When a product fails to live up to its advertising claims, we're not surprised. But when the same product may produce additional engine wear, we feel duped.
After reading through the following examples, you should notice a trend. Before shelling out $50 or more on a replacement air filter claiming horsepower gains, ask yourself this: "If the manufacturer could add horsepower to a car with a simple air filter change without any ill effects, wouldn't they do so?"
Exhibit A
Honda Tuning magazine conducted "The Great Intake Test" in their October/November 2002 issue. When the test Acura RSX Type S' original equipment Acura air filter was tested against a K&N 'drop-in' replacement, there was a .1hp decrease in power output and no measurable change in torque output.
Exhibit B
When BMW engine management guru Jim Conforti started work on his aftermarket engine software for the 99-00 BMW 3series, he visited Boston Performance Group's in-house chassis dynamometer. (We remember this because we ran the place at the time).
Before his arrival, the staff at bpg performed baseline tests of the 99 BMW 328i test car, using both the stock paper/foam filter and the K&N oiled cotton 'drop-in' replacement. While everyone attending was expecting to see an increase in power using the K&N filter, the car consistently produced less power and torque (~4hp and 6lb-ft.) using the K&N 'drop-in' filter element.
The staff noted similar results on various other cars tested over the course of a year of dynamometer testing. There's more to an air filter's effect on performance than how much air can pass through it, especially on modern cars.
Exhibit C
K&N Filters
by Mike Perkins, Owner of Bavarian Machine Specialties
(originally published in Houston Chapter BMW Car Club of America Newsletter, circa 1998)
"The promise of inexpensive horsepower gain is always an effective method of promoting an aftermarket product. However, as with most products claiming improvements beyond factory specifications, there is commonly a compromise to go along with the gains. K&N filters are no exception to this, and I do not recommend them to everyone.
There are basically two types of K&N filters. The first and most common filter is the flat type. This filter fits directly on the original air filter housing. The second type is a cone shaped filter which requires removal of the stock air filter housing and requires a special system to take its place.
Both types of K&N filters improve volumetric efficiency of the engine because the filtering elements are less restrictive compared to the original equipment. The cone type is more efficient due to its greater available surface area. Both filters operate identically in that less restrictive, less dense, cleanable pleated paper filter removes larger debris and grit from the air. In addition, the paper element of each filter is coated with a film of special oil that enables the filter to trap minute particles without the need for additional restrictive paper pleating. So far, this is all good stuff and it works well. However, there are compromises that must be considered.
As you can imagine, an oiled filter can clog up rather quickly. When the filter becomes clogged, two problems occur: 1) the filter becomes more restrictive than the stock unit; and 2) the filter untimately traps less particles. The immediate concern is that filters must be cleaned every 3,000 to 6,000 miles, depending on conditions. Routine cleaning, to be effective, requires a pressure cleaning system combined with a cleaning agent to clean the element, as well as adequate drying time and re-oiling of the pleated paper element. (A cleaning/oiling kit can be purchased separately).
In an independent study that provided comparison testing of a K&N filter and a stock unit, the results showed unequal levels of engine protection. Presupposing both units were cleaned or replaced respectively on a regualr service interval, the K&N unit allowed more microns of dirt/grit into the engine per mile. Over a long period of time, this translates into worn rings and cylinder walls.
The final issue regarding the use of K&N filters concerns the cone type. Typically the stock housing is removed to install this unit. Our own testing of this type of setup, in fact, produced less horsepower at the track than the stock unit. The stock system will only allow air to flow into the filter from the front of the car, usually from behind the grill. This means cool, fresh air. On the other hand, the cone filter, left to dangle from the air flow meter or air mass meter, receives hot engine compartment air, or worse, hot radiator air. Engines simply make more power with cool air...
In conclusion, K&N filters can be a excellent and sometimes necessary upgrade to be used in conjunction with other engine modifications or race engines. This, of course, requires the strictest of maintenance schedules. However, the benefits for a street car are questionable, especially when an extremely effective original equipment air filtering system already exists."