Air Filter Installation

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This page covers various folk's experiences with installing air filters on their combustion air.

Lr jp plenum 1.jpg


John Tvedte posted the following on the Reflector

Ken Parser, in Speed with Economy, suggests an inlet size 10% greater than the throat area of the carburetor.

Don at Airflow has suggested creating a diverging duct - 7.5 deg. included angle.

And Gary Hertzler suggested expand to 2 times the fuel inlet area (Minimum)

Kurt Winkler posted the following
I have had very good results using K & N filters in all my vehicles... and that includes my cherokee 235. Challenger has STC's for K&N filter use in aircraft. They cost a bit more, but they are head and shoulders above the old Bracket type or the paper ones. Plus you can clean and reuse them over and over.

I put the K & N on my 235 and, while it may have increased the power (didn't notice that), it did lower my fuel burn about 1/2 Gallon per hour. Now days, I think that pays for itself Smile

Bottom line, if you use a cessna type filter setup, order a K & N filter from challenger (or find the number and buy it from an auto store for your experimental).

Challenger Aviation Products (

I would also make sure you size whatever filter you are using correctly for not only the engine displacement, but power as well.


Also, this from Lou Stedman
For what it is worth, I am using a K&N filter listed for the O-360, 200HP, in the Van's Aircraft Catalog. The part number is 33-2060. Van's is the best price by far.

Lou Stedman
Velocity N7044Q
Olean, NY

Jack Sheehan
I have had very good performance with the alternate air system from airflow performance I actually use it differently. The photo is looking up at the engine the straight tube ( without the filter has the butterfly valve controled by a cable. In the open position the induction air comes thru the air scoop under the belly through scat tube making a S bend up to the fuel controller. This provides Ram air. If the environment is bad ie dust rain sleet etc (ground operations), I close the ram air and induction air is thru the K&N filter. The K&N is a RU-2790

There is almost 1 inch difference in manifold pressure due to the Ram air. Take off on the filtered air or unfiltered(ram ) is essentially the same since the effect of ram at slow airspeed is very little.. I thought the increased temperature of using air from within the engine compartment would cause an issue but it has not. The idea was to keep the filter protected from the elements so it would not clog due to ice dirt etc.

the air scoop I used was one velocity sells it was a mold used under the wings of older models.

Race 55

Andy Millin
The factory does not install an air filter on their aircraft. The reasoning is pretty good. The intake air is taken from the top of the aircraft. Theoretically, it should be clean. I have been trying to convince myself that I should do the same. I couldn’t do it. I have too much invested in the engine, and the cargo will be worth so much more.

I started doing my research. A K&N filter will flow 6 CFM for every square inch of filter. My engine will need 421CFM @ 2700 RPM. I chose a K&N RX-4140. It should flow 490 CFM.

Next step is building an air box. I flashed the 6” tube from the outside of a piece of 6” HVAC plenum. The end caps were shaped with blue foam and received 3 bid. You can’t see the base of the filter. There is a nifty adapter 3 ½” O.D. to 3” O.D; the base of the filter is 3 ½”. I used a 3” male-to-female PVC coupling as a mold. The inside diameters were perfect.



Here are some pictures from the Reflector on a Jean Prudhomme's airplane. Also below are installations from my aircraft, which includes an alternate air door from x.