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Dual Pipe Induction - Part Two

Designing and installing a secondary air intake passage

By Michael Knowling

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At a glance...

  • Final of two-part series
  • Designing and installing a secondary air intake passage
  • Before and after results
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This article was first published in 2006.

In the first part of this series we identified that 68 percent of our Nissan 180SX’s total intake restriction is caused by the plastic snorkel feeding the airbox. That’s huge. We also discovered that, due to limited space, it isn’t feasible to install a big-bore airbox feed pipe, so we decided to take the slightly unusual approach of installing a secondary intake passage. Why limit yourself to having only one feed pipe into the airbox?

Now let’s get down to the design and installation.

Designing a Secondary Air Intake

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One of the first considerations is how much power you expect to squeeze from the engine – the more power you want, the bigger the cross sectional area of the airbox feed.

If you’re anticipating a power output up to around 200kW you can achieve next-to-zero intake restriction with an airbox feed having a total cross sectional area of 46cm2 – the equivalent of a 76mm (3 inch) diameter pipe. In the case of our 180SX (with a standard output of 130kW), a 46cm2 airbox feed is ample.

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In most vehicles, you’ll need to fit a secondary intake passage that feeds directly into the airbox (as shown in this photo). Let’s assume you’re chasing up to 200kW. If the existing airbox feed pipe measures, say, 51mm (2 inches) diameter you’ll need to bore a new hole into the airbox and add a secondary feed pipe with a diameter of 57mm (2 ¼ inches) to meet our 46cm2 goal. The secondary intake pipe should then be routed so that it draws a cool supply of induction air.

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In the case of our 180SX, we were fortunate because the standard entry to the airbox is more than big enough to meet our power requirements – it has a cross sectional area of around 70cm2! This meant there was no need to bore an extra hole into the airbox - instead, we were able to connect the secondary intake passage into the rearmost section of the factory intake snorkel.

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The rearmost section of the snorkel connects directly to the airbox entry and, therefore, has the same 70cm2 cross sectional area. However, the cross section of the snorkel gets smaller as it nears the air pick-up so it’s important that the secondary intake isn’t installed too far along the pipe.

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This photo shows the air pick-up of the factory snorkel which has a cross sectional area of just 25cm2 – the most restrictive part of the entire factory intake. Given the standard pick-up’s 25cm2 cross sectional area, we chose a 51mm (2 inch) diameter secondary intake pipe to achieve a 46cm2 total pipe cross section.

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In our 180SX, the inner guard cavity was chosen as the air pick-up for our secondary intake. We chose this area because it is insulated from under-bonnet heat and there are plenty of gaps in the plastic guard liner that allow the inner guard to ‘breath’. It’s also very easy to run a short length of pipe from the rearmost section of the factory snorkel, through the inner guard and into the inner guard cavity.

Installing the Secondary Air Intake

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The first step in installing a secondary air intake to our 180SX was to loosen the bolts securing the airbox and remove the factory intake snorkel.

With the snorkel removed, we used a centre punch to mark where we want to run our secondary intake pipe through the inner guard. Care was taken to ensure the new pipe wouldn’t be fouled by the intercooler pipes that exist almost directly beneath.

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Using a hole saw, we drilled a 54mm hole through the inner guard. Note that a slow drill speed is required to avoid rapidly blunting the hole saw on metal surfaces.

Once the hole was drilled, we enlarged the opening with a file so that our 51mm flexible hose was a tight push-fit. The edge of the hole was then painted to avoid corrosion.

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Next, we grabbed the hole saw (now slightly blunt!) to bore a hole through the bottom wall of the plastic intake snorkel. Again, the hole in the plastic snorkel was enlarged with a file to achieve a tight push-fit with the secondary intake pipe. The newly created hole in the snorkel aligns closely with the hole through the inner guard – almost directly above.

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With the necessary holes drilled, we inserted the secondary intake pipe into the factory snorkel, being careful not to excessively penetrate the snorkel and cause airflow restriction. Next, we passed the secondary intake pipe through the hole in the inner guard and reinstalled the snorkel. The airbox was also tightened to its mounts.

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After partially removing plastic guard liner, we were then able to route the secondary intake pipe into an open volume inside the guard cavity. This ensures the pipe can breathe effectively without obstruction.

The air pick-up of the secondary pipe was secured in position using a cable tie and the plastic guard liner was reinstalled.

Job done!


So how does our 180SX’s secondary air intake perform? Pretty well, even if we say so...

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With the Magnehelic gauge again hooked up to the intake we saw pre-airbox restriction slashed from 17 inches of water to just 4 inches of water. This represents a 52 percent drop in the overall intake restriction – we’re now down to 12 inches of water from 25 inches of water! Intake air temperatures remain pretty close to ambient but there does seem to more resistance to temperature rise during extended idle periods.

Hand-timed 0 – 100 km/h acceleration is also improved from 7.6 seconds to between 7.5 and 7.4 seconds. And, finally, the intake mod is stealth - you’d never know someone had fiddled around with it unless you look r-e-a-l-l-y hard.

Not bad given the upgrade cost less than AUD$10 in materials and took only a couple of hours labour! Step aside big-bore airbox feed pipes that replace the factory system – you’ve just been aced.

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