I can see that the ratio of combustion chamber size to total cylinder size doesn't change, but doesn't total compression change depending on how much vacuum is present?

Ex. If I had a compression gauge and cranked an engine over with the throttle closed and then again with the throttle valve open, wouldn't I get a higher psi reading with the throttle open while cranking?

I should have said higher compression psi in my original post.

Yikes.

I thought it was already established that a warm air intake was better for FE than a CAI.

you're right, but thats not usually talked about in those terms. you just described what we've been talking about with pumping losses and throttle plate restsrictions but backwards.

I am testing different temps on my 99 Metro 3 cyl 1.0L as we speak.

I just received my ScanGauge a few days ago and I am waiting to burn through a tank to properly set it up for measuring FE.

I will play with different temps and let you all know what I find.

I would think the ideal is hot engine + cold air. Or, rather, dense air. Cold is one way to get denser air, lower elevation is another, and turbochargers are another. You don't see the benefits when they are masked by a long warm up time because it's colder outside. It also obscures things if the vehicle has lousy aerodynamics.

Turbo and super chargers make the engine more efficient by ramming more air in. True, that allows more fuel to be burnt, and consequently more power, but it does improve efficiency. Think about it: moving those pistons back and forth takes energy. The less movement there is per unit of power produced, the more efficient the engine. Now most people use this to burn more rubber instead of less gas. One would need to make the gearing taller to really benefit.

In the absence of a 'charger, colder denser air gives a little of the same benefits. The only reasons for hot air is fuel vaporization, and ever so slightly less air resistance. Better aerodynamics makes air resistance less important. For cold air, the advantages are more power per power stroke, and possibly a slight push from the air expanding even before combustion just from being rapidly heated by the engine when it enters the chamber during the intake stroke. As for vaporization, air is easy to warm up. Once warmed up, a properly designed engine should have no problem with that even in extreme cold such as -40 degrees.

Let's ask a similar question. Is FE better at sea level, or at 10,000 ft elevation? Anyone have any numbers on that? I expect sea level is better.

Yeah, but it's fun to argue! I think it has been established that it does, but now they're arguing why?

Well, I live in CO around 5200 feet. I regularly go driving up into the mountains and usually see a noticeable drop in FE. Mainly because I have to slam the pedal down a lot more to achieve the same speeds at 5k feet.

It may not be as noticeable going from 5k to sea level tho. I would assume the o2 sensor would, to a degree, regulate the air/fuel mixture.

Yes. Taking readings to regulate the air/fuel mixture is it's job.

The air sensor located between the air filter and throttle body does its best to tell the computer how much air is entering the engine. Fuel is injected to match the amount of air. Oxygen sensor reads O2 present in the exhaust to detect whether the mixture is too lean or too rich. Mixture is adjusted accordingly, so it's a feedback-loop system.

Ya get more air and thus more fuel into the cylinder if it's colder air. But it burns somewhat more efficiently if it's warmer. Dunno the ideal temperature but I'm pretty sure it's well above the 20-40 degree temps we typically have in winter in my area!

By now I've got the system pretty well restored.

Hot air hose is in place, wrapped with pipe insulation which is spiral-wrapped with duct tape. Cold air supply is also in place. Added thermometer sensor is in place inside the air box. New tube directs driving airflow at the small air inlet at front of airbox which feeds cold outside air to the thermostat inside. (I discovered the airbox thermostat is designed to read the outside air temp, not the resulting mix).

Intake air temperatures are in the mid-50's F with outside temperatures around 30 deg. I think warmer would be better but I haven't been able to achieve that.