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Dual Pipe Induction - Part One

A slightly different approach to pre-airbox intake enhancement

By Michael Knowling

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At a glance...

  • First of two-part series
  • Introducing an innovative approach to pre-airbox intake enhancement
  • Probing the standard Nissan 180SX intake system
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This article was first published in 2006.

Modifying your car’s air intake system is one of the most cost-effective upgrades you can make. For well under AUD$100 you can fit a big-bore feed pipe into the factory airbox and enjoy improved throttle response together with a smoother, stronger power curve. It’s a process we’ve proven time and time again.

So when we decided to enhance the intake of our ’89 Nissan 180SX the decision was easy – once again, we wanted a big-bore feed pipe into the airbox. The idea was to purchase a ‘universal fitment’ cold air induction system through eBay, bung it into the Nissan and present it as an article. Something along the lines of a low cost, low fuss, ready-to-install system – great for people who don’t want to hunt around for parts and materials.

But it wasn’t to be.

A Big-Bore Feed? Not in this Case...

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After stripping the guard liner and various ducts from our 180SX it became obvious there isn’t a lot of useable space in the vicinity of the airbox. A single big-bore feed pipe wasn’t going to fit without absolutely massacring the metalwork, a tortuous pipe route, relocating an under-bonnet fuse box (and probably extending its wiring loom) and blocking the factory airbox intake.

So, yes, it could be done but the final product would’ve looked daggy and we were straying further and further away from the original ‘low fuss’ concept. An alternative approach was needed.

A Secondary Intake Passage – Why Not?

In recent months we’ve seen two performance workshops – MRT and Silverwater Automotive Services - take an innovative approach to intake system modification. Nothing groundbreaking but an approach that looks perfect for our application.

The idea is to leave the factory intake system in place and introduce a secondary intake passage to the airbox. By adding a relatively modest diameter secondary intake passage you can achieve a similar airflow capacity as achieved with a replacement big-bore feed pipe. But there are several advantages to this approach – a secondary intake is easier to incorporate where space is limited (a small diameter pipe is much easier to bend into shape than a big one), it maintains more of the standard under-bonnet appearance and it’s even cheaper than using a big-bore pipe.

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This photo shows the air intake system used in a MRT enhanced MY06 Impreza WRX. It might look standard, but that’s not how it performs! In the standard car, induction air enters a pick-up above the radiator, flows into a resonant chamber inside the left front guard and into the airbox. MRT modifies the in-guard resonator so that it becomes a secondary air pick-up – this reduces restriction caused by the standard over-the-radiator pick-up and lowers the pre-turbo intake air temperature.

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Similarly, this photo shows the air intake system used in Silverwater Automotive Service’s modified Ford XR6 Turbo. As you can see, the factory over-the-radiator snorkel is supplemented by a secondary intake snorkel that draws air from behind the left headlight. Combined with a K&N air filter, this set-up achieves a huge flow gain over standard.

So that’s the concept. Now let’s examine the design and performance of the 180SX’s existing air intake system.

The Standard 180SX Air Intake Design

It’s all too easy to take a quick look at an OE air intake system and come to the conclusion it’s all cheap and nasty. But the 180SX proves there’s often more to it than meets the eye...

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The 180SX air intake begins with curved plastic snorkel that feeds the lower half of the airbox. The entry of the snorkel is sealed against an opening behind the left pop-up headlight assembly – an area separated from under-bonnet heat. Interestingly, there is also a noise suppression section incorporated near the front of the snorkel. A series of holes through the wall of the pipe let induction air enter a small chamber that’s filled with sound absorption material – it works like a muffler.

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After passing through a right-angle bend at the end of the plastic snorkel, induction air enters the bottom half of the airbox. Again, the inside walls of the airbox are lined with sound absorption material in a further attempt to reduce induction noise. Not the sort of thing we expected to find in a 180SX – a Lexus or BMW, maybe...

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The 180SX air filter is the same as used in the contemporary Nissan Pintara and many other garden-variety cars – a Ryco A360 panel filter. Clamping onto the filter is the top half of the airbox which, unlike the bottom, isn’t lined with sound absorption material. But, as seen in this photo, there is a plastic resonant chamber T’d in.

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Also inside the airbox lid you’ll find a bell-mouth entry that improves airflow into the airflow meter. This metal bell-mouth is the same as you’ll find in cars such as a Holden VL Turbo, Pintara and early WRX. After passing through the bell-mouth, induction air enters a hot-wire type airflow meter. This photo shows the meter’s two airflow passages - one very small and another almost 50mm diameter.

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The final section of the intake system is the turbo induction pipe. In the 180SX, this pipe incorporates hose fitting for the rocker cover breather and the ‘waste’ air from the boost control solenoid. The pipe has a very smooth internal surface but it does contain a wire spiral that helps the pipe keep its shape.

So that’s the design of the 180SX intake system – how does it perform in terms of airflow and intake air temperature?

Performance of the Standard 180SX Air Intake

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With our trusty Magnehelic pressure gauge we were able to the measure pressure drop at various points throughout the 180SX intake system. Testing was performed with the automatic transmission in second gear and with the engine at full power. This table shows the results...

Intake Section

Airflow Restriction (Inches of Water)

Plastic snorkel


Lower half of airbox and air filter


Top half of airbox, airflow meter and part of turbo induction pipe




From this table it’s obvious that the factory plastic snorkel causes a huge percentage of the overall intake restriction – 68 percent in fact. The bulk of remaining restriction is caused by the section incorporating standard airflow meter and it’s interesting to note that the filter element and post-filter resonant chamber cause bugger-all restriction.

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Using a Jaycar LCD temperature meter, we also measured intake air temperature in the lower half of the airbox. With air drawn from behind the left pop-up headlight, the measured intake temperatures typically hover around ambient temperature. Extended idle periods cause the intake air temp to rise about 10 degrees Celsius above the 25 degree Celsius ambient temperature.

So what can we determine from this testing?

Well, we know that we can achieve big gains by improving the airflow into the airbox – the standard intake snorkel is extremely restrictive. But, more importantly, we also know that the intake air temperature encountered using the standard air intake is quite acceptable – by drawing air from behind the pop-up headlight, the pre-turbo intake temperature is close to ambient temperature.

This is probably the most important point in deciding whether a secondary intake passage is the best approach for you.

If we had measured a relatively high intake air temperature there would be no point in adding a secondary air intake – the engine would still be breathing a proportion of hot air. In this case, you are better off installing a replacement big-bore feed pipe that picks up air from, say, inside the wheel arch. Fortunately, the 180SX intake is the perfect recipient for a dual pipe intake.

Stick around – in Part Two we’ll be designing and installing a secondary air intake passage!

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