When we began thinking about designing an intake for the twins, we first wanted to evaluate the claim that “the stock intake is good enough.” Its general knowledge that in the last ten years or so, that factory OEM intakes have become very good in design, and are often difficult to improve upon. There are several ways to evaluate this claim, and we wanted to start out with looking at the design of the entire intake as both an overall system, as well as the sum of all of it’s parts.
A visual inspection doesn’t tell an absolute truth about the system, but it does give you a place to start evaluating. The first source of restriction you’d look for is sharp or abrupt entry points. Air entering a pipe without a flared entry (think velocity stack, or a funnel shape) produces a restriction, compared to one that does have a flared entrance or transition. Just the same, when air has to traverse a larger and larger angle bend, there is an increase in restriction. The same can be said for when air has to pass over surfaces that are not smooth, etc. All of these situations add restriction, which can be measured as a drop in pressure. The ideal case to move air from point A to point B would be a perfectly smooth, straight length of pipe, and even that will have a pressure drop as the length of the pipe increases.
So from a visual standpoint, lets break apart the sections of the intake: There is a snorkel, front of airbox, air filter, rear of airbox, MAF housing, intake elbow, and throttle body. The entire system can be looked at as being the area before the snorkel (behind the bumper cover) to just passed the intake elbow (right at the throttle body). Measuring the difference in pressure between these two points will give you the overall restriction of the system. But in order to identify where the weaknesses in the system are, one would be more interested to measure the difference in pressure between components in the system. For example, to measure the restriction the air filter has on the system, you would measure the pressure before and after the filter. And if you add up the pressure differences between all parts of the system, it should equal the overall restriction.
Back to the visual inspection of the system, what do we see as a potential problem area, and why do we want to choose these locations to test? The first part of the system that air sees as it enters is the snorkel. The inlet of the snorkel looks good; there is a well formed velocity stack that has minimal extra material from being molded. It’s a slight oval shape, roughly 2.25in x2.5in. About 10 inches down the air’s path, the snorkel starts to make an approximate 90 degree bend to it’s exit. The bend is very smooth, and all the while the shape is transitioning to a flatter oval, while at the same time increasing in overall cross sectional area. At the point where the snorkel transitions into the air box, it is roughly 2in x 5.7in. The snorkel contains two resonators along the first section, in two different sizes, each containing a small drain hole at their lowest point. The snorkel is sealed to the air box with a strip of foam that interfaces the outlet of the snorkel to the inlet of the front airbox.
The front face of the airbox is angled at the bottom, and contains a circular emboss. Both features are in place to maximize area before the filter, while still clearing the radiator and fan. There is also a large resonator to the left of the entrance. The front airbox has a hole at it’s lowest point just right of the entrance, as does the large resonator, both for drainage purposes. The inside of the front of the airbox is very smooth across all surfaces. The only noteworthy point from a flow standpoint is at the entrance. The half of the entrance below the snorkel has a smooth radius flowing towards the filter. However, the half above the entrance is abrupt, and looks different than you would expect from viewing it from outside the box. Outside the box, just above the exit of the snorkel there is a hump which looks to exist as an area to smooth airflow going towards the filter, but just the opposite appears to be true as there is a void here. One can only assume this is for strength, or some phenomenon that is hard to explain.
The air then flows through the filter, which is not your typical paper filter, and has only 14 large ribs. I am unsure of the media of the filter, but it is similar to a fabric like cotton. The ribs on the front side are longer than those on the back to increase filter surface area.
After the filter is the rear of the airbox, which contains mostly smooth transitions, with a taper at the opposite side to the exit that should promote flow towards the MAF housing. The only noticeable source of restriction in this piece are several protruding ribs that run lengthwise in the rear of the airbox, however small. The exit of the airbox is technically the mass air flow, or MAF, housing. The entrance to the MAF housing appears to have been optimized, as it is one of the most important parts of the entire engine. The rear face of the airbox has a section “dug out” to smooth the transition into the MAF, and the opposite side of that feature has a molded plastic velocity stack. Immediately at the entrance is a plastic matrix that is commonly referred to as an “air straightener.” This is specifically put in place to help the MAF provide the most accurate reading as possible by modifying the flow of air before it. The thickness of the pieces of this matrix is 2mm, and the diameter of the entrance here is roughly 68.5mm. The entire MAF housing is only about 70mms long, and places the MAF sensor about 25mm, or about 1in after the air straightener. The inner diameter at the MAF sensor is 70mm, and the diameter at the outlet of the MAF housing is about 72mm. So there is a taper through the entire section, albeit minimal.
At the exit of the MAF housing is the entrance of the intake elbow. The entrance to the elbow is just under 3in in diameter, and has an immediate 90 degree bend. This bend is very tight, and has a centerline radius significantly under 3in. This most likely means that the diameter of the cross section does not stay a constant 3in as the bend progresses. There are ribs on the outside of the part for strength, but they do not exist on the internal surface of the elbow. There is a tube exiting the elbow for the sound tube, just opposite of the intake elbow’s entrance, and a resonator toward the bottom of the engine bay, both located directly on the bend. Immediately after the bend is a roughly 2.25in long flex section. This section contains 5 smooth ridges that exist on the inside of the tube, and extend outwards of the tube less than .125in. After this flex section is a 5in long straight section, smooth on the inside, with ridges on the outside. This terminates at the entrance of the throttle body.
Based on this visual assessment there isn’t much to expect in the way of restriction. From the entrance of the system to the exit, we expect to see a restriction from: 90 degree bend of the snorkel, air filter, decreased size (in comparison to the air box volume) of the MAF housing, the tight 90 degree bend on the entrance of the intake elbow, and the flex section located right after the previous bend.