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Air Intake Design and Conical vs Square/Rectangular Filters

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Hi! New to the forum and am trying to soak up as much information as I can. In the process of converting my FD into a time attack car and am trying to thoroughly plan out the airflow through/around the car. I'm still running the stock twins for now, so need to route two pipes for intake to the turbos. I'm installing a vmount with proper inlet/outlet ducting, so running two 2.5" pipes past all of that ducting to the front bumper will be near impossible. Most people terminate the intakes at the front of the engine bay, but then the filters are still sucking engine bay air rather than air from the front bumper. My plan is to have both of the intake pipes stacked on top of each other feeding into a 3d printed air box that would have a some custom shape to grab air from the front bumper and snake it to the box location in the engine bay.

Two questions:

1. Two 2.5" inlets would need a surface area on the front bumper inlet of ~10" for full flow. Is there any advantage to slowing down the air velocity/increasing the static pressure in the air box by having a smaller front bumper intake area and having an expansion to the air box?

2. Instead of two massive conical shaped intakes in the air box, it would save a lot of space to have a rectangular panel style air filter in the box mounted vertically in front of the intake pipes. I would think the air flow would be better with less filter obstructing it, but all aftermarket cold air intakes I see use the conical shaped filters at the end of the pipes, even in an air box, rather than a flat rectangular one. Curious on what the advantages/disadvantages are to having one panel air filter instead of two conically shaped filters?


Hi Kellen, welcome 😃

The ideal situation is to take air from a cold (relative to the engine bay, anyway), clean and high-energy source. For us, that means somewhere on the front of the car.

As you mention, you do want to increase the static pressure by adding a gradual expansion along the duct. Theoretically, the expansion ratio should be in the order of 7 degrees max. Packaging is always a problem and very rarely do you have the room to expand cleanly and as gradually as you would like.

It also pays to keep all radii as large as possible, this is particularly tyre of the actual inlet. Sharp corners are no good here.

As far as the filter placement, a sensible arrangement I am most familiar with is an expanding duct that leads to a rectangular panel filter that sits quite close to the engine. However again, the practicalities of packaging will also play a big part in this. Below are some images of the kind of layout used on the current gen LMP2 cars to give you an idea.