You may be thinking of a barometer. Barometric pressure is affected by altitude at approximately 1HG for every 1,000 ft. The pressure at sea level is pretty close to 30hg, here at 6,200 feet it is pretty close to 24hg, even though the weathermen report it adjusted to sea level.
I could be wrong, but I don't believe barometric pressure will have a significant affect on the operation of an internal combustion engine which has been tuned for best performance in its current environment.
