Agreed - but having had one when I was younger, thinking my little Saturn was a hot rod, and noticing that I did get better mileage with the cold air compared to the stock one when I watched my throttle for a tank - I do believe that the reduced throttle requirement outweighs the 3% smaller opening into the intake manifold, since a 20*F decrease in temperature yields about 3.5% more oxygen per cubic meter.

Like I said - if you're very careful on the throttle, you can see a benefit - but not as good as a warm air.

to be completely honest, before I did my WAI, I could tell you what the outside temps were within about 3 degrees just by looking at my IAT reading. maybe my intake is designed better than most. the only benefit to a CAI that I can see is a freer flowing intake witout the baffles of some intake boxes (which mine doesn't have) and a less restrictive filter (there again, crud in the engine).

I also remember hearing somewhere that a less restrictive intake and exhaust will actually push your torque peak farther up in the RPM range which will hurt your FE. not sure about that one but if it did reduce your torque in lower RPMs then you would definitely get worse economy.

I've had a philosophical problem with the WAI idea for a long time, but have held my tongue. Correct me if I'm wrong but you're saying that the primary gain in efficiency is through a decrease in air density, right?

If decreased density improved efficiency, then it would follow that you'd get better fuel economy at high altitude. In my life I have lived primarily at two altitudes; 1000' asl, and 7000' asl, and I've owned the same vehicles in both locations. I can tell you for a fact that I get an average of 2-4 mpg lower gas mileage at 7000 ft than at 1000 ft. I've kept paper gas logs on my vehicles for the last 17 years, so my degree of confidence in this statement is very high.

Unless you can describe some other reason that a warm air intake will increase FE, I'll remain unconvinced. The one thing that I am convinced of with the WAI is that you'll decrease your warmup time which would probably yield a noticeable difference in your MPG on short drives and in cold weather.

There's a bit more involved than just that. With the intake temp, primarily where the source of air is, and where the sensor is, is of primary concern. My little saturn sucks it from behind the driver's headlight, and has an IAT a few inches downstream. How much that air is heated up as it travels down the tube, I've no clue. I'm sure it's under 10 degrees, but it's an unkown in the equation.

The exhaust design considerations has a much bigger impact than the tube going to the intake manifold does on where the powerband is. When I was doing Formula SAE, we would dyno the piss out of the header design to tune the torque curve. We went through about 12 header concepts. We went through 3 intake manifold concepts, with the biggest changes being throttle response and intake temperature. Scavenging, exhaust temperatures, and exhaust velocity all had huge roles in the torque curve.

The intake tract, especially without access to a dyno, fluid flow analysis software, and a lot of patience, will only effect the intake temps and oxygen consumption - not the shape of the torque curve itself.

The effect on torque isn't about how restricted the intake and exhaust are. It's about the length the air has to travel between the throttle and the valves, and EGV.

Restriction wouldn't matter to most hypermilers anyway; if the stock system can flow (somewhat restrictedly) enough to make three or four (or more!) times as much horsepower as you'll be using, it's got to flow completely freely under the conditions in which we use it. For us, and probably for most others, the throttle is the biggest restriction and everything else flows freely to and from the throttle.

Not only is there the decreased volume of oxygen going into the engine, and reduced warmup times but the gasoline should vaporize better in warm air than in cold promoting more complete combustion.
I'm sure there are other factors at work here as well.
-Jay

This is DEFINATELY true. Take a peek under the hood of any C/K chassis, R/V chassis, or any muscle car and look for the thermo tube going from the carb hat to the exhaust manifold, as well as the flapper valve for it.

However, the decrease in oxygen content from 6000 feet of elevation difference tosses another player into the role - barometric pressure. At lower outside pressures the engine has to work harder to create a vacuum to keep going given the same air intake (least this is according to my thermodynamics professor - I haven't taken the time to do the math, nor do I really care enough to do so).

So the difference may very well have to do in your case with not only the decrease in oxygen per cubic meter, but also in the decrease in air pressure. Just something to think about.

That heat-scavenging effect maybe also increases efficiency after warmup -- instead of discarding that heat energy, it's put back into the engine. Additionally, the warmer air may enhance fuel vaporization.

Just a couple guesses.

There could also be other factors at work for your high-altitude observations -- for example, the lower pressure air outside the engine would increase pumping losses effectively nullifying the WAI's density advantage. Hills could cause you to drive less efficiently, climate might require the fan to be used a lot, etc

Edit: D'oh, I was too slow. :)

It's not the restrictive part of the intake and exhaust that moves power around. It's all in the velocities. Smaller pipes yield higher velocities and more low-end torque at the expense of top-end power. Exactly the opposite of large pipes.

I have to agree with the altitude statement. I lived in San Antonio at 650ft and my best tank using coasting and such was 33.8 mpg all city. Here, forget it. My best in town tank so far here at 5800ft has been MAYBE 27. And that's because 1/2 of my drive to work can be spent in neutral with the engine shut off. I have to press the gas down more here to get the same power I would need to use at 650. According to the WCF on the dyno i'm down 20% of the power at sea level as is.

If you had two engines, one here and one at the coast, and ran them both WOT the engine here would use 20% less fuel and make 20% less power.

And this is exactly why Willies used engines with .2 higher c/r for high altitude dealerships than those closer to sea level. :D