pictures/questions about my Maxima filter

[NissanMaxima]

Hi all: Changed the air filter on my Maxima. They were both Nissan OEM filters and come oiled. Don't know exactly how many miles on the filter but suspect around 20k. Driving conditions are mix of around town and freeway around Colorado - no dirt roads. Was surprised how dirty the filter looked and took a few pictures for the forum (hey... first picture post!!) including overall picture, closeup, and holding filter outside to the sky to see how much light shows through. Questions:
1. was I due for a change?
2. I've always wondered why my filters show these dirt "patterns". Not having looked a lot of used engine air filters, is this typical and why does it happen? - just how the air flows through the filter box into the filter?
3. does anyone have a guesstimate as to how many cfm flows through a filter? I know this is a tough question that depends on a lot of variables but let's take a guess for my car - 3.0 liter, 6 cylinder, running 2500 (highway conditions). I was thinking about bathroom fans the other day that are speced in terms of cfm and wondered what an engine air filter experiences.

Thanks!

 

[Ethan1]

Displacement x RPM, I guess?

 

[FlyNavyP3]

3L engine at 2500 RPM is 132.42 Cubic feet per minute of airflow. Or, not very much.

Pretty simple math. It takes a 4 cycle engine 2 complete revolutions to displace it's full volume. So take the engine speed divide by two multiply by engine displacement then convert to CFM.

Also this assumption is based upon 100% volumetric efficiency which isn't the case at part throttle and low engine speeds.

 

[Jetronic]

lLook at your instantaneous fuel consumption and speed, and realise that you're dancing around lambda =1... 14.7 pounds of air for 1 pound of fuel. Factor in the air density and you'll know.

 

[NissanMaxima]

Originally Posted By: FlyNavyP3
3L engine at 2500 RPM is 132.42 Cubic feet per minute of airflow. Or, not very much.


Thanks! so about the same as an economical bathroom vent fan which are usually 50-100 cfm last time I looked.

 

[SirTanon]

I was just about to post what Navy said.. but he said it so well there's no need.

As far as the dirt patterns are concerned - these would be due to the natural air flow patterns that occur within the airbox. Based on the shapes of the box itself, the intake snorkel tubing leading to it, as well as out of it, the air is naturally going to follow a certain path, and that pattern is basically the 'shape' that the airflow is most heavy when it crosses the plane of the filter.

 

[JoelB]

I get similar patterns on my car. They line up with the intake tube coming into the air box. So it makes sense that most of the dirt will be placed there. I swap it around every few thousand miles to even it out.

 

[SHOZ]

Originally Posted By: NissanMaxima
Originally Posted By: FlyNavyP3
3L engine at 2500 RPM is 132.42 Cubic feet per minute of airflow. Or, not very much.


Thanks! so about the same as an economical bathroom vent fan which are usually 50-100 cfm last time I looked.
And that is at 100% Volumetric Efficiency. If you have an OBDII scanner look at the load value at WOT (pressurized intakes will need to look for Absolute Load) and near peak torque rpm. This will give a value equal to the air flow and the percentage is the VE.

 

[ARCOgraphite]

I had to clean my MAF due to oil contamination from the overly oiled OEM filter.

Whats up with that!? I replaced it with a less oily STP.

 

[NissanMaxima]

Thanks for the info on the patterns. I'm going to take better attention to the orientation next time I check the filter to see if I can make sense of the air flow out of curiosity! Any comments on the dirt level or is it hard to tell from the photos. I think it was good to change it out. The dirt had an oily/greasy feature to it too (from the oiled filter media?).