I have a 90 GTU, and I already have a K&N drop in.

What I want to know is, is it worth it to get a cone style filter or should I just get an extra box cover from the junkyard and drill 1-1 1/2 holes into the cover instead.

Which one of those choices would be better?

 

The stock FC airbox is quite restrictive. Throw it away and install a K&N pod filter BUT make sure you also make (or buy) a heat shield and duct some cold air to the filter or it will be sucking in air that can exceed 140ºF (measured).

 

The best setup is www.mariahmotorsports.com 's

Custom cold air box (good quality), stamped headlight airscoop, K&N intake.

 

The K&N or Apexi cone intake filter is worth around 5HP with a cool air box over a K&N or FX filter in the stock filter box and about 7HP over the stock or a standard paper filter in the stock airbox.

The poor man's air box of drilling or cutting up the factory box and a K&N or FX filter is okay, but won't yeild more than a couple of HP.

Of course those yeild are based on a motor in perfect tune running correctly, so before you consider any mod always make sure the motor is running correctly. Things like Plugs/plug wires, O2 sensors, timing, small vacumn leaks are often overlooked and need to be taken care of first.