Hot air or cold air better for mileage?

[Talyn]

Is hot air or cold air better for fuel mileage? The reasons I am asking is because i have been thinking of installing a vacuum valve (maybe a supercharger bypass valve) to pull warm air from the engine bay for better mileage when cruising. The valve would close when vacuum drops. However, I would also be concerned about when the valve does close would the hot air heat the intake enough to warm the now cold air?

 

[old_man]

All things in moderation. Normally cool air is better for an engine, (superchargers and turbos over heat the air) but if it gets too cold, the computer tends to have problems regulating the mixture. It is common for gas mileage to drop when it is cold.

 

[Talyn]

I know cool air is better in terms of power, but would the same hold true for mileage? Hot air is less dense so it would require less fuel per cylinder fill. However, cold air is more dense, which would require more fuel, but make more power, possibly requiring less air to make the same amount of power as with hot air.

 

[Milford Cubicle II]

Better question: is there such thing as cold air on a 4.0?

 

[Talyn]

Better question: is there such thing as cold air on a 4.0?

If you do it right, yes. http://naxja.org/forum/showthread.php?t=1064001 Also, insulating the intake manifold helps

 

[hubs97xj]

Even a "cowl snorkel" setup can get you much cooler air.

I can't really make any definitive claims, but I believe that I got the best mileage with an open conical filter. That was also the setup with the highest intake temps most of the time.

Why would they have gotten rid of the cut-out in the front crossmember? My friend's 96 has it, although it's got a plastic cover from the factory. My 97 does not. I've wondered if that was to force the engine to draw hotter air from the engine compartment, presumably to reduce emissions/increase mileage. It could have just been a cost saving measure, but the change from a duct, to just the opening, to a cover, and then finally removing the cut-out altogether makes me wonder.

 

[Talyn]

I can't really make any definitive claims, but I believe that I got the best mileage with an open conical filter. That was also the setup with the highest intake temps most of the time.

The CAI and cone filter companies to advertise more mileage, but is it due to less pumping losses from less filter restriction or from the cooler air?

Why would they have gotten rid of the cut-out in the front crossmember? My friend's 96 has it, although it's got a plastic cover from the factory. My 97 does not.

I don't know either. It is puzzling as it just opens the airbox into a wall and kills flow. One of the first mods I did on my 2000 was to go with Rusty's setup. Immediate and considerable increase in throttle response... don't know if power actually increased or not, but I definitely felt it.

Also, ceramic coating the header helps temps as well.

 

[Milford Cubicle II]

If you do it right, yes. http://naxja.org/forum/showthread.php?t=1064001 Also, insulating the intake manifold helps

What a shameless, filthy plug :twak:

Personally, I'm doing a cowl intake. Also, you don't want to insulate the intake manifold, you want to reflect as much radiant heat off the bottom of the manifold from the exhaust while allowing as much excess heat as possible to be dissipated off the top of the intake manifold, since most of the heat is conducted from the head anyways and there's not much you can do about that - short of maybe some kind of silicon manifold gasket. But yes, insulating the intake tube is a good idea if it's metal. Plastic obviously doesn't absorb or transfer much heat, and since the actual intake tube is fairly well insulated from the metal throttle body, you don't have to worry about it heating up the intake tube.

Back on topic, the relative complexity of what you're toying with wouldn't be worth the (if any) potential gains, IMO. You would just be trading less load for less fuel - which is the same thing since less load automatically decreases fuel consumption and less fuel automatically increases engine load which of course increases fuel consumption. So it's kinda pointless. Good thought though.

 

[Talyn]

Also, you don't want to insulate the intake manifold, you want to reflect as much radiant heat off the bottom of the manifold from the exhaust while allowing as much excess heat as possible to be dissipated off the top of the intake manifold, since most of the heat is conducted from the head anyways and there's not much you can do about that - short of maybe some kind of silicon manifold gasket.

Lol.. thats pretty much exactly what I did. I borrowed Bryson's idea (and image here) and covered the bottom in heat reflective material as well as coating my header in ceramic.

But yes, insulating the intake tube is a good idea if it's metal. Plastic obviously doesn't absorb or transfer much heat, and since the actual intake tube is fairly well insulated from the metal throttle body, you don't have to worry about it heating up the intake tube.

My air tube is plastic, but there is a "sock" of sorts made to cover a metal air tube.

Back on topic, the relative complexity of what you're toying with wouldn't be worth the (if any) potential gains, IMO. You would just be trading less load for less fuel - which is the same thing since less load automatically decreases fuel consumption and less fuel automatically increases engine load which of course increases fuel consumption. So it's kinda pointless. Good thought though.

Its not really that complex. Once I decide on a valve (doing the proper research and testing as well) its a simple matter of cutting a hole in my air box (which I don't want to due for 0 gains), installing the valve, routing a flex pipe to the area of the exhaust manifold, and routing a vacuum line.

There is also a complex way of doing it which I though of. Some cars have a flap in their air box that opens at WOT. Steal flap, install in air box, run power to arduino multi gauge and tap in to the TPS to close valve at certain TPS value... alternatively I could set the AEM FIC to control that too with its 12v out.

 

[Milford Cubicle II]

I wish I woulda ponied up and bought my header with the ceramic coat :doh:

And I purposefully used the word "relatively" when I said it was complex. Meaning that it's complex compared to have no bypass system at all. It's really not that complex, I just don't think you'll see a difference. Also, I would think that if there were something to the idea, it would already be employed by other auto manufacturers - which to my knowledge, it is not. I could be wrong though, good ideas always start somewhere, and it's not always with the the big time pros.

All I can say is either try it, or don't. But if you do, document it well - ya never know.

 

[techno1154]

I know of people who had their intake manifold ceramic coated. Would that help performance seeing the intake is insulated by the ceramic coating therefore provides cooler air?

 

[Johnnie Walker]

So, you basically want a better version of the early, stock thermostatic air cleaner valve?
http://www.4x4trailhunters.com/media/pictures/article_16_1._Thermostatic_Aircleaner_Valve.JPG

 

[Talyn]

I though Renix had a device like that. Judging from the name of the image it is for cold starts though.

 

[CherBear]

Theoretically hot air would give the highest mpg as long as:

1. You keep your foot out of the pedal, don't go anywhere near WOT.

2. The air is not so hot as to cause problems with FA regulation.

Although in reality you probably wont see any major change because your vehicle still needs a certain amount of power to accelerate and maintain speed. Therefore, if it is hot out, you will use less fuel per combustion cycle, but require higher RPMs. This would negate all savings IMO.

One other thing to consider is that most of the energy a car makes at highway speeds is used to move the air out of it's way. Especially with a vehicle with such poor aerodynamics as ours, a lower density fluid (aka hot air) would require less energy to drive though.

Alright well I think I have done enough armchair engineering for one day...

 

[bmpweXJ]

One other thing to consider is that most of the energy a car makes at highway speeds is used to move the air out of it's way. Especially with a vehicle with such poor aerodynamics as ours, a lower density fluid (aka hot air) would require less energy to drive though.

That's a great observation. I had never considered this before. I wonder if it makes an actual quantifiable difference...

 

[Dr. Dyno]

Is hot air or cold air better for fuel mileage? The reasons I am asking is because i have been thinking of installing a vacuum valve (maybe a supercharger bypass valve) to pull warm air from the engine bay for better mileage when cruising. The valve would close when vacuum drops. However, I would also be concerned about when the valve does close would the hot air heat the intake enough to warm the now cold air?

What you've just described is a similar principle to the EGR system. The EGR recirculates hot exhaust gases to the intake system so that warm-up time is faster (shorter open loop time) and pumping losses are reduced, thus improving fuel economy under part-throttle acceleration and at constant throttle cruise. I doubt you'll gain much more than 1mpg and you can gain at least that much just by moderating your driving style.

 

[Shorty]

What you've just described is a similar principle to the EGR system. The EGR recirculates hot exhaust gases to the intake system so that warm-up time is faster (shorter open loop time)

I'm not so sure that is the principal theory behind the EGR system. I believe it was designed to recirculate exhaust gasses into the combustion chamber to reduce the creation of oxides of nitrogen through decreased combustion chamber and exhaust temperatures.

 

[Talyn]

Thats right. The EGR is to reduce NOx levels by cooling the combustion chamber. Its purpose isn't for mileage gain, but there may be a small impact.

 

[offroadordnance]

The reduction of combustion temp allows leaner mixtures which saves fuel. Also, the exhaust displaces fresh incoming charge and fuel necessary to maintain safe A/F ratio. the more Egr that can be introduced the greater fuel savings. Of course this is only feasible at high vacuum/light load which is why Egr is regulated by intake vacuum.

 

[bigalpha]

I thought engines were more efficient at WOT?

Also, read an article stating that accelerating to your cruising speed a little quicker is better for your mpgs because you reach a better gear ratio quicker.