Compression changes with altitude?

[shortfuse]

I was told that the compression of a motor changes varying on the altitude? For Example if a car was compression tested in Denver where the elevation is extremely high, and it reads 6.5 all around both chambers, and then is tested in lets say Los Angeles where elevation is very close to sea level, that same compression number will rise to maybe 7.0 - 7.5 ??? Is this true? Any feedback is appreciated Thanks

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[shortfuse]

bump

[shortfuse]

anyone got an answer to this???

[Maximum]

The compression ratio does not change because that's a volume ratio, but it is true that the maximum pressure reached in compression is lower at altitude. Denver is the "mile high city", so the altitude is about 5280 ft. At that elevation, the atmospheric pressure is about .8 atmospheres as given by http://www.personal.usyd.edu.au/~gerhard/pressure.html . So if your compression tester measures 7.5 bar at sea level, you will see about 6 bar in Denver. Feel free to tell me if I'm not considering something here.

[shortfuse]

okay, i understand, so the compression of the motor DOES change but not the actual compression ratio. So if the car was tested at about 6.5-6.7 I should see around 7.8-8.0 compression at sealevel, which i believe is very good.


Thank you
anymore feedback is appreciated.

[ehos]

When you do the compression test, you have to factor in RPM's, temp. and altitude.

For example, FD's have/show lower compression and are still 'fine' up here in Edmonton. Ie, I had a FD with 80PSI all around. That's 'good' up here, but it might be marginal at sea level.