At 05:26 PM 5/12/98 -0400, ENRIQUE CLAURE wrote:
>I was wondering, if measuring car compression at sea level and at high
altitud say like Pikes Peak in Colorado will give same values? I was told
that measuring at higher than sea level will give lower readings? any
comments?
Atmospheric pressure decreases with altitude. And in the range of altitude
of interest (driveable roads) the pressure drop is nearly linear, so you
can easily interpolate between the numbers below.
The easiest number to work with is the ratio between pressure at altutude
compared with pressure at sea level. If you use this ratio, all the rest
of the factors cancel out. These ratios are as follows.
Atmospheric Pressure Ratio (APR)
At sea level 1.0000
At 5,000 Ft 0.8320
At 10,000 Ft 0.6877
At 15,000 Ft 0.5643
Starting with the compression _gauge_ reading you get at sea level, just
multiply by the APR for the alitude of interest to get the _gauge_ reading
at that altitude. For example, suppose you get a pressure reading of 140
psig at sea level. For other altitudes you would get the following results:
Compression Reading With Altitude
At sea level 1.0000 x 140 = 140 psig
At 5,000 Ft 0.8320 x 140 = 116 psig
At 10,000 Ft 0.6877 x 140 = 96 psig
At 15,000 Ft 0.5643 x 140 = 79 psig
Because of temperature drop at higher altitudes, the air density is not
reduced as much as the pressure. The density ratio is also nearly linear
within the range of interest:
Atmospheric Density Ratio (ADR)
At sea level 1.0000
At 5,000 Ft 0.8617
At 10,000 Ft 0.7385
At 15,000 Ft 0.6292
These numbers indicate the reduction of the mass flow of air that your
engine can take in at various altitudes, and in turn are roughly
proportional to the power that your engine can make in the thinner air.
Suppose your engine can make 80 HP at sea level. You can multiply the sea
level power output by the ADR to get the expect power output at other
altitudes:
Power Output With Altitude
At sea level 1.0000 x 80 = 80 HP
At 5,000 Ft 0.8617 x 80 = 69 HP
At 10,000 Ft 0.7385 x 80 = 59 HP
At 15,000 Ft 0.6292 x 80 = 50 HP
As I recall, the access road to Pikes Peak Park starts at about 8,500 feet
and ends up at a little over 14,000 ft. The result of the thin air at
these altitudes is that you enter the gate with about 62 HP and reach the
top with about 52 HP. Now you can see why people who drive a lot at higher
elevations can be such great fans of turbo chargers.
Barney Gaylord
1958 MGA with an attitude (and 72 HP an a good day)
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