Cold air intake design

Nissan Micra over the top style cold air intake design Nissan Micra over the top style cold air intake design Nissan Micra over the top style cold air intake design

Fundamentally, there are two main areas to improve on in terms of air intakes for your vehicle.

They are:

  • Air flow
  • Air temperature

In short, you want the shortest, smoothest and straightest section of pipe bridging the gap between the source of cold air and your growling throttle body. When I say "shortest", this is not always purely the case. In the case of tuned length intake pipes, sometimes it is more desirable to select a specific length intake duct as this may significantly reduce turbulence and operate in cooperation with the pulses of waves inside the intake duct improving the efficiency somewhat. But the general idea is to go for a 50cm straight duct instead of a 2 metre duct full of curves that runs around the perimeter of the entire engine bay.

Sharp bends form a source of heavy restriction in the intake duct and thus must be avoided! It is almost always more desirable to even add another 50cm of straight length to your intake duct if it will mean avoiding a very sharp bend. To give an example, A sharp 90 degree bend in a 3" section of intake duct creates the same flow restriction as approximately 2.3 metres of straight duct! This demonstrates just how critical it is to minimise the number of bends in your cold air intake duct. Slight bends are ok, but as always straight is better if possible! Also: be careful not to make your air filter too exposed to picking up too much dirt or water from underneath the car etc.

Regarding inlet air temperature, its very simple, the hotter the air entering your vehicle's engine, the more power you lose and the higher the risk of detonation: BAD. You want the coolest air possible, that is: as close to the ambient temperature outside as possible. Ideally the air should be colder than the ambient temperature, but unfortunately present cooling technology is unable to do this without robbing at least as much power as you would gain from the engine. You want to minimise any heat absorption by the entering air from the engine as it passes through your duct and into the engine. This can be achieved by choosing the best path for your duct to be positioned in, but additionally by effectively utilising heat shielding to protect your intake duct from heat soak. For example, shielding your exhaust manifold (and turbocharger if you have one) by way of HPC coating or some other form of heat shield is a must as this is the greatest source of heat in an engine bay. This will reduce the amount of hot air floating around the engine bay getting sucked into the air filter or being soaked up by your duct.

Now, in the case of the K11 micra, I found from experience with trialing many different designs that the best design was a 3" flexible PVC pipe in combination with a 90degree steel mandrel bend ran directly from the throttle body over the engine cam cover and attached to a pod-filter positioned just behind the grille to face incoming cool air.Unfortunately, with the position of the throttle body from the factory meant that it was virtually impossible to get past using a 90degree bend exit from the throttle body. [Note: On my current turbocharged Super S micra the inlet plenum exits sideways which provides an excellent entrance point for the charged intercooler piping without the need for tragically sharp bends.] This design was also recommended to me by Paul Brell at BD4s performance in Sydney who has had enormous experience with cold air intakes and the like. I also kept a formula in mind which used to be recommended by K & N air filters: As a rough rule, it is desirable to have approximately 130% the capacity of your engine in post-filtered air already available past the filter to give you good throttle response. As the CG13DE is 1.3 litres in capacity, this formula states you should have about 1.7 litres of post-filtered air in your intake duct. I didn't calculate the volume of my intake duct exactly, but judging by the size of 1 litre milk cartons, my intake duct roughly satisfied this rule, in fact I went slightly over this figure.

One of the main reasons that the factory issues the car with a reasonable volume air box is for the throttle response of having that much air already filtered and ready-to-go.

Also, luckily in the case of the micra, Nissan have placed the air-flow meter directly inside the throttly body as opposed to being positioned somewhere in the intake duct. This gives you the key advantage of being able to remove the entire factory airbox and ducting and replace it with your own design. I found that the factory micra airbox was not ideally suited to being modified and running your own ducts to it, so I removed it completely.