K&N Open Element Air Filter

In an effort to gain a little bit more power from the engine, and to get that classic K&N cone filter sound, I removed the stock air box and purchased a K&N air filter to replace it.

I had two concerns when doing this swap.  The first is complete equality as original in terms of reliability and drivability.  This includes a glass-smooth idle; smooth and consistent acceleration, idle to redline; availability of cool dry air (thus, the heat shield); and a perfect fit.  Sometimes when an open element air filter is installed, the vehicle can idle rough or have an inconsistent power band.

I also had to make sure that the filter had a supply of cool dry air.  The factory air box is good for isolating water from the intake system, and for drawing cool outside air into the system.  I attempted to replicate this as best I could with the materials I had to work with.  I fabricated a heat shield out of very thin gauge sheet metal with a pair of straight-cut tin snips, primed it, and painted it low-gloss high-temp BBQ grill paint.  I trimmed some of the edges with split vacuum line, to prevent chafing and help with insulation a little more.  This keeps most of the heat of the radiator and exhaust system away from the filter, and allows the filter to breathe air that (mostly) originated outside the vehicle.  It also prevents huge amounts of water from coming in contact with the filter.  Even if driving through a big puddle, water doesn't have a direct line to get to the filter element with this shield in place.

With the K&N filter installed, the performance band has been moved up in the rpm range a little bit.  The engine still pulls well in the upper half of the rpm range, although I'm not convinced the gain is worth the costs.  The WOT (Wide Open Throttle) high-rpm power seems to have increased by a moderate amount, but the throttle response in the low and mid ranges seems to have decreased a moderate amount.  None of that is absolute because I don't have any dyno results.

Very very little of my driving is done at redline (the same for most people I imagine), so I decided to put the original air box back on.  After removing the K&N filter, I looked around on the web and found a few interesting resources.  An air filter test at bobistheoilguy.com revealed that despite K&N's claims, their filter might not provide better filtering than the OE filter.  I continued to surf through the forums at the same site and a lot of people reported increased silicon levels (dirt) in their used oil analyses.  I admit that the air filter test above seems rudimentary and less than scientific, but it was the only objective test I've seen that attempted to actually compare filtering efficiency.  Then I happened on another filter test performed to an ISO standard that found the same results.  In fact, it said the K&N filter captured only 37% of the dirt the AC filter did, passed 18x more dirt, and did it in only a third of the time (because it plugged up that quickly).  That really confirms the poor filtering claims by many in the automotive world who don't take K&N's marketing department for their word.

I'm happier overall with the stock air box and stock AC paper air filter.  The car has the best throttle response this way and the mileage is better (due to me using less throttle than I had to with the K&N filter).  The open air filter seemed to give me slightly more WOT power, but due to decreased mid-range power, I had to keep the engine spinning faster all the time to make use of that power -- so my mileage actually decreased overall.  Combine this with a filter that has LESS filtering capacity than an OE paper filter, and the decision for me was a no-brainer.  Plus, if you have a car with the PCM under the hood, like many Northstar-powered Cadillacs, you need the airflow across those cooling fins to keep that module cool.  Without that benefit of the factory air box, you run the risk of overheating your PCM.

In short, I recommend the stock air box and stock paper filter for everyday use.  Maybe if you go to the track and have a dedicated race car, put on the K&N, but for normal use...the air box definitely works best for me.

Pictures of the filter and the heat shield, installed in the car (click for larger):

The pictures above were taken the day I installed it, and I didn't have the IAT (Intake Air Temperature) sensor installed in the intake tract.  The pictures below show the intake tube after I drilled it for the sensor, and with the whole assembly installed back in the car.

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