Modify Stock Air Filter

The performance of a stock engine will only satisfy us for so long.  Being the gearheads we are we'll want to start modifying our engines.  The most common area that gets addressed first is this stock air filter assembly.  Due to it's very restrictive nature this stock setup is choking down our engine and holding back power.  Installing an air filter adapter and high flow air filter is the best alternative however if you have to keep the stock air filter box for whatever reason, it is possible to modify this setup for better air flow!
We start by removing the stock air filter from the filter box.  
Remove the prefilter by simply sliding it off in one piece.  Once the prefilter is removed you are left with a filter that looks like this with the paper element inside.
Remove the paper element.  The use of needle nose pliers helped with this process.
We cut a piece of UNI prefilter to fit.  
Now we re-install the filter.  Due to the high flow design of this UNI prefilter we had to install a larger main jet.  We used a #80 main jet for this GX160 with a modified stock exhaust and increased compression.  
When complete you have a high flow air filter assembly contained in the stock filter box.  You can also modify the box itself to allow more air to reach the filter.  This is just one example of what can be done with the stock air filter setup.