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My question is actually quite simple. Do you get any performance/sound/feel differences while driving with multiple air filters? For example, the stock filter that's built into most engines:
and an external air filter
If there is no (clear) disadvantage. Would it actually be of any good to use multiple filters. Would you actually get 'cleaner' air in your engine?
If driving with multiple filters, your vehicle will lose performance. Your car will have a greater restriction in the intake system from having to pass through two separate elements. You would, however, have cleaner air coming into the system, but at a cost of the performance. You'll have to have more gas pedal to compensate which would cause other issues as has been explained.
Aftermarket manufacturers do have what is called a "cold air intake", which can replace the entire system from the air box and filter all the way to the throttle body. This would be used as a single filtration system (only one filter). Depending on the manufacturer, this type of filtration system can actually perform better in both flow and in how well it cleans the air (you'll have to do your homework to figure out which are which). This can provide you with a small performance boost (it's actually freeing up power which was is already there, but that's another story), as well as better protect your engine from particulate matter. The mass air flow (MAF) sensor, manifold absolute pressure (MAP) sensor, and oxygen (O2) sensors will tell the computer how to adjust for these variations so as to give the engine the fuel it needs to stay at its target air/fuel ratio for optimal efficiency (given the fuel map in the computer).
Some will tell you that the manufacturer will put together the optimum system and fuel mapping for your vehicle. I would tell you this is a complete fabrication. Unless you are talking about high performance vehicles, vehicle manufacturers use parts and settings which are a trade off between comfort, economy, and performance. They will give you parts which will work well for your vehicle, but are cheap to mass produce, and provide you the most comfort from your vehicle (whether it is noise or whatever). There is always room for improvement over what comes stock. Fuel mapping is done conservatively on the rich side so as to prevent dreaded engine knocking or pinging. There's even room for improvement in high performance vehicles. The conservative nature of the fuel mapping is also done so you get better longevity out of your engine. I will say fuel mapping has improved over the past decade so as to provide better fuel economy, but as I said, there is still room for improvement over what comes stock in almost any situation.
If you use more than one air filter, you will make it harder for the engine to suck air into the intake manifold, meaning you will lose power. Less restrictive filters will give you more performance, but will allow more dirt into the engine.
If you use something like a cone filter (like in the second picture), you will have a change in sound, especially when you rev high. This is simply because the filter in the first picture is inside a box which muffles the sound a bit. You can get the same type of sound by drilling holes in the box, if you just want the sound and don't care for "performance".
When your engine and its parts, including the air filter, were put together the whole assembly would have been matched to perform to the optimum and then mapped to the engines ECU. Changing any parts for differant non-standard parts will always effect the performance of that particular system, simply by coming out-of-map. If you look at the filter in picture 1, the black rubber surround is to seal the air inlet past the filter. If you change the filter, and the replacement filter does not include an arrangement to maintain this sealing, the first thing to suffer will be the MAF sensor(s). This in turn will effect the O2 sensors and you will have codes for MAF faults, O2 sensors, and usually 'engine running weak' faults. To do the opposite and add 'more' air filters will produce the same faults but they will include 'engine running rich' codes. Finally, if the whole system is at optimum then there is really no need to change it.
