COOL AIR VS. PRE-HEATED AIR:

[Stewart G Griffin]

Now, we know that cooler, denser air to the carb is going to make more power.  Hence, the hood scoops etc. at the dragstrip.  What i'm wondering is, does cooler air also improve mpg?

As you may or may not know, since the 60's to probably late 80's(i'm not sure how efi setups handle this), most cars came with air cleaners that mixed in underhood air with outside air to get the inlet air temperature around 77-115 degrees.  This helped with emmissions and supposedly helped the mixture burn better.  That's what the circular thing on the snorkle is for.

So what's the poop on hotter air for mpg?

[VileZambonie]

The idea behind warming intake air is for better complete combustion to lower emissions not to improve performance. The best economy comes from getting the most power out of the air fuel charge and using less fuel for the same distance traveled. So keep your foot light on the throttle and tune your engine accordingly for best mpg's. improving VE is the key.

[TexasRed]

I think you forgot the "discuss in great detail" part.

The thing about the track is that you see them changing jets and air bleeds to get the most part out of the engine depending on environmental factors. If you had colder air going into the engine then technically you could get more air into the engine, thereby leaning out the mixture. Colder air is denser so it'd be like having jets in your carb for denver, driving to houston and the engine would run leaner as Houston is below sea level and denver a few feet above (like a mile). That's assuming that temps at both are the same.

The point about controlling the incoming air temp is to improve driveability as the jetting is optimised for that air temp (kind of like a density control). It is somewhat true that a carb can deliver an incorrect fuel/air mixture at every RPM range, the snorkel and related crap is designed to increase consistency. Most drivers don't care about getting peak power when the stars align, they care about having power that's pretty much the same today, as it was yesterday and the same it will be when you floor it to impress those girls in the car next to you that you hope are not underage.

[Stewart G Griffin]

Sooooooo........you're saying the if i jet the carb correctly, or the carb may already be right for cold air only, then mpg will rise with cold air only as opposed to pre-heated?

[DnStClr]

Theoretically colder air should give an increase in mpg. More air(denser), more oxygen to burn. But the carb will adjust for the increase in air and pull more fuel into the airstream and the power increases. So Stewart feels the new power and steps on the throttle harder because that's what we guys do when we feel more power. The truck goes faster and the mpg's go down drastically, absolutely smashing our pretty theory to pieces.
 That's the trouble with turbochargers. They increase the temp of the incoming air charge, but with the newfound power they create, there goes the mpg's..
A guy can't win.      

[Captkaos]

My brothers 2000 Silverado actually picked up 4mpg after adding the turbo.

[Dragon]

My brothers 2000 Silverado actually picked up 4mpg after adding the turbo.

Sure you can easily pick up MPG's with a Turbo, Subaru has been doing it for years, but again once your foot finds the power, then there goes those new MPG's

[fishead]

I just had a friend over the other day looking at my truck, and helped me work on my Carb.  He is a pilot, and said that the air coming off the manifold is so that in the winter time, your carb won't ice up.  He was saying that airplanes have heaters built into the carb for that purpose.  His truck has an aftermarket air cleaner, and he said that he quite often has problems in the winter.  (Calgary, Alberta).  He said the only way to get it to run is to start it, stay on the throttle until the engine heat comes up, then shut it down for 10 minutes.  This gives the engine heat a chance to work it's way up into the carb.  Otherwise, with all the cold air racing into the carb, it will never warm up.  Once it gets all warmed up, he can run it fine all day.

He said a lot of other cool stuff that I can't remember because I was drinking beer. 

But my truck runs better...

[DnStClr]

Actually, when the pilot allows his engine to warm up it is heating up the intake manifold much more than the carb.