Photography: Diesel World staff and courtesy of the Manufacturers
Myth-Busting the Science of Airflow and Filtration
Opinions are like bellybuttons … everybody has one, and they are all different. When the topic of automotive air intakes, airflow and filtration hits the Internet diesel forums, that analogy comes to mind.
So how can you separate the BS from the real deal when it comes to understanding what your diesel needs in terms of airflow and filtration? Even the aftermarket companies tell different stories.
To separate fact from the fiction, Diesel World Magazine asked the experts. The results of that polling could best be called “Flow and Filtration 101.” Let’s get started.
Air Filter Basics
The primary purpose of an air filter and the corresponding air box that holds it is to keep contaminants out of the engine. According to the experts, 90 percent of engine contamination comes from the air intake side, including dust and engine blow-by through the crankcase ventilation system, depending on the engine. And on a diesel, toss in some soot and fuel blow-by, and the oil gets dirty quick.
To minimize the contamination from dirt and dust, a filter element is added to the air box. In general, factory air filter elements are made from a cellulous material similar to paper. The reason this material is used is for efficiency and longevity of the powerplant, since it filters out more than 99 percent of the dust and dirt, right down to the finest particles.
Filter Materials and Characteristics
Over the years, new filter materials have been developed to include nanofiber technology, synthetic, multi-media and the good, old standby, oiled cotton gauze, which is the basis for most performance air filters and air intakes. The next question is, how does a filter stop the dirt?
Essentially, there are two methods: surface capture (i.e., paper) and depth loading (oiled cotton gauze). In the surface capture model, all the dirt sits on top of the filter medium, since the openings in the paper are too fine for it to flow through. The tradeoff is airflow. The downside is that as the filter loads up with dirt on the surface or gets polluted with crankcase oil blow-by, airflow decreases dramatically. Filtration performance is also affected by the properties of the filter material, pleat spacing, size and shape.
In the depth-loading model, multiple layers of say, cotton gauze or synthetic materials retain the dirt in the filter material fibers. And in the case of oiled cotton gauge, the oil acts as an attractant. In an ironic twist, the more dirt that accumulates in a depth-loading filter, the better it filters, because the dirt becomes a de facto filter medium.
Another great example of a depth-loading filter is the giant, blue canister element found on the 6.0L Super Duty and late-model Duramax engines. Developed by Donaldson Filtration Solutions, the Power Core G2 design has great dirt-loading characteristics while maintaining airflow, primarily due to its internal design. Donaldson was also the company that developed the massive air filter system for the M1 Abrams tank and many of the earth-moving machines used today.
Testing and Efficiency Ratings
Air filters are rated on efficiency, air flow and dirt-holding capacity. The gold standard for testing is the ISO 5011 testing procedure. In a simplistic nutshell, special testing equipment is used to draw a measured amount of test dirt, which is measured right down to the gram, through an air filter. The remaining dirt is collected, and based on the weight of the filter before and after, testers can calculate the “efficiency” of the filter—or how much dirt it can really hold, based on the percentage of dirt that was actually captured.
So, if efficiency numbers are high, that’s the filter you want, right? OE paper filters have a ridiculous high-efficiency rating across the board, but you’ll never read about their actual numbers. The black magic in the efficiency ratings comes from the fact that nearly all aftermarket air filter companies use coarse test dust, which, according to K&N, ranges from 5.5 to 176 microns. While most companies use an independent lab for testing, K&N has moved its product testing in house for better quality control.
Factory Air Box
OK, now that we know what goes into the factory air box, let’s take a look at the box itself. First of all, there are the physical properties and limitations, such as how big it can be to fit in the engine compartment and where it draws air. Secondly, how much air can it flow at its maximum capacity?
The OEs usually calculate the maximum airflow in cfm (cubic feet per minute) of the application, based on engine displacement and rpm; and in the case of turbo-charged diesels, that cfm rating has to include how much boost the engine can make at wide-open throttle.
Usually, the factory box meets these numbers very closely—or, in some cases, falls short due to packaging limitations, which means your engine might not get as much air at full song as it needs to. Other elements that come into play regarding the stock air box are the size and shape of the tubing leading to the turbo and any ducting or noise-attenuation devices that could reduce flow in the interest of controlling intake noise.
Here’s where the most common misconception on diesels comes into play. Say the factory air box on your diesel is calculated to flow its maximum at wide-open throttle at 15 pounds of turbo boost. So, you go out and pick up a 100hp tuner that turns the boost up to 30 psi. What do you think happens inside that factory air box above 15 pounds of boost?
There’s a lot more vacuum, since the opposite side of the turbo is sucking in air at a higher rate than stock. We’ve seen factory filter-minders in the red with perfectly good air filters in the box in this situation and even factory filters that have been collapsed due to the added vacuum. So rule #1 is: Think air intake and exhaust before you turn up the boost.
Editor’s note: for the full story on the truth about air intakes, filters and air flows, pick up a copy of the May issue of Diesel World that goes on sale March 13th.