Compact cold air intake

18 April 2004

Have a feature packed micra and struggling for a cold air intake solution?? Read on..

For a while I'd been thinking about slamming a nice cold air intake onto my girlfriend's white Super S. Originally, I'd overlooked the fact that I wouldn't be able to install an "over the top" style intake duct like I did on my first micra. The reason being that this car wasn't the same poverty pack that my first LX micra was - This Super S has some extras: air conditioning and ABS, with a power steering conversion highly likely in the short to medium term future. I sat there staring at the engine bay for about 20 minutes just thinking about all the possible placement options for a well designed cold air intake duct.

  • The potential power steering installation had ruled out the left hand side of the engine bay
  • The air conditioning voided any chance of an "over the top" design.
  • ABS and air con pipes also ruled out the left hand side of the engine bay.
45 degree bend

The only 90 degree bend near the bottom of the duct

Initially I was quite disappointed and thinking things like "what is the point of air conditioning if it locks you in to having a slow micra!". Then finally, I plotted out a possible path for a nice fat duct diving straight down under all the congested engine bay componentry and popping its little trumpet head out down nice and low to pick up a direct stream of oncoming cold air. Pretty sure that this would all fit, I raced down to a Bunnings hardware store and raided the PVC fittings section for an assortment of nicely made smooth mandrel bends. PVC is ideally for cold air intakes: it is reasonably lightweight, its very tidy and easy to spray paint any colour you so desire, and it comes in a whole range of diameters and varying angled mandrel bends to suit your desired application.

In total I purchased: -

  • 1 metre of 65mm PVC downpipe (~ $12)
  • One 65mm 45 degree mandrel bend (~ $4)
  • One 65mm 90 degree mandrel bend (~ $4)
  • One cool 100mm downto 65mm reducer which I spotted as a perfect trumpet for the pickup end! (~ $4)

Total cost: About $24.

Additionally, I had a few design goals in mind: -

  • Capture the greatest amount of direct oncoming air flow as possible. i.e. minimise anything being in between the duct entry point and the oncoming air.
  • For that air to be as cold as possible.
  • To absolutely not having to remove any of the extras such as air cond, ABS etc from the vehicle
  • To end up with a tidy factory-as-possible appearance inside the engine bay to cause minimal questions and problems with rego checks or curious police officers etc.
  • To increase the diameter of the intake duct substantially compared to the factory item.
  • To not allow any engine bay heatsock to affect my cold air stream.
  • To consider throttle response and therefore always have a reasonable volume of post-filtered air available on tap.
  • I wanted some kind of bellmouth/trumpet/flange on the pickup entry point to assist air into the duct as this is a proven design to assist with flow efficiency.

I did want to minimise the number of tight bends in the design, I was fortunate enough to get away with a smooth 45 degree bend up high in the engine bay, however a 90 degree was needed down low to angle the pickup horizontally to face the oncoming air. I could have used a 45 degree bend here, but it would have made the pickup at a very bad angle to the oncoming air as well as making it lower unless I shortened the main vertical section of the pipe.

Factory air box in place Looking at the entry into the airbox before modification Enlarging the airbox's entry duct The finished enlarged airbox entry duct Measuring the throttle body diameter

So you ditched the factory airbox ofcourse right?

Well no actually. The factory airbox serves two excellent purposes irrespective of the downsides it brings.

  • It holds a decent volume of post-filtered air, i.e. air that's already passed through the filter, this gives you excellent throttle response.
  • It completely separates and protects air from high engine bay temperatures. This in turn reduces the probability of detonation and lets you make the most power possible for that given volume of air because it keeps cool.

So the factory airbox was ideal for 2 of my design objectives. The problem with the airbox however was that it only catered for some completely tiny duct diameter and as a result is quite restrictive. Luckily that was easily remedied with a jigsaw. I expanded the entrance point into the airbox to accomodate my chunky new 65mm diameter pipe. The 65mm pipe I think actually refers to the PVC internal diameter, the external diameter of the fittings was larger than that. So its not far off 3" in diameter, however the next step up being 80mm clearly would not have squeezed into the small spaces I was working with and was deemed unsuitable.

I measured the throttle body and it was slightly under 65mm, closer to the 60 mark, so I deemed that 65mm would be a good choice in duct diameter being larger than what it was feeding. Considering this was the largest diameter PVC that I could fit, I considered it a great compromise.

Additionally, with my very low pickup point in mind, it would be pretty silly to position a pod filter so close to the ground.

Lining up the duct inside the airbox out of the car Installing the duct with the airbox lid removed Installing the duct with the airbox lid and air filter removed Checking the clearances underneath the car are safe from the gearbox and driveshafts Showing roughly how the duct will sit when in place relative to the car

Then I began to make a few measurements and start to line up the pipework to ensure it would fit. After a couple of refinements and shortening of the pipes, I had it pretty good.

As you can see in the picture left, I cut the end of the pipe that entered the airbox at an angle so that it would fit in tidily and not intrude on the air filter. This worked great, the pipe sat in there happily and still allowed the filter to sit firmly in place without contact.

The trumpet entry from the 100mm to 65mm reducer was ideal. I was pretty happy when I came across this at bunnings. Originally, I was going to heat up the end of the 65mm pipe and force it down over a ceramic pot or funnel etc to give it a bellmouth edge. However the amount of bellmouth flange I could have achieved doing that wouldn't catch anywhere near the same amount of air volume as this 100mm fitting. So that saved some time and looks very tidy/performance oriented. I think the 100mm pickup subtly sitting down behind the bumper looks very subtle and cool. Might make some onlookers think "hmm that's not factory" heh.

Looking into the duct Making use of some black spray paint The finished painted result Sealing the gaps with some $5 gap filler

Then I pulled out the assembled duct and sprayed painted it with matt black on some newspaper. It came up looking really good and tidy just like a factory intake duct - this is certainly the result I was after. Due to a surprising request from my girlfriend, I was ordered to leave the 100mm pickup white so that it was a bit visible. Hehe, not something I would normally do, but I think it still looks cool. The main thing is that everything visible from within the engine bay now looks totally factory!

After fitting, the final stage was to use some acrylic gap sealer inside the airbox with its lid and filter removed. There were tiny gaps in between where I'd cut the enlarged hole and the 65mm pipe. The cartridge gun took care of this in no time and sealed it up perfectly. This was important in ensuring that the airbox had no "easier option" to source its air from. I.e. any such gaps in the airbox would offer a less restrictive source of air that what could be pulled out of the long duct, this would almost guarantee some amount of hot air being sucked in from the engine bay.

So how did it turn out? Excellent! I'm extremely happy with the final result and for $24 plus a bit of acrylic gap filler and a can of black matt spray paint, its a bargain. This will give the car the top end air flow it needs once the extractors are fitted and its fuel pressure is increased. There is definately a small improvement in performance even now without extractors or increased fuel pressure. The car has better throttle response throughout the rev range, and seems particularly better at top end pulling ability. For example doing 100kmh in 4th gear up a long hill the car will actually accelerate reasonably well now, before this same hill caused the car to just sit steadily and it wouldn't accelerate until further up where the hill flattened out more. I plan to prove this with my g-tec meter sometime by timing a couple of different scenarios and comparing results.

The end finish to me looks extremely factory inside the engine bay. I think there would be a lot of rego check guys and people with no micra-specific knowledge who would look straight past it with no second thoughts. Its very functional and still allows space for all the extras like aircon, abs, power steering without hindering performance. The low pickup point has the potential to pickup some water from deep puddles of course, however I think that the approximately 2 foot long vertical section would prevent most of it even reaching the filter. This a risk I took to be low and ofcourse, you need to drive sensibly and according to the conditions. The regular checking and cleaning of your air filter is part of good vehicle maintenance.

The finished result - duct peering out from under the front bumper The finished result - tidy factory-like finish in the engine bay The finished result - tidy factory-like finish in the engine bay