Tom Wilson
January 1, 2001

Step By Step

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138_90z 1991_ford_mustang Left_rear_view138_91z 1991_ford_mustang Air_filter
On a stock car, such as ours, the K&N-versus-stock-air-filter power gain is small, but previous experience has taught us the power goes up in proportion to the other hot-rodding parts installed. The more power you are making, the more a K&N is worth. That, and you don’t have to buy stock air filters anymore.
We tested three filters. As shown from the bottom left they include a brand-new Motorcraft paper filter, a new K&N, and the dirty K&N that was already on our car.
At left is the new K&N—at right our dirty original. As you can see from the accumulated dirt, there is a sweet spot in the center of the used filter, which indicates most of the air rushes through the center of the element.
When buying a new K&N filter, you’ll be happy to see it comes pre-oiled from the factory. All you have to do is put it in the airbox. You’ll have no worries about over-oiling, or even need to buy oil. You can purchase a cleaning kit 30,000 or so miles later.
138_95z 1991_ford_mustang Left_rear_view
K&N’s dyno cell is an in-ground Dynojet with all the safety and cooling issues well handled. These tubes help evacuate the exhaust gases.
138_96z 1991_ford_mustang Rear_view
Typically, cooling the engine is a problem on Dynojets, as duplicating the airflow over the radiator and oil cooler is impossible with the usual shop fan. As a result, much time is wasted cooling and warming the engine when trying for repeatable data. At K&N a large roof-mounted fan blows a huge volume of air at the front of the car, assisted by one propeller-type and two big, squirrel-cage shop fans. Then, to help evacuate all the incoming air, these two box fans blow the room air outside. Thus, cooling was rapid and the data quickly taken.

True superstars, it is said, are recognized by their first names only. For our grandfathers, George, Paul, John, and Ringo need no further explanation. Your dad knows who Elton is, and if someone says Madonna, you know who they're talking about. The same seems true with hot rod parts.

If it's a superstar, then just the name will do; you don't need to say what it is. Say "Vortech," and "centrifugal supercharger" doesn't seem necessary. Or if you hear "Flowmaster" no one has to explain "muffler." And certainly the name "K&N" connotes "air filter."

But, as with household-name megastars, often what everyone seems to know about some of the most famous hop-up stalwarts is incomplete. To counter that, we're taking an in-depth look at one of the most familiar hot rod superstars ever--the K&N air filter. It's the first speed part most anyone adds to their 5.0 or 4.6 Mustang.

What Is An Air Filter?
A dumb question, perhaps, but a good place to start. An air filter strains the air entering the engine to trap any harmful particles, from gravel to microscopic dust. Removing this dirt keeps the engine and its oil much cleaner, and thus extends engine life by tens of thousands of miles. The most noticeable benefit of this is an increase in piston-ring life, although rod-and-main-bearing longevity is directly affected by intake-air cleanliness as well.

Air filters also act as induction mufflers, helping quiet the hiss, whoosh, and roar of air entering the engine. Ford, as do all major manufacturers, uses a paper-element air filter. It's a good filter, trapping tiny pieces of dirt, and it makes a good air muffler. You could also say it's cheap, but considering how many must be bought over the life of the engine, the true cost is much higher than the $10 per filter the dealer receives.

What's A K&N Filter?
Instead of paper, the K&N filter uses cotton gauze as its filtering medium. As with stock paper filters, the gauze is folded into pleats to increase the available surface area. However, unlike a paper element that is tightly made, the K&N fabric is a wildly random collection of fibers ranging in size from hair-like to invisible without optical aid. This leaves a much larger area between the fibers than the holes in paper, and so much more air can easily pass through.

To capture dirt, the K&N fabric is oil soaked. The oil does not fill the space between the fabric fibers, but lives on the fabric strands, ready to grab and hold the dirt passing through the filter. It is quite effective at this because the air, which is light and turns small corners with ease, passes right through the fabric almost as though it wasn't there. The dirt, meanwhile, is much heavier and thus has much more inertia, understeers into the bushes--er, fabric--and sticks to the oil.

Now, it is true that a paper filter blocks more dirt--that is, it traps smaller particles than the fabric K&N design. However, the K&N traps any dirt large enough to do damage in an engine. The more practical result is a K&N filter remains effective and highly efficient at passing air long after a paper filter would have clogged up and turned into a choke. Also, the paper filter can tear when bombarded by heavy dirt--think dirty, sandy roads--and then all sorts of grit makes it through. The stouter K&N will not tear unless mishandled by a mechanic.

Finally, the K&N is washable and reusable. This saves money in the long run and is more eco-friendly than tossing out several paper filters during the life of the car. A K&N panel filter for an '86-'93 5.0 can be had for $34.95 from Mustangs Unlimited [(800) 243-7278;]. So after buying three paper filters, you could have had more power with a K&N. You will have to buy cleaner and oil, however, and Recharger kits containing these items can be had for $11.95 from Dallas Mustang [(800) MUSTANG;].

Horse Sense:
The day after our K&N dyno test, our LX suffered a frozen-tight alternator. Guess those 6,000-rpm dyno blasts were a bit much after the old man's driving. Stranding the editor-at-large's wife didn't gain us any points either.