*I've just re-read this before posting and it sound more like a motor science lecture! But if you're interested, read on. Hope I don't bore you all....*
Chris,
If you mean the metal/ foil pipe from the exhaust manifold to the airbox, then yes, it does do a job and isn't just for show. If you feel the hole where it goes to the airbox, it will appear to be blocked off. This is because there is a spring loaded flap that covers this hole from inside the airbox.
This flap is temperature sensitive, and the idea is that when the engine is cold (and so is the air temperature in the airbox) it will allow hot air to be sucked in from the top of the exhaust manifold to help the engine heat up quicker. The flap also tries to keep the incoming air at a constant temperature, because if the air is too cold it will cause carburetor icing.
There is also the main cold air pipe going into the airbox, which is the main air source. This is balanced by the hot air inlet as mentioned above which helps keep the incoming air at a constant temperature.
Icing is a more common problem with carburetor engines, rather than fuel injection which tends to suffer less from fuel 'icing' problems. In fact, I read somewhere that multi point fuel injected engines don't suffer from icing. Also with multi point injection the more cold air going in the better, as most of the air temperature is controlled by the injection system. I think the fuel freezes less because it is pumped in under pressure rather than entering by a suction process, and the fact that there aren't small jets to get blocked as used in a carburetor.
However, the injection system in the Uno (non turbo) uses a single point injection system. This is really some kind of hybrid carburetor so it may be subsceptable to icing just as much as a real carburetor. The turbo uses multi point injection having four seperate injectors firing directly into the inlet manifold, so as far as I know Turbos never suffered from icing problems.
Carburetor icing occurs when moisture in the air/ fuel mixture is allowed to freeze. This causes ice crystals to form, which end up blocking jets. This happens gradually hence the problem of a car losing power the more it is driven on a journey in cold weather. The engine idle is also the first to be affected because the idle jets in the carburetor are the smallest so tend to get blocked first. This is why the first sign of icing is when the engine stalls when the car is brought to a stop after a run, yet fires up straight away with a bit of throttle but won't idle. Despite being injected, I think the single point system used on the Uno still has a number of jets inside the throttle body therefore suffering just like a carburetor.
It doesn't have to be freezing outside for icing to occur either, because icing can be formed by the wind chill factor of the speed of the air travelling through the intake. Just like in normal weather, it can be 5 degrees above freezing, but with the wind travelling at 10mph (this is a guess) it can actually have a chill factor of minus 5. The air speed travelling into the engine means the temperature inside the carburetor is below freezing despite the outside and engine temperature, causing it to ice up. This is why the airbox has a hot air inlet to try and stop this from happening.
However, as cold air is more dense than hot air, it allows the air fuel mixture to be more potent which actually equals more power through better combustion. That is actually how turbo cars produce more power by forcing a greater amount of air in (making it more dense through pressurising) for the ratio of petrol. With non turbo engines this is no-where near as noticeable, but if you've ever driven through a patch of fog on a late summer/ autumn evening you might notice that the engine seemed to be smoother and just a little more powerful for a few seconds! That's the effect of a greater air density.
As for losing all the air out of the hot air manifold pipe hole, that won't happen. On a norminally aspirated engine air is drawn into the engine by a SUCTION process caused by the air displacement of the engine pistons. The suction occurs at the carburetor or fuel injection throttle body. Therefore, this causes air to be sucked from the cold air and hot air inlets at the front end of the airbox.
A lot of people have mentioned that the cold air pipe going to the airbox appears to be blocked at the grill/ headlight end. My Uno (a carburetor model) originally had a pipe going to just below and to one side of the offside headlight. It was clipped into a plastic 'Hoover nozzle' type thing that exited at the front of the panel behind the front bumper. The idea here being that it takes an inlet of air away from the engine and also outside the engine bay, hence drawing in colder air.
On my car the blanking panel next to the radiator had a rubber flap that was supposed to be kept in place by the radiator, but due to air pressure coming through the front grill it kept blowing inwards and didn't seal against the radiator. This meant that when it rained, water and muck kept getting past it and all over my clean engine!
So while pulling an alternator out of a mk2 Uno in a scapyard, I noticed it had a different inlet arangement. This had a solid plastic panel with a cold air inlet pipe hole, that held itself securely in place next to the radiator and was a straight swap into my mk1. The cold air inlet hole IS partly covered by the front panel, but there is in fact plenty of space to allow air to enter through this hole from the front of the car.
Don't forget that this cold air inlet pipe is only supposed to be sucking in ambient air, and is NOT designed to be working like a ram air or forced induction system. It's only meant to draw in air from somewhere cooler than the engine bay, and the space behind the front grill is ideal. That's why it doesn't appear to be directly facing the cars airstream.
It is possible to fabricate some kind of ram air system for the Uno, but it is not without it's problems. The first issue is to have a big enough air scoop positioned so that it can catch a large amount of air at the front of the car. The mouth of the Uno's cold air inlet pipe is way too small to have any real effect. The scoop would have to have a much wider mouth to catch as much air as possible, tapering to the size of the inlet pipe to force the air in.
Then you would have to make sure the entire airbox and inlet pipework were totally free of air leaks. The idea here is that you are trying to pressurise the air intake system, and the slightest leak would cause the pressuring effect to be lost. You WOULD have to block off the hot air intake hole in the airbox, and also the engine breather tube inlet and the air bleed tube to the base of the carburetor/ throttle body. You would also have to make sure as well that the seal between the two airbox halves is completely air tight. It's designed to seal under SUCTION and is not meant to be pressurised!
Even silly things like the hot air flap sensor needs looking at, as on my car it is screwed in on the underside of the front casing. Where the screw hole is you would have to make sure this was sealed air tight too.
The cold air inlet pipe would also have to be much more substantial than the standard brittle plastic pipe originally fitted. The original pipe would be likely to crack under pressure, and in fact when they are a few years old they do a good job of cracking on their own anyway! Ideally a metal pipe would be best as rubber pipes tend to bulge under pressure.
Anyway, let's assume you managed to make an air tight airbox and inlet pipe, and placed the air scoop in a position where it can catch a large amount of air from the front of the car. You will now have one other problem, and that is a ram air effect doesn't work until quite high speeds are reached. There's a lot of arguments about exactly how fast you'd need to go, but some people reckon you'll have to be travelling well over 100 mph for any significant gain to be felt. It would also be dependant on how big and efficient the scoop is, and whether it is placed in such an area that isn't slip streamed by the cars aerodynamics.
Remember that a carburetor/ fuel injection throttle body can suck in very large volumes of air very quickly, so the amount of air travelling into the airbox would need to be much greater than that to be able to create a pressuring effect. And as soon as you opened the throttle wide open most of the pressure would disappear very quickly into the engine giving a momentary 'turbo' effect.
On top of this, you would no longer have the hot air inlet and could well end up having icing problems in colder weather! You may also find that as the intake/ air box side is now totally sealed, in wet weather water might enter into the carburetor/ throttle body from a combination of being forced in through the ram air effect AND the suction from the engine. Apart from making your air filter go soggy, it could cause rough running problems if it mixes with the fuel.
On the normal cold air intake, air is sucked in and the inlet pipe travels upwards towards the airbox. This is so that any water/ rain that may enter the inlet pipe will just drain out as the suction isn't great enough draw it up the pipe into the air filter. If damp air is forced in by a ram air effect then it may be possible to have water ingress problems. That's also why a lot of cold air inlet tubes are of the ribbed type, as air travels through no problem but foreign objects or water droplets are more likely to get stuck in the ribs. Water is way heavier than air and is less able to travel upwards over the bumpy ridged surface of the tube.
Assuming that you did get a ram air system to work you also face yet another problem. If you force more air in than normal for the ratio of fuel, you make the mixture weaker. That in turn causes 'pinking' or pre-detonation, which can lead to burnt valves, holed pistons and overheated engines. That's why turbo or supercharged cars have knock sensors to detect this, and they adjust the fueling and timing when it happens to stop the pinking and weak mixture problems. A bog standard non turbo Uno can't do this, so will continue with a weak mixture pinking away merrily until something goes bang!
To cut a very long posting short, yes, ram air DOES work, but only at very high speeds that allow more air to be forced into the air box than is needed by the engine hence keeping it pressurised. This theory was kind of proven by a bike magazine when they tested a Kawasaki with a ram air system (I think it was a ZZR or ZXR 750, going back over a decade). They did some top speed runs, and found that going by the measured power of the engine, the weight of the bike and also taking into account aerodynamics the bike went much faster than it should have done.
As they measured the power on a dynometer where the bike was static on a rolling road device, they concluded that the ram air effect was coming into play and making the engine produce greater power at higher speeds. Obviously, on a dynometer no ram air effect was happening at all as the bike was effectively standing still. It is worth bearing in mind that the bike was reaching speeds well over 150mph where a ram air system is likely to start working, a speed way more than a non turbo Uno can ever hope to achieve......
Also worth bearing in mind, is that if a ram air system really did work on the average road car then car manufacturers would fit it. The fact that they don't says it all really. Then again, they fit spoilers, but the laws or aerodynamics say they have no effect until speeds of over 90mph are reached. And most factory fitted spoilers have hardly any effect at all as they are designed to look good rather than create downforce.
To make a ram air system for a standard Uno is really a waste of time. It might look good, but is unlikely to have any effect at all. Best to keep the standard airbox and make sure the hot air pipe from the manifold is in place. That way you can drive all year round without any problems.
If you really do want forced induction on your Uno, go find and fit a supercharger. Or to save the hassle, go find a Uno Turbo which has all the hard work done!
Think I'd better get to bed now after the science lecture......