Last week in Part 3 (see
Negative Boost Revisited,Part 3
) we were in the hunt for those pesky negative boost critters.
They’re the ones who hide around corners, in changes of diameter and in sharp
bends, waiting to rob your engine of power. We’d gone through each part of the
pre-throttle intake system on the EF Falcon, measuring the cross-sectional areas
of all the bits and pieces that go to make up the intake system, right from the
gap under the leading edge of the bonnet (that allows air to enter) through to
the diameter of the tubes between the airbox and the throttle.
In fact, to remind you, this graph shows what we
found. As can be seen, the cross-sectional area of the bonnet gap is huge
relative to the others. After that, the airbox entrance appears least in need of
change, however the airbox exit duct and the snorkel mouth could apparently both
do with changes. After those, the throttle duct (the twin pipe system) would be
the one to alter.
But all is not necessarily so simple.
While cross-sectional areas are very important,
other factors also come into play. For example, despite the airbox outlet
looking bad, flowbench testing that we’ve performed in the past shows it to work
well (on the flowbench anyway!). And despite the intake area of the gap between
the bonnet and the bumper/headlights being large, at that position the airflow
is wrapping around the upper edge of the bonnet, so potentially creating a low
pressure area - exactly what you don’t want at the location of the engine air
intake!
The only way to find out is to get out the trusty
old manometer or DMDPG – the Dwyer Magnehelic Differential Pressure Gauge....
Test Points
Some people get a bit excited when you suggest
drilling pressure tapping holes in the intake system. But the holes are tiny and
after you’ve finished testing, a wipe over them with black silicone will yield
them invisible to everyone. So it’s no big deal. Sometimes you can temporarily
pull off breather hoses or suchlike and use these openings for pressure taps but
in the case of the Falcon, no such handy points were available so we drilled new
holes. Drilled them where then?
Pressure Point 1 was located just before the
throttle body. The pressure drop measured here takes into account the flow
restrictions throughout the intake system – from the bonnet gap right through
the filter, airbox, intake trunking; the lot. Pressure Tap 2 was located on the
outlet duct from the airbox, Pressure Tap 3 on the filter side of the airbox,
Pressure Tap 4 on the intake side of the airbox filter, and Pressure Tap 5 was
located in the mouth of the intake snorkel.
At each of these points a small hole was
drilled...
...then a small hose fitting was temporarily pushed
into place.
A small diameter hose was used to connect the
fitting to the Dwyer Magnehelic gauge, with a 0-8 kilopascals gauge used. (We’ve
converted the readings of this gauge into inches of water.) The gauge was
located in the cabin and read off by an assistant. The testing was done at 5000
rpm in second gear – of course at full throttle!
The Results
Let’s start of with the worse first. In other
words, just how big is the foul-mouthed, matted fur, yellowed teeth, smelly and
grumpy negative boost that lives in the Falcon’s air intake? Not that big,
actually. The peak pressure drop through the intake system (as measured at
Pressure Point 1) was 16 inches of water.
Maybe a comparison is in order. A standard Subaru
Liberty RS has over 31 inches of water pressure drop, the 5-cylinder Audi S4
(which was the subject of the original Negative Boost series) has a total nearly
30 inches of water, a Nissan Maxima V6 Turbo also has 30 inches, a near standard
Holden VL turbo is much the same at 29.5 inches of water, a Toyota Crown
Supercharger has 20 inches, a NHW10 Toyota Prius hybrid just 10 inches of water,
and a JE Camira a measly 9.6 inches of water. (Note that other than the Camira,
all of these cars use airflow meters.)
So for a standard car, the Falcon certainly
doesn’t have a bad air intake. But there were still 16 inches of water of
pressure drops through the intake system – and there doesn’t have to be any!
(Well, there doesn’t have to be much – eliminating all pressure drop is usually
not worth it. Yep, some hairy critters will remain.)
But what made up these pressure drops?
|
No. |
Location |
Total Pressure Drop |
Difference |
Description of Section of Intake |
|
1 |
Before throttle |
16.0 |
|
|
|
|
|
2.8 |
Dual duct between box and throttle |
|
2 |
Outlet duct of airbox |
13.2 |
|
|
|
|
|
6.0 |
Airbox outlet |
|
3 |
Outlet side of filter in airbox |
7.2 |
|
|
|
|
|
1.0 |
Filter |
|
4 |
Intake side of filter in airbox |
6.2 |
|
|
|
|
|
3.0 |
Snorkel |
|
5 |
Intake of snorkel |
3.2 |
|
|
Let’s take a look at this table cos it tells the
whole story. By looking at the pressure drops at each point within the intake
system, we can quickly conclude how much each section contributes to the total.
So (and this is the classic case that I love
repeating each time we do a story like this!), by looking at the measured
pressure drop after the filter (7.2) and the pressure drop before the filter
(6.2) we can see that the pressure drop across the (new) filter is just 1 inch
of water, or 0.036 psi! In fact, as it is always the way when you measure
pressure drops, the filter contributes stuff-all to the total intake flow
restriction. In all the testing we have done, we have never seen a factory
filter contributing more than a trivial amount of restriction to the total –
never! So, upgrading the filter in the standard airbox is a complete waste of
time.
So what mods should I do? The pressure drops
caused by the other sections of the intake can be seen in the table, but it’s
easier to see when it’s graphed. The chief hairy monster is the airbox exit –
over the few centimetres between the inside of the airbox and the outlet duct,
there’s a pressure drop of 6 inches of water - that’s nearly 38 per cent of the
total intake restriction! But what about the previous flowbench testing? Well,
apologies to those who have followed that test in the modifications they have
done on their cars, but I don’t reckon it stands up to scrutiny. The outlet duct
of the EF airbox is the smallest cross-sectional area of the intake system and
it creates the greatest pressure drop. That means it flows badly!
Next up on the smelly list is the snorkel. Or is
it? Let’s take a step back. We measured the pressure drop at various points
through the system to calculate the contribution of each part. But we missed
one. Look at the last line in the table above – the intake of the snorkel had a
pressure drop of 3.2! That means the air is not able to adequately flow through
to the mouth of the snorkel – there’s a negative pressure hiding in the
gap between the bonnet and the bumper! The bastard!
This graph includes that pressure drop as well. As
can be seen, addressing the way that the air gets to the snorkel is in fact just
a bit more important than the snorkel itself – but both need to be changed. And
hell, not far behind at all are the dual ducts that connect the airbox to the
throttle. In fact, just about the only thing that can stay unchanged are the
airbox and filter...
Conclusion
The results of the pressure testing stacked-up
reasonably well with the cross-sectional areas we measured last week. The
exception – and it’s an important one – is the gap between the bonnet and the
bumper/headlights (despite calculating out as a large cross-sectional area)
imposes quite a major flow restriction – in fact, 20 per cent of the total.
So the results of the 30 minutes of zero-cost
testing show changes needed to the way air gets to the snorkel, the snorkel
itself, the outlet of the airbox, and the ducts that connect the airbox to the
throttle body. Hmmm.....
Next week: making some changes
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