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Negative Boost Revisited, Part 4

Finding the flow losses

by Julian Edgar

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At a glance...

  • Measuring actual flow restrictions through the intake system
  • Isolating the exact causes of flow restriction
  • Working out which bits to change
  • Low cost, accurate real-world testing
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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.

Click for larger image

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?

Click for larger image

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.

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At each of these points a small hole was drilled...

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...then a small hose fitting was temporarily pushed into place.

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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?



Total Pressure Drop


Description of Section of Intake


Before throttle



Dual duct between box and throttle


Outlet duct of airbox



Airbox outlet


Outlet side of filter in airbox





Intake side of filter in airbox





Intake of snorkel


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.

Click for larger image

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!

Click for larger image

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...


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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