<VV> Fuel milage and the cold

[Mikeamauro at aol.com]

"...Does colder temperatures give you worse or less gas mileage?? I  have 
to drive my '63 84/pg year-round and have noticed my mileage drop from  21/22 
on average to 17/18.? Comments?? Physics reasons??..."


Well, Randy, in cold weather the chokes are "in" for a longer time,  and 
this means the engine is seeing richer fuel, longer. "Richer" generally  means 
lower fuel mileage numbers. I suppose, if one takes a lot of short  trips, 
then a significant drop in fuel mileage may be seen in colder  weather. To 
lessen the time the engine is being "choked," one would want to be  sure the 
choke and fast idle adjustments are correct, and the bellows  thermostats, 
with the engine cold, are fully closing the rear air outlet  doors.
 
Now, about the effect of cold weather on air-fuel mixture... colder  air is 
denser (this means, for an engine, at a given elevation, more oxygen  to 
work with). Therefore, with a carbureted engine (after the chokes are  fully 
open), as the air temperature falls, the AF ratio increases (the mixture  
becomes leaner). For example, my 67 coupe is equipped with wide band O2  
sensors and digital AF gages in the cabin... in the summer the AF ratio, at  
cruse, is consistently 14.5 to 15.5:1. Now, with the air  temperature primarily 
below 60-degrees, it is not unusual to see AF ratios  above 16:1 at cruse. 
When I get into the petal, as I'm using the later, before  smog, fuel 
enrichment carbs, the ratio drops to a safe range  for acceleration (but that's 
another topic). With colder meaning  leaner, one would think colder would 
automatically mean better fuel mileage...  maybe, and this is purely 
theoretical-speak: there is a point for any  engine-vehicle combination where a too lean 
condition (besides causing engine  damage) can cause efficiency to "fall" 
off the performance curve.  Finding and holding the "sweet spot" with a 
carbureted engine is a challenge;  hence, precisely the reason fuel 
injection--especially the modern,  microprocessor-controlled version--was  developed.     


Mike Mauro
of several Corvairs  http://community.webshots.com/user/mikeamauro