The stock air intake system has that box with a paper filter in it with a puny plastic pipe that extends into a small hole with an air resonator box in that empty space in the bumper. It actually routes cold air from the fender well up into the stock air box. It was a good idea in a sense that the air is colder but the flow is so restrictive that it doesn't do a lot of good. If you plan on just installing a drop-in K&N filter into your stock air box then you might free up a pony or two at the most. If you take the whole box off and put a K&N cone filter there, you'll get much better air flow but the terminal part of the tube where the air actually enters is placed in a much hotter part of the car, so you trade flow for air temperature. That's about another pony or two, same as the stock box.
Generally you would want a cold air intake for the best air flow and coolest (relative to the outside) air temperature. It's mandrel bent piping that extends from the trottle body down through a hole next to the battery and ends with a cone filter right behind the hole for the driver side foglight. The low-restriction piping allows good air flow and the placement of the filter is in a much cooler spot in the car to pull in denser air, which means more oxygen to burn in the cylinders. That's somewhere in the midst of five horsepower. Sometimes more, sometimes less, depending on the rev-range and your own varying circumstances.
As for the cold air applications, Hotshot, Place Racing and Injen make very good setups for our cars. Search around the forums, sentra.net and nissanperformancemag.com for pictures and other information on it.
how about this... i have a cone filter in, but it's stuck to the stock tube and MAF, so what if i fabricate a tube leading into the resonator bay? I took the whole air tube and resonator out, would it help a lot even if i left the cone filter INSIDE the engine bay?
the length/diameter of the intake tube has a greater effect on the power/torque of an engine than the placement of the filter. and yes, leaving the cone filter in the engine bay will give more power/sound than the stock system.
decide......sound or function??? bypass valve, no bypass valve?? yes, it seems the bypass valve is just a foam ring, but it costs way less than a new longblock or headjob, just an advise though, i personally run on wai, and take out my left headlight when racing, not on the street though......