Last winter during the installation of my 406, I replaced the air filter with a large K&N part# RF-104. I liked the filter because it was larger than the cone shaped filter I was previously using and thought that it would flow more air.

For a computer, I use a DFI. In the fuel map, the base map shows engine load and rpm. During the tuning process, the motor would never show 100% load which means that vacuum was still present in the manifold. This generally points toward an intake restriction.

Over the weekend, I changed the filter back to the large cone filter #RE-0870. I immediately felt a sop difference and now the fuel map shows 100% load which means more airflow through the motor.

I can only think that the design of the cone filter is better for airflow even though the surface area of the RF-104 is greater.

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Interesting. I'am using the cylindrical RU-1785 on my 350. Was thinking about the cone style. I did a search of the K&N web site but I could not find the RF-104 filter. Also did a search for RF-1040 and got an oval filter. Is that the correct filter number? Allen

 

1989GTATransAm - Yes that is what the RF-104 filter looks like minus the chrome K&N plate. The filter I am now using is a long tapered filter part # RE-0870. The filter length is 9" and has a 4" flange.

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Thanks for the information. K&N makes a filter the same as your RE-0870 but with a 3.5" inlet which will work on my setup. I did not achieve the ET or MPH with my new setup that it should. Granted it still needs tuning but the filter may be part of the problem. Allen

 

Update: Received my K&N RE-0920 air filter today. It is of the cone design and I can tell you it is a much better design than the RU-1785 cylindrical that I was previously using. It has a radius on the inlet and a step that matches the tubing I'm using and it is larger. The first two items alone have to help the air flow compared to my previous setup. Allen