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
But it wasn’t to be.
A Big-Bore Feed? Not in this Case...
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.
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.
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...
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.
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...
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.
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.
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
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...
Airflow Restriction (Inches of Water)
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.
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
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
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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