Air Intake Advice
Sunday, May 6, 2007 - Rob Arnold
If your shopping for a high flow air intake system or thinking about making your own consider the following:
First, the intake air needs to come from outside the engine compartment. Too many kits locate the new filter in the same place as the old air box assembly but do nothing to route incoming air from outside the engine compartment. This may reduce any flow restrictions that exist but at the expense of a higher intake air temperature. More air flow at a higher temp (less density) doesn’t do us any good. Look for a design that takes advantage of factory ambient intake air routing.
Second, mass air flow placement in a replacement intake tube is generally a matter of convenience instead of deliberate design. The mass air flow sensor (MAF) is generally regarded as an air meter and while this is accurate it is also a rudimentary description of it’s duties. MAF sensors measure air mass and temperature, they do not measure air speed or volume. Schools of thought on MAF placement, especially with forced induction vehicles, varies from tuner to tuner. Most agree that the MAF should be placed where the smoothest and most consistent air flow is. Typically this must be before the turbo inlet or after the intercooler near the throttle body. On an aftermarket “cold air” intake it is best to allow around 12 inches of straight, unobstructed tube to place the MAF in. Depending on your engine dynamic, longer than necessary tubing can actually amplify intake resonance and reduce performance. If possible measure your intake resonance frequency across the throttle range and try to aim for tubing length that is at odds to that frequency length. Why not keep the MAF in the factory mounting location? The factory intentionally located the MAF to be in an optimal position within their design. With any drastic changes, MAF placement can become a serious consideration.
Third, we look at intake resonance. It is caused by the pulsing draw of air into the engine. The engine does not draw air like a shop vac at a steady or smooth rate. Each cylinder pulls in air for only one fourth of its rotational duration during one engine cycle. Considering the average four cylinder one might say, “four cylinders each pulling for a ¼ of its rotational duration equals a consistant draw”. This is fundamentaly correct except that the intake draw from each cylinder is progressive, that is to say, the intake draw is relative to piston position and speed. To further prove this point, listen to any vehicle with an aftermarket intake tube and high flow filter and you’ll notice a much louder and hollow ‘growling’ sound eminating, especially at idle. While this exists with factory fitted parts as well, the factory intake system has been designed and tuned to account for this. Additionally the factory inlet tube from the front of the vehicle that feeds the air box with ambient air has been designed to account for both intake resonance and incoming air buffeting. Intake resonance and air buffeting can affect the MAF sensor readings, jittery or bouncing readings are a sure sign of this, especially during moderate to wide throttle opening.
Last but not least we look at air buffeting. An example of this can be best experienced when the sunroof is not quite opened fully during higher speeds. The cyclic pressure change in the cabin is very much like what can happen with a poorly designed intake setup. Consider what the MAF must feel like in this same situation. Common designs for isolating the cold air intakes’ supply air can come from, hood scoops, lower cowl or wheel well placement. While all of these solve the problem of drawing in ambient air, some may develop the problem of buffeting. Is your intake experiencing air buffeting? Hard to say, MAF readings will surely clue you in. A well placed MAF will not see buffeting and steady MAF readings will indicate a smooth intake air flow at that point.
A final note to consider is the fresh air inlet placement in respect to the height of the vehicle. Intake air should be as clean and dry as possible. Rainy areas with puddles are common here in the northwest and care must be taken when choosing a mounting location. We certainly don’t want to suck in road spray during a rain storm or a puddle splash from standing water and potentially hydraulic the engine.