What is an intake system?

For the sake of this discussion, the phrase "intake system" is being used to describe the duct work feeding air to an engine prior to the throttlebody, including the large duct, filter, and any other parts thereof.

OEM

The original equipment intake system on a car consists of a resonator, lots of plastic duct work, a large metal or plastic air filter canister, a paper air filter element, and a rubber accordion hose between the filter canister and the throttlebody. Most often, the intake system will pick up air from behind the fender or bumper, from the very end of the plastic duct work and resonator. If this is the case, this is advantageous because cooler air allows the engine to make more power than hot air, as the air inside the engine compartment.

However, there are several disadvantages to the OEM intake system.
The resonator and the plastic duct work can be very restrictive to air flow. These pieces are designed to reduce intake sound, and performance is not the greatest concern. removing the resonator and enough of the duct work such that the system still pulls in air from behind the bumper or fender can do a great deal to improve air flow.
The paper filter element is designed with money as its chief concern. Paper does not flow as well as cotton gauze or cloth, as is used with aftermarket performance drop in filters, such as those made by K&N. These filters provide a great deal more air flow with the added advantage that they are reusable and can be washed, reoiled, and reinstalled lasting over ten years, as opposed to the paper filters which are thrown away every year or two with proper maintenance.
The filter canister itself often has inlet and outlet passages that are quite small and constrictive, but eliminating this restriction is a bit more involved and will be discussed in the next several sections.
The accordion hose between the filter canister and the throttlebody does not encourage very good air flow. The ribs of the hose extend into the air flow and cause turbulence, reducing flow. Eliminating this restriction will also be covered in the next several sections.

Hot Air Intake Systems

This type of intake system gets its name from the temperature of the air that is being drawn into the engine. Hot air intake systems are probably the most common and least effective "upgrade" to an intake system. These involve removing the resonator, plastic duct work, paper filter, and air filter canister and replacing these items with a cone filter mounted within the engine compartment.

Cone filters are cylindrical or cone shaped filter elements with a flanged in that is designed to be clamped onto a pipe, drawing air from all around and into the pipe. They are most often cotton gauze cloth and reusable.

The simplest of these hot air intake systems involves mounting a cone filter to the end of the accordion hose. This has the dual disadvantage of drawing in hot air from inside the engine compartment as well as continuing the use of the accordion hose which restricts air flow.

The next step up from this would be the what many call a "short stack" intake involving the removal of the accordion hose and replacing it with a short piece of pipe which the cone filter is then attached to. This eliminates the restriction of the accordion hose, but the disadvantage of drawing only hot air into the engine remains. Mandrel bent pipe is desirable as there is not reduction in interior volume (no bottle neck) as is found in squeeze bends, which will loose one third of their interior volume in a right angle bend.

Another type of hot air intake system involves cutting holes in the side of the OEM air filter box which is erroneously thought to increase air flow to the filter. Unfortunately, all of the air drawn into the filter through these holes is hot air from the engine compartment

The main disadvantage to all of these is the fact that they are drawing in hot air from inside the engine compartment. Intake air temperature is very important whenever performance is a concern. It is a general rule of thumb that a engine will loose one horsepower for every additional ten degrees that the intake air temperature is increased. The engine compartment of an automobile is a very hot place, with heat radiating off of the engine block and head, the exhaust pipe, and air passing through the radiator which is full of 180 degree water. it is no surprise that under hood temperatures can easily reach over 220 degrees. Even on hot summer days when it is 100 degrees, the potential intake air temperature with these hot air intake systems is 120 degrees over the temperature of the outside air. It is very easy to see that any performance gain from reducing restrictions of air flow is quickly lost from increased intake air temperature.

Sport Compact Car and Import Tuner Magazines have done several dyno tests of intake systems. All have shown the short stack hot air intake systems to increase power output of the engine by .2-7 HP, but this only tells part of the story. In the September 1997 issue of Sport Compact Car, Dave Coleman goes into great detail to describe the testing procedure that they used to test these hot air intake systems. All cars were tested with the hood up, meaning that this was by no means a test of real world conditions, where by the hot air in the engine compartment of the car would be held in the engine compartment by the hood. In addition, Dave Coleman goes on to explain that the radiator fans of the test car was unplugged and disabled, because the thermal electric switch turned on one of the fans half way through a dyno run and the horsepower needle took a nose dive as the fan blew hot air from the radiator onto the cone filter. Unless a driver plans on driving around with the hood open and the radiator fans disabled, count on never seeing most of the .2-7 HP that these systems were tested to produce.

In fact, under real world conditions, meaning operating the car with the hood down and the radiator fans operating, loss of power from increased intake temperatures will, more likely than not, cause a net loss of power with the use of these products.

Performance will be worst when attempting to accelerate from a standing start, as engine compartment temperatures will be highest when the car is not moving, lowering only marginally with the air being blown through the radiator as the car is moving, because, remember, the radiator is full of 180 degree water and the job of the radiator is to transfer heat from that water to the air passing through the radiator.

Many people will vigorously defend hot air intake systems, claiming that they can feel a great deal of power improvement, but much of their claims can be attributed to the placebic affect of a louder intake sound. "It sounds louder, it must be making more power" is the theory, but actual testing tells a very different story, that these are the least effective way to improve engine performance.

Air Bath Systems

Air bath systems are used in conjunction with hot air intake systems as an attempt to offset the problem of hot engine compartment air. These most often consist of a long piece of dryer hose that is to direct air from outside the engine compartment to the filter, often using a scoop to be mounted on the front of the car or in the grille work. Some people will erroneously call these "ram air" intake systems, which they are not.

These systems are at best a very poor band aid and at worst a total waste of time and money. The engine will not draw air through a dryer hose that is near or even on a cone filter unless the filter is sealed in a container with the dryer hose as its only opening to draw air through. This should sound familiar, this is exactly what the OEM air filter canister is. The only readily available cone filter canister is a product made by PRM, which is quite expensive.

Systems with scoops to catch air from the grille are equally ineffective, as air will only be blown around the filter when the car is moving at highway speed. As discussed earlier, when the car is sitting still, there is no air movement through the air bath system, and this is when under hood temperatures are highest, resulting in the same poor power output when attempting to accelerate from a standing stop. In addition, blowing air around a filter is somewhat less than effective, as these cone filters are designed to draw air from all around the filter, and there is no mechanism to prefer cooler air blowing from a dryer hose onto 1/4 of the cone filter versus the hot air that is around the other 3/4 of the cone filter which is drawing in hot air from the engine compartment.

Cold Air Intake Systems

A cold air intake system is named such because it draws air in from outside the engine compartment where the air is comparatively cold as compared to the hot air inside the engine compartment.

The most common and most effective design is to use sections of mandrel bent pipe, connected with turbo hose connectors, leading from the throttlebody leading out of the engine compartment to the area behind the bumper or behind the fender, where a cone filter is fitted to the pipe to draw in cool air from outside the engine compartment. The combination of the cooler intake air and the reduction in flow resistance results in significantly higher power improvement than any of the other intake systems discussed here. In addition, the intake system will typically be three or more feet in length, tuning the intake charge and effectively acting as an extension of the intake manifold of the engine, almost as if it were a header for the intake side of the engine, improving low and mid range torque. If all this weren't impressive enough, the added length of the pipe work also encourages something called "laminar air flow effect" whereby the air passing through the pipe is unobstructed and begins to act somewhat more like a liquid than a gas, gaining momentum as it passes down the pipe and resisting anything that would stop its flow, acting with a air ramming effect as it passes through the throttlebody.

Somewhat less common are cold air intake systems made up of dryer hose and other materials. Dryer hose has ribs in it and much like the accordion hose discussed earlier, it causes turbulence and resistance in the air flow, negating any of the laminar air flow effect.

There are few, if any, disadvantages to the mandrel bent pipe cold air intake systems. The air intake point is typically lower than the OEM system, meaning that the car can not be driven though deep standing water (puddles or streams deeper than a foot) and there should be a provision in the design for a splash guard to keep water from being thrown off the tire and onto the filter.

Testing by Sport Compact Car and Import Tuner Magazines showed 9-13 HP improvement in engine performance from this type of intake system, with no difference in performance if the hood was open or closed or if the vehicle were moving or not.

Some will claim that heat from the engine compartment will be transferred through the pipe of the intake system and into the air passing through the pipe, but this is not true. Testing by Sport Compact Car showed negligible change in intake temperature between where it enters the filter and when it passes through the throttlebody. Their conclusion was that the air is passing through the pipe at a high enough speed that it does not have time to pick up any heat from the pipe itself. Further evidence of this is the fact that the Ice Man brand intake system, made of heat insulating PVC plastic, tested at 9 HP while the AEM brand intake system, made of heat conducting aluminum, tested at 12.9 HP. The conclusion is clear that the design of the intake system is more important than what type of pipe is used to make the intake system.

Ram Air Intake Systems

Ram air intake systems derive their name from their function of ramming air into the engine using scoops to pressurize air through the intake system and force it through the throttlebody. This is done by constructing a long intake system similar to a cold air intake system, with a scoop to catch air in the grille work or through the hood. The entire system must be sealed air tight with no leaks through drip holes, connections, etc. Anywhere that air pressure is allowed to escape will nullify any ram air affect. Air filtration can be either by placing a cone filter in the air scoop or by sealing the filter in a canister, similar to the PRM filter discussed earlier.

Ram air intake systems take advantage of drawing in cool air from outside the engine compartment, but they do have several serious disadvantages.
The air ramming affect is seen only when the vehicle is moving at highway speed or faster. There is no performance advantage in starting from a standing stop, aside from drawing in cooler air.
Decreased filter life is inevitable with bugs and road debris being forced into the intake system and into the filter element. Cleaning the filter will become a weekly task instead of a monthly or annual task. Damage to the filter by bugs and debris will cause filter failure with more frequency.

Shady Deals and Scams

There are a number of items on the market which are somewhat less than effective in increasing performance.

Air swirling devices, often called Vornados or Tornados, consisting of a ring of fins made of metal or plastic, designed to be placed in the intake pipe or duct, have a rather dubious reputation. These devices are claimed to swirl air in the intake tract to aid gasoline atomization and causing the air to pick up speed and "supercharge" the engine. Testing by Sport Compact Car tested one of these devices and showed it to inhibit air flow, causing turbulence and resistance in flow, and decreasing engine power.

Electric superchargers are another rather suspicious item. These are electric motors that are attached to the intake system to pressurize air like a turbocharger or a belt drive supercharger. The only problem is that electric motors run on twelve volts of direct current are not strong enough to pump enough air to pressurize the intake system of a gasoline engine at idle, much less at 6,000 RPM, when even a small 1.6 liter engine is pumping somewhere around 5,000 liters of air through it each minute.

Air catalyst systems consist of blankets, wraps, or tubes of materials which are wrapped around or filter elements or packed within filter canisters around filter elements which are purported to release chemicals into the intake air that supposedly improve combustion. While the chemical value of introducing a chemical into the intake air is questionable, there is no question that packing material around a filter will definitely inhibit air flow and reduce air velocity in the intake system, reducing power output of the engine.

General things to look for

An intake system that draws in cold air from outside the engine compartment.

Mandrel bent pipe that will not restrict air flow.

Cotton gauze air filter element.

Splash guards to keep water off the filter element.