No offense, but you should never recommend driving a car without an air filter on. The smallest pebble can be the demise of a healthy engine.
Also, to say you don't need a filter larger than the smallest part of the intake is also erroneous. The larger the filter, the less air restriction.ÂÂ
The best way to show this is to think of a huge filter and a small filter. The engine is going to try to draw in X cubic feet of air per minute, no matter what (that's a function of displacement and compression ratios). With the small filter, the engine is trying to pull in X cubic feet of air through a tiny opening; more air is going in through a smaller space (a single square inch of the filter is responsible for more air flow). The more air you have going through a small space, the more restriction there is (basic physics). With the larger air filter, the engine is still trying to draw in X cubic feet of air, but each square inch of the filter is responsible for less air (since there is more surface area, each square inch does less work). This means that there is less air restriction on a larger filter. Restriction kills horsepower.ÂÂ
Like I said in my example, there was a 5-8rwhp difference in horsepower between two filters of different sizes, even though the smaller filter flowed more air per square inch. Had these two filters been the same size, the Apex-i would've undoubtably performed better than the JWT. But that wasn't the case... each square inch of the JWT had less work to do because it was a much larger filter - as a result, the JWT performed better. The intake piping on a 300ZX is only 2 inches in diameter... the JWT cone filter has an ID of 6 inches and a height of about 8 inches.
I just thought of a good example. Try to breathe through a piece of gauze that is the size of your mouth. You should notice the restriction (if you don't, make it thicker until you do). Now make a cone of gauze that is several times the size of your mouth and the same thickness as the smaller "filter," hold it up to your mouth and breathe now. It's going to be easier to breathe with the larger "filter" because you're sucking in less air per square inch.
In an engine, the easier it is to breathe, the more power the car makes.