I'm looking to buy a K & N Filter Kit for my 2001 4.2L and want to know if any of you guys have experience a change in horsepower or MPG, I have done some research in the internet and have not convince me since this are not cheap product.
Does this work for a smaller engine 4.2L?

 

i got mine for my 5.4 on ebay for 40 bucks shipped its not k&n but who cares even if i put a actual k &n filter element on it its still under half price

 

Never saw a MPG or Horsepower boost but I did note that my throttle response felt better and its nice not having to buy a new filter every 3 or 6 months.

 

Basically it's a wast of time and money. Not going to do anything but look pretty. Will not increase your power or mileage.
If your gonna do it anyway , get the cheep ebay one.

 

First off, before you even consider purchasing one, ask yourself, what is the purpose of an air filter. The purpose of an air filter is to remove any harmful particles that might otherwise get into the engine and cause damage over time. If this part does not improve either performance or mileage, is it worth the risk of engine damage. Are their filters out there that will protect the engine and is washable?

Ok, so lets start with this. This is a test of numerous air filters using industry standard test equipment, which interestingly enough, most performance filters prior to this were never tested on.
ISO 5011 Duramax Air Filter Test Report

AFE makes a performance filter, that while not shown on this test, was later tested at AFE's expense. The filter is known as the ProDryS because it uses a dry filter element (no oil). It is made from synthetic media, so it can be washed and reused repeatedly. According to the test AFE conducted, the filter is 99.8% efficient, flows up to 30% more airflow than a paper filter, and takes about as long as a paper filter to clog and become restrictive. Thats a lot better than K&N did on the same test. The K&N clogs faster, lets more dirt through, and becomes more restrictive than a paper filter very quickly.

I personally use the AFE ProDry filter and love it, its easy to clean, it cost less than a comparable K&N, and I like that it protects my engine like a paper filter.

 

Dose the AFE go right in the stock air box?

 

Mine does, they have cone filters too.

 

I always like the drop in filters. There usually less expensive then buying a hole kit and most times work better.

 

all the after market filters do for you is lighten your wallet. Keep the factory filter and your money and do something worth while.

 

Technically they save you money in the long run. I haven't bought a throw away air filter in probably 8 years. Last time I did they were around $15, a K&N pays for itself pretty
quick in those regards. I ran a K&N for several years then got a AF1 FIPK.

 

Yes I definitely did,

BUT

that was paired with a performance exhaust system - I think I needed the both together to see the results I got

 

ps
For what it's worth to you - K&N really stands behind their product very well and if you take care of your filter properly they really will last you a lifetime. I have one that's almost 30 years old.