How much horsepower gain can one expect from a K&N filter... i was thinking 3-6hp but my friend says its like 15hp????
(he was talking about putting it in a Grand National Turbo - if that changes anything)

Thanks

 

just a k&n filter alone in the stock box will add only 2-3 hp for the most part. if you are doing the whole k&n performance intake, you will notice more horsepower, but i don't know if you can get 15 hp out of the performance intake either.

 

I would say 2-3 horsepower for the K&N drop in if you are lucky and depending on what Honda it's in it will be more or less. If the stock intake is well designed like the Type R, or if it is VERY restrictive like a Civic Dx? It all depends. The same goes for the Fuel Injection Performance Kit made by K&N- Those are pretty good, especially since they come with a heat shield.

I, from personal experience, now have a K&N drop in panel filter in the stock box with a removed resonator, and from the seat of my pants, I feel that it is working better than the Weapon R I used to have. The Weapon R was way louder, but the car pulls more with the K&N drop in. I believe that with the drop in, you have a much wider power band than with the Weapon R. By the way, this is on a 96 Accord. Other cars may differ.

 

just what was said before except your friend probably wasn't wrong because the differences of opening the airflow on a NA car and a turbo car a totally different. The grand national is turbo so putting a free flowing filter would definately give those types of gains

 

What I said applies to an NA car. An open element on a turboed car is ok. Since the air is being forced in, it doesn't matter the if the filter is exposed to engine air.