[Healeys] Elevation effects on SU carbs

[Bob Spidell]

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 at mac.com> 
To: "healeylist" <healeys at autox.team.net> 
Cc: "Michael Oritt" <Awgertoo at aol.com>, "Michael Salter" <michaelsalter at 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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