Hey -

I'm a regular forum guy on another Volvo board but figured I'd run this by ya'll.
I have a 96 Non Turbo Volvo 850. I've noticed SOME 96 850's have this, and some don't. My air box has two vacuum lines on the cover side facing the firewall. They're both placed vertically. One line runs direct to the vacuum tree mounted above the throttle body. The other runs to a port on the LOWER half of the air box which appears to control a door on the airbox. I accidentally knocked this vac. line off previously and it KILLED my gas mileage. I was just curious how this vacuum line controlled the door. When I noticed it was off, the door was stuck open. So I'd assume the mpg should have been fine....but it was horrible. When I reconnected it, gas mileage went back to normal. I wondered if it because of the Vac leak, or because of the door itself, and wondered HOW it controlled the door.... for those of you that don't have the same setup, or don't understand my description I put a picture below.

 

Motorcyclists and car enthusiasts use the term airbox for what might more properly be described as an air intake chamber. Older engines drew air directly from the surroundings into each individual carburetor. Modern engines instead draw air into an airbox, which is connected by individual hoses to each carburetor, or directly to the intake ports in fuel-injected engines. This allows the use of one air filter instead of many, and allows the designers to exploit the properties of air to improve performance. For example, modern high-performance motorcycles have the airbox draw air from the front of the bike, so that air is forced in, rather than having to be sucked in. To the same end, designers exploit a property of air cavities known as Helmholtz resonance.

Hope this helps your understanding...

 

So basically, if you don't have the hose on, you can't get the air to "exploit the properties of air to improve performance".

I'm not really familiar with what they're refering to, but there's your problem.

 

If the question I asked was answered in the text book definition you just gave, that'd be fine. But it had NOTHING to do with it.
I know the explanation and function of an airbox. I asked how the vacuum in the selected spot A controls the door, and B has such a great effect on MPG.

 

Damn man, sounds like you're still holding a grudge since I wasn't able to post from a blackberry which lines you were referring to.

It's probably has something to do with air pressure, and the way that the box is able to hold and release pressure to close and open the door.

Do you know how a t-stat works to open and close that swivel? I sure as hell don't, but if your t-stat is stuck open, you're going to be experiencing a huge power loss.

I assume it's the same prinicple.

Edit: I think I was right too...It has to do with air pressure.
From the link I posted.

"When air is forced into a cavity, the pressure inside increases. Once the external force that forces the air into the cavity disappears, the higher-pressure air inside will flow out. However, this surge of air flowing out will tend to over-compensate, due to the inertia of the air in the neck, and the cavity will be left at a pressure slightly lower than the outside, causing air to be drawn back in. This process repeats with the magnitude of the pressure changes decreasing each time."

I assume the door facilitates this process.