Better breathing means more power and torque. That's the lesson magazines and the automotive aftermarket have been preaching for years. Whether gas or diesel, the power principle remains the same: More air in and more air out equals improved horsepower and torque. To understand this philosophy, you have to get down to basics.
First and foremost, an engine, whether it be gas or diesel, is essentially a giant air pump. The up-and-down motion of the pistons moves specific volumes of air through the engine. The quantity of air depends on many factors, including engine displacement, the size of the intake system, the camshaft configuration, and the efficiency of the exhaust side of the powerplant.
One of the biggest restrictions...
One of the biggest restrictions in the factory intake system, aside from the intake box itself, is the stock element. Here's a cut-away of an H2 intake with the stock Donaldson-style filter inside.
On a typical four-cycle engine, the atomized fuel mixture is drawn into the cylinder, past the open intake valve by the downward motion of the piston. As the piston moves upward, the air/fuel mixture is compressed and ignited. The force of the explosion pushes the piston downward. This is considered the power stroke. On the piston's return upward, the burnt gases are pushed out through the open exhaust valve, and the process begins again.
Sounds simple enough, right? But where's the "hidden" horsepower in this equation? By improving the efficiency of the engine in any of the critical areas (intake, valve timing (camshaft), ignition, or exhaust), you can make more power. For the purpose of this discussion, we'll focus on the intake side of the engine.
This is the same H2 intake...
This is the same H2 intake with a high-flow Airaid SynthaFlow replacement air filter, which flows more cfm than the stock unit while still fitting in the original housing.
Most manufacturers face quite a challenge when it comes to designing intake systems for today's trucks. First off, the systems have to fit the platform (model). Secondly, they have to thoroughly filter the incoming air to the engine. Finally, these intake systems have to be somewhat quiet to reduce the vehicle's overall NHV (noise, harshness, and vibration). What's left is what you get on your new truck. Does it work? Yes. Is there room for improvement? Most definitely.
On the stock intake system, the main airflow restrictions are usually the design of the intake box itself. This is the actual air inlet, where the box meets either the inner fender or radiator bulkhead, with the factory paper-style filter inside the box. The easiest upgrade to any intake system is to simply change the filter from a paper-style element to a high-flow filter. The most common performance air filter is an oiled cotton-gauze style, which is capable of flowing more air while dirty than the stock filter flows while clean. Airaid Filter Company has taken the standard cotton-gauze filter to the next level by adding a synthetic layer of filter material, which improves filtration (down to 2 microns) without affecting airflow. The company's patented SynthaFlow layer is now standard on its line of direct-replacement filters as well as all intake system filters.
One of the easiest ways to...
One of the easiest ways to improve flow while retaining the stock intake is to install a high-flow filter in the stock box. Airaid goes one better with the '04 Ford F-150 by offering both a high-flow direct-replacement filter and an upgraded intake tube that attaches to the stock box. On the dyno, this Airaid Junior setup was good for an additional 12 rear-wheel horsepower and 18 lb-ft of torque.
The next step to better breathing is to change out the complete airbox for an aftermarket intake system. Since the stock airbox and airbox lid on most trucks are major restrictions, removing them and replacing the assembly with a high-flow air intake usually generates a big improvement in not only horsepower and torque, but also throttle response. Sometimes just changing out the lid yields substantial results, as this flow bench data from Airaid indicates.
A typical airbox on an '04 Chevrolet Colorado pickup equipped with the 3.5L inline-five powerplant flows approximately 397.88 cfm of air at 20 inches of water. The Airaid QuickFit air intake, which retains the lower-half of the stock airbox but replaces the lid with a high-flow filter and cool-air dam, flows 615.98 cfm at 20 inches of water. That's a 54.8 percent increase compared with stock.
On the Power Stroke diesel found in many of today's Ford Super Duty pickups, the flow bench results also yielded gains when the entire intake was swapped out. The factory Ford intake system on the Power Stroke-equipped Super Duty is one of the best factory systems on the market. It uses a fresh air snorkel through the radiator core and a large commercial-style air filter in front of the turbo inlet.