Calculating compression based on elevation?

[slipnfall]

Hi folks, I'm curious if anyone can tell me how to scale the compression numbers in the FSM to reflect my current altitude. Is this assume to be at sea level? I did a compression test on the east coast in March('92 S13), and it was 175-180 across the board. This was at ~550'. The elevation(roughly) in NM is 5300', where I want to re-test. I'm wondering where I should be at. I'm looking for a calculated number, not just a guestimate. A friend's '93 DE tested at 120-130psi here, but I don't want to base off just someone else's results.

Any thoughts? Thanks.-Jamie

[PoorManQ45]

compression is not affected by altitude.

Take the volume of the combustion chamber dead bottom dead center and the volume at top dead center and you've got your ratio

[slipnfall]

How does the compression ratio relate to cylinder compression? Do you just take the C/R times atmospheric pressure(14.7), to arrive at the expected cylinder compression result?

[slipnfall]

Going along with the above, because I am at a different altitide, my atmospheric pressure will *not* be 14.7psia. Just using a quick reference chart(http://www.uigi.com/Atmos_pressure.html), I'm sitting at about 6000' above sea level, which equates to an atmospheric pressure of around 11.8psia.

So if a C/R of 9.5 is at sea level, one would expect a cylinder compresion of 139.6psi.

At 6000', cylinder compression is 112.1, which is a 20% reduction.

So how does the FSM arrive at values of 151psi minimum to 179psi standard?

[PoorManQ45]

oops, sorry, I thought you were talking about compression ratio, not compression.

Then yes, you're correct that the compression(pressure) will change with altitude.

Sorry for the bad response

[slipnfall]

No problem, I had a feeling that's what you were getting at.

So the question still stands then, how does an engine achieve 179psi from a C/R of 9.5:1, at sea level?

[UK-SRi]

Because when you compress it you also heat it up, so the pressure rises more.

Mike