i've noticed a significant variance in air intake temps and subsequent mpgs as well comparing morning vs afternoon commute. the route is exactly the same except that the morning drive has less traffic and fewer stops. the morning ambient temp has been consistently ~20*F lower than the drive home. in the summer months the difference can be much more varied. morning and afternoon intake air temps also vary about as much.
besides temp, volume of traffic and fewer stops, all things are equal. but, i'm getting ~5mpg avg BETTER in the afternoon. this would make sense if it were winter when it could be near freezing in the am, and in the 60s pm. this is scan gauge verified, of course, but the 3 or 4 vehicles i've used with SGII didn't produce these type of results.
my experience has been that i used to get near equal numbers(am and pm) when the a/c was used for pm drives home. and if i used no a/c, my yield was greater on the way home, but not 5mpg!
so does this car(97 civic LX 5 speed), actually have significant gains when warmer air is introduced, even unmodified?
well, today(after work) i twisted my air intake tube towards the engine and exhaust, instead of it's stock position of away. i'm interested to see if my iat goes up and if that produces more mpg. for reference, morning ambient temps are in the 60s not quite yielding 40mpg, and afternoons are in the 80s yielding ~45.
this effect can be attributed to pumping loss. large displacement engine wont have as much effect on mpg as you would on smaller displacent. the idea is to rob the engine power to its minimum power so the throttle can be open more, that increases the load but also minimize the vacuum, and vacuum mean more power from the engine to produce. cold intake temp means dense air= more power, hot air is to more disperse air molecules yet yielding the same afr. this can be tested by installing a vacuum gauge or by putting a volt meter on the throttle signal/map sensor so you can check the difference. in reference you can ask mr.pgpro since he cruises on 103 kpa on his 60 something mpg civic
do not deny the suns importance of intake temps, when the sun beats on the black pavement we drive on, this is also another source of warm air, same with the sun hitting your hood if its darker.. when the clouds come out this decreases road temps and rain just eliminates heat completely, keep in mind that the sun will heat your car interior even when its cloudy out, cosmic gamma dowhatshickyrays
living in suburbia where the malls have paved over our part of the planet, you can feel the different a driveway sealcoat and fields of pavement makes when you get out of your car on those 90 degree days boston providence baby cookie cutter strip malls
Your 20 degree warmer afternoon temps mean thinner/lower viscosity engine oil and transmission fluid, wheel bearing lube, etc. My ABA testing of a WAI showed no improvement from increased intake air temperature.
75 degree Spring weather means I can get mid 70 mpg on my test loop v. low 60 mpg in 55 degree Winter temperature. Warmer intake air doesn't improve those mpg figures.
i've driven 3 other vehicles using the scan gauge. NONE have had this great a difference in regard to exact route, morning vs afternoon.
no elevation difference--it's all flat, w/ exception of 2 overpasses.
in regard to ambient temp difference...i've not seen this great a difference w/ exception of those crazy florida days when we have freezing mornings and 70s in the afternoon. there's not even that much variance comparing a typical florida summer day...say 60 overnight and near 100 during the day w/ the exception of a/c usage being a factor. and i did test my usage w/ air in another 4 cyl vehicle--the consumption gave a loss of no more than 2mpg if i remember correctly.