs (assuming sea level) is related to altitude and how much from engine wear.
I read the thread on measuring the engine output decrease in horsepower but could find nothing in the archives on compression.
Thanks in advance for any guidance.
Can anyone tell me how to calculate decreased cylinder compression of a naturally aspirated internal combustion engine at various altitudes? Actually, I will work the formula backwords:i.e. I have measured compression numbers and want to determine how much the decrease from OEM specs (assuming sea level) is related to altitude and how much from engine wear.
I read the thread on measuring the engine output decrease in horsepower but could find nothing in the archives on compression.
Thanks in advance for any guidance.





RE: Engine Compression at Altitude
You will likely not be able to tell the difference. Another factor of a compression test is variance between the cylinders. If it is really that important, you should be doing a cylinder leak down test instead of a compression test.
RE: Engine Compression at Altitude
RE: Engine Compression at Altitude
Yes, perhaps I ought to start over with a leak test and lab oil sample analysis but I have this compression test data and need to make conclusions about the unit's future. Time for additional tests I do not have.
Clearly, the per cent variance between the cylinder compression numbers is probably most important of all.
RE: Engine Compression at Altitude
al1
RE: Engine Compression at Altitude
CODE
REM Written in BBC BASIC using free demo version from
REM http://www.cix.co.uk/~rrussell/products/bbcwin/bbcwdemo.exe
REM Formula from http://en.wikipedia.org/wiki/Atmospheric_pressure
DEF FN_pressure(altitude)=10^(5-altitude/15500)
ratio = FN_pressure(7800 * foot) / FN_pressure(0 * foot)
PRINT "Pressure at 7800 feet is ", ratio / percent, " percent"
PRINT "155 -> ", 155 * ratio
PRINT "160 -> ", 160 * ratio
Prints
Pressure at 7800 feet is 70.2452195 percent
155 -> 108.88009
160 -> 112.392351
RE: Engine Compression at Altitude