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Negative Boost Revisited, Part 3

Measuring areas to find intake restrictions

by Julian Edgar

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At a glance...

  • First steps in intake system modification
  • Measuring and calculating cross-sectional areas
  • Zero cost but gains useful information
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So you've read Negative Boost Revisited, Part 1 and Negative Boost Revisited, Part 2 and you're indignant about the presence of negative boosts (ie pressure drops) in your intake system. Why? Cos you know that every one of them harms flow - and that means that your engine develops less power than it should. In fact, you were so excited about them that you've gone off and built your own manometer (or bought a DMDPG - told you to pay attention to abbreviations!). Now whadya do with them? Well, before we get to using those instruments, let's go do some observing and measuring.

The Guinea Pig

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The car that we’re gonna apply the technique to is an EF Ford Falcon, which uses a 157kW in-line, 4 litre six cylinder engine. This engine was produced with plenty of different factory versions of the intake system from which we can pick and chose bits and pieces. But, until we find where the negative pressures actually are, we’ll leave everything stock as. I mean, why would you want to change things at random?

So what’s the air intake system look like? This is where people start losing the hairy critters – they take a casual look at the intake system and then make changes. But that approach is not for us – instead we’re gonna inspect every single part of the system. It doesn’t cost a cent and takes only little time, so why not?

Bonnet Gap

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The air pickup point (ie the snorkel opening) is high on the front of the car - tucked under the leading edge of the bonnet in fact. This is a common location for cars of the last 10-15 years, with the high point good for stopping the entry of water, and the partial concealment behind the bonnet lip good for cutting down dust ingestion.

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The air that flows to the engine air intake must all come through the gap between the bonnet and the bumper/headlights. That’s because...

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...a rubber seal prevents air being drawn from the engine bay and...

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... air cannot flow up from in front of the radiator because of the presence of this panel (and another that’s hidden beneath it).

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So how big is the area through which this air can flow? Is this small gap likely to be causing a flow restriction? One indication of whether this is the case is its cross-sectional area.

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Blu-tac is a cheap and easy way of accurately finding the width of this gap. A cylinder of it is rolled and placed so that it will be squashed when the bonnet is closed.

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This is done and then the thickness of the squashed Blu-tac measured. The process is repeated in different spots until a feel for the average size of the gap is gained. The smallest measured gap is 12.6mm and the total length of the opening is about 1500mm. Multiply these two measurements together and you have a cross-sectional area of about 19,000 square millimetres.

Area of bonnet gap: 19,000mm square

Snorkel Mouth

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The airbox snorkel connects the bonnet gap to the lower half of the airbox.

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Assuming a few millimetres for wall thickness, at its smallest dimensions it’s about 38 x 98mm. That’s a cross-sectional area of 3700mm square, substantially smaller than the bonnet gap area.

Area of snorkel mouth: 3,700mm square

Air Box Entrance

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The entrance to the airbox – ie where the snorkel attaches – uses a round pipe with an internal diameter of 78mm, giving a cross-sectional area of 4,800mm square – bigger than the intake mouth of the snorkel.

Area of air box entrance: 4,800mm square

Air Box Exit

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The airbox exit uses an unusual extended snout, finishing in a small bell-mouth opening. The minimum size of this exit duct is just 56mm, giving a cross-sectional area of only 2,500mm square.

Area of air box exit: 2,500mm square

Air Box > Throttle Duct

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A very interesting duct design is used between the airbox and the throttle body. (This car uses MAP sensing, so there’s no airflow meter.) As can be seen here, a single duct exits the airbox and then smoothly splits into two, each with an internal diameter of 52mm. These ducts are connected to two sections of convoluted hose before they rejoin prior to meeting the throttle.

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It’s the underside view which is even more interesting. A blind-ended duct joins the main duct just before the throttle. This tuned-length duct is almost certainly present to act as a Helmholtz resonator to decease intake noise – but it may also aid cylinder filling.

The two smallest sections of this system, each with an internal diameter of 52mm, result in a total cross-sectional area of 4,200mm square.

Total area of dual ducts: 4,200mm square

The Areas

All the measurements used in this article are approximate – it’s simply not worth it at this stage making super-accurate measurements. What we want is an overview of flow cross-sectional areas. However, when we use the manometer, we’ll increase the accuracy considerably!

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As can be seen in this graph, the cross-sectional area of the bonnet gap is huge relative to the others. (But all may not be as it seems – we’ll come back to this in a moment.) In fact, with them all on the same graph it’s a bit hard to see what’s what....

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...so let’s delete the bonnet gap area from the picture. On the basis of these figures, the airbox entrance is least in need of change, however the airbox exit duct and the snorkel mouth could both do with changes. After those, the throttle duct (the twin pipe system) would be the one to change.

However, if it were all this easy, the hairy horribleness of negative pressures would be simple to find. And often they’re not. Take that airbox outlet duct, f’rinstance. Back in 1998 we tested a bunch of airboxes on the flowbench, including this very same EF Falcon design. In comparison to an AU Falcon airbox, which doesn’t use the long internal snorkel but has a larger exit, the EF Falcon airbox out-flowed it by 18 per cent! So maybe this design actually works – as opposed to just looking a bit average. Or maybe the flowbench – working at just 1 inch of water pressure differential – couldn’t suck enough air through the box to get really meaningful figures? The answer is to get out the manometer or Dwyer meter and do some real world testing.

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And here’s another one for you to think about. The intake area of the gap between the bonnet and the bumper/headlights is large – in comparison to the other cross-sectional areas, huge in fact. But hold on, it’s also directly connected to the flow over the front of the car – the flow of outside air. And airflows wrapping around the curve of the leading edge of the bonnet often generate a low pressure area – exactly what you don’t want at the location of the engine air intake! So despite its cross-sectional area being large, the fact that it’s nowhere near the stagnation point (the location of max pressure, which typically is about halfway down the bumper), may be harming intake airflow heaps! Again, get out that manometer or DMDPG...

And that’s just what we’ll be doing next week.

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