Lin,
Air density--the number of molecules per (any) cubic area--becomes less with
altitude, temperature and humidity, and also varies with barometric pressure
(essentially an altitude change at the same spot). The nominal elevation
adjusted for these parameters is known in aviation as "density altitude" and is
essentially the altitude an aircraft "thinks" it is flying at. At sea level, on
a hot, humid day the density altitude could be equivalent to a couple thousand
feet (a "standard" day is 15degC/59degF and 760mm/29.92 inches of mercury in a
column barometer).
Because of this you will lose power no matter what and the engine will run a
bit richer (SU carburettors compensate for this somewhat because the less dense
air will not raise the pistons and jet needles as high for the same throttle
opening). I live at sea level; when I drive above 3,000 feet--and expect to
stay there awhile--I'll open the throttles (slow-run valve on HD carbs) to get
the idle back up to 700-800rpm and lean a bit. Make changes you can remember;
e.g. "raised jets 4 flats to lean" and "opened throttles one-half turn." Then,
you can reset the carbs when you descend (HD carbs make this really easy, I'm
less familiar with other SU models).
Bob
----- Original Message -----
From: "Linwood H Rose" <linwoodrose@mac.com>
To: "healeylist" <healeys@autox.team.net>
Cc: "Michael Oritt" <Awgertoo@aol.com>, "Michael Salter"
<michaelsalter@rogers.com>
Sent: Saturday, May 16, 2009 10:44:25 PM GMT -08:00 US/Canada Pacific
Subject: [Healeys] Elevation effects on SU carbs
I am normally driving my car in Virginia, but I am now in the
mountains of Colorado. I anticipate some negative impact on engine
performance tomorrow. How should the carbs be adjusted to compensate
for the elevation - 7,200 ft?
Lin
1960 The Bloody Beast
1959 bugeye
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