My truck when i got it has a K&N filter what kind is the best to replace it with? it is a 99 F150 5.4.

 

Why would you want to replace the K&N filter?

 

I used STP stock type filters when I had the stock type filter in mine.

 

I would replace it with a Motorcraft, Purolator, or Wix OEM paper filter. Since it's been run with the K&N, known for passing dirt and oil, make sure you clean your MAF sensor. Running the OEM paper filter, you won't have to worry with it any more....

 

K&N filter is to allow more airflow into your engine and increases hp.
You will need to ensure the filter is clean if not you can by a clean up kit sale by K&N The kit contains a degreaser, oil, and complete instructions on how to do it.

 

Independent studies show the OEM paper filter will flow all the air the 5.4L will pull. This can be verified by putting a vacuum gauge where the IAT sensor is in the intake, and look for a vacuum at WOT, and high RPM. The thing to remember, with the porous filter media of the K&N (hold it up to light and you can see the holes in it), it will flow more air (and dirt) than the OEM paper filter, but unless the engine is requiring more air than the OEM paper filter will flow, there WILL NOT be any performance improvement with the K&N.

 

The oil actually traps many of the fine particles. My intake tract on my 88 Mustang is still as clean as new and has never soiled the MAF. I installed the K&N at 9000 miles and it now has over 160,000 miles. I've never had to clean the throttlebody or IAC either.

 

The environment plays an important part. If the environment is clean, then the K&N probably won't be a problem. It is interesting, however, that K&N recommends a "pre-charge" filter to be used with their regular filter in dusty
conditions, which indicates that K&N is aware that their regular filter does not filter well.

Check out this K&N URL:

http://www.knfilters.com/wraps.htm

 

Well, the Mustang doesn't get off-road much