The drop in K&N Air Filter requires no modification, no component removal, and no visual change at all. The K&N part number for all Z31s is: 33-2010. The list price for this item is $75.09.
The Fuel Injection Performance Kits (FIPK) is the special cone style air filter that fits directly onto the resonator and requires removal of the original air box. The K&N part number for all Z31s is: 57-6002. The list price for this item is $189.26.
There is also a replacement air filter part number for the above FIPK. The direct replacement part nubmer is RF-1011. The list price for this item is $79.59.
The difference between the 57-6002 and the RF-1011 is the following are included in the FIPK:
Please note that it is possible, but not required to remove the resonator and replace it with a 3" outer diameter piece of pipe. Replacing the resonator will net you NO performance gains. All that will happen is a slightly different intake growl. The resonator is essentially a muffler for your intake. The length of the pipe is irrelevant. It simply needs to be long enough to allow the filter and the other intake pipe to clamp to it, and short enough for the filter to fit in the area without hitting the radiator hose. So somewhere between 1.5" and 9".
With resonator:
Without resonator:
K&N also provides a page with links to several online retailers of their air filters. I suggest you check them all for the cheapest price. Don't forget to include shipping, handling, and tax charges in the total price.
Dyno graph showing 3hp gain from K&N filter on a non turbo:
