From digest.v7.n51 Tue Aug 5 11:05:21 1997
From: Jerre Goodmon <goodmon_at_bostech.com>
Date: Tue, 5 Aug 1997 11:35:05 -0400
Subject: Compression Test Results

I just responded directly to a post regarding compression test results and I sensed some concern in that the figures were "off spec". I picked up that the Bimmer-under-test "lived" in Phoenix and it reminded me of the first time I ran a compression check after moving to Albuquerque. The results were low, about twenty percent low; and then, I did some thinking about the effect of altitude on compression -- Albuquerque's altitude is 5,280 ft. (Denver doesn't have a corner on this market). If you've ever jogged above sea level, you know the air is thinner (less atmospheric pressure). You would then expect that compression figures would be less and, in fact, they are. So what's "normal" for your location. If you know your elevation, it's relatively easy to adjust the figures for sea level or vice versa. Here's a table that will help. I know it's not perfectly accurate, but you may interpolate for intermediate elevations. We're not dealing with brain surgery. Sea level standard pressure is 14.7 psi or 29.92 in. Hg. So, you could expect the following:

Sea level (14.70 psi) - normal values straight from the book. Let's use 145 psi

        1000' elev. (14.17 psi) - 96% normal or 140 psi is normal
        2000' elev. (13.66 psi) - 93% normal or 135 psi is normal
        3000' elev. (13.17 psi) - 90% normal or 130 psi is normal
        4000' elev. (12.69 psi) - 86% normal or 125 psi is normal
        5000' elev. (12.23 psi) - 83% normal or 120 psi is normal
        6000' elev. (11.78 psi) - 80% normal or 116 psi is normal

That should just about do unless you live in Nepal in which case, half the earth's atmosphere is gone at 18000' and so is half your compression...

Thanks,
Jerre
'87 325ic 247k (and still strong, head's never been off)