Here are the aftermarket air filters we tested in this '04 1/2 Dodge Ram 2500 equipped with a manual transmission. Clockwise from the top left are: K&N, Volant, AFE, Airaid, True Flow, and S&B.
Each of the intakes we installed provided a marginal horsepower upgrade over the stock setup that scored 309 hp at the rear wheels (see the chart). From our testing it appears that improving airflow alone to the turbocharger inlet on this particular Cummins engine will not instantly provide a boost in engine power. Why? In this case, we believe the engine needs more fuel to take advantage of the improved airflow. So is an aftermarket air intake a waste of your time and money? No. But depending on your truck and engine, it may need other engine modifications (injectors, performance calibration, or bigger turbo) to truly reach its potential.
Ever spent a whole day installing seven air filters in a stock Dodge Ram and running the truck on a dyno? Well, now you don't have to. With help from the folks at Westech Performance, we tested a bone-stock '0411/42 Cummins with a stock air intake and filter and then compared the results to the leading intake systems available for this truck.
Westech's Eugene Walde strapped the Ram to the dyno and performed the tests. He made multiple runs for each filter, and as an added touch, he used a zip tie to bind a shop rag to the exhaust catcher so its metal post wouldn't scratch the paint.
Our results were shocking. We hoped for massive increases over the stock airbox and filter, and we thought this article would read like hate mail to the design team at Dodge, with criticisms about the ugly factory paper filter and seemingly restrictive intake setup full of bends and baffles. Boy, were we wrong.
It turns out the engineers at Dodge did just what they were supposed to when they designed a system that is quiet, very effective at filtering dirt, and provides the maximum airflow that can be used by a stock Cummins 5.9L. Keep in mind the potential for more performance is possible with a modified truck, but all of these intakes eventually narrow down to the inlet size of the stock turbo. Since we learned it's not possible to significantly improve the horsepower over the stock setup (without the addition of a larger turbo, more fuel, bigger exhaust, and so on), we'll concentrate on how each setup sounds, what's included in the kits, the ease of installation, and how the intakes look when you pop the hood.
"Three-Oh-Nine, every time," became a running joke at Westech Performance on test day. Here is a dyno chart with the aftermarket filter results laid over the horsepower reading of the stock Dodge filter. Don't bother trying to tell them apart. The average results of the dyno runs ranged between 308.7 and 310.4 hp, and each filter had at least one run that was in the 309s. In fact, the stock air filter scored exactly 309.0 hp. Torque figures ranged from 566.1 lb-ft up to 573.0 lb-ft, which is not very different from the stock reading of 570.0 lb-ft.
The sealed stock Dodge airbox grabs air from inside the fenderwell, so you could say the Ram comes with a cold air intake from the factory. All of the aftermarket systems utilize this factory air inlet. The stock accordion-style tubing leads air through a sound baffling device before a 90-degree rubber elbow joint attaches to the turbo.
The factory silencer is mounted between the airbox and the turbo inlet tube. It works much like a muffler for the intake using baffles to reduce the "noise" of the Holset turbo when it spools up. | 
The stock Mopar air filter is made of paper pleats covered with a pre-cleaning layer of "mouse fur." This unit was less than a week old and had already captured some dust and a few leaves. One thing some people forget in their quest for diesel power is that a stock paper filter like this one might trap particles that are significantly smaller than those that can slip through a high-flow filter into your engine. | 
This white plastic insert near the turbo inlet helps route air through the 90-degree bend in the factory intake. |