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Eliminating Negative Boost - Part 3

Finding negative boosts on a real car - just where are those ugly little critters that are taking away my power?

by Julian Edgar

Click on pics to view larger images


So you've read Parts 1 and 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, let's get outa here and go and do some real life measuring.

No, not on a dyno - I said real life.

The Guinea Pig

Click for larger image

And what machine are we going to suss out? My Audi S4 - all 2.2-litre, 5-cylinder turbo of it. So what's the intake system on this car comprise anyway? Well, before the tur-, nope, let's start at the atmosphere end. The air pickup point is high on the grille - tucked under the leading edge of the bonnet in fact. This is a common location on late model cars, 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. The area of the opening is about 19 x 3cm - or a little bigger than the cross-sectional area of a 3-inch pipe.

Click for larger image

From its pick-up point the duct curves down into the guard, from which it reappears (but it's not the rubber duct that you can see in the above pic) and goes into the side of the vertical air-box through a 3-inch hidden convoluted duct. (There's no resonant box or anything inside the guard.) The bottom half of the airbox is h-u-g-e, and the paper airfilter is pretty bloody big too. It's both large in area (at 30 x 17.5cm it has a plan surface area of 525 square cm) and also has 5cm deep pleats, giving a very large effective area for filtering.

Click for larger image

Inside the airbox on the other side of the filter is placed a 70mm bellmouthed pick-up, which extends fairly closely towards the opposite wall of the airbox. This duct feeds a 75mm Bosch hot-wire mass airflow sensor, which is equipped with wire mesh screens at each end. From the airflow meter, a curved rubber duct takes the intake air straight to the compressor of the turbo.

Fitting the Pressure Taps

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Now you might be thinking - no way in the world am I gonna drill holes all over my intake system so I can measure pressure drops - no way. Well, you don't need to drill everywhere - just some places. I measured the pressure drop at three points, and drilled two small holes to allow me to do that. And aren't I worried that the formerly pristine airbox has now got two tiny holes drilled in it? Nope - a smear of black silicon and you'll need a magnifying glass to find 'em.

The first - and worst reading - pressure tap was placed just before the turbo. I accessed the intake duct at a PCV return line fitting, with the breather hose removed while I was taking measurements. This 'Total' pressure drop measuring point takes into account the restriction of the intake snorkel, airbox, air filter, and airflow meter. That's why it's called 'Total'.

Click for larger image

The next pressure tap I placed in the airbox on the engine side of the filter. I drilled a small hole and then screwed a miniature plastic barbed irrigation 'hose joiner' fitting (tiny black thing worth 22 cents from hardware store) into the hole. Then, cos it was gonna be a bastard to get at with the airbox back in place, I attached a short length of plastic hose to it with another hose joiner stuck on the free end. This pressure tap measures the restriction of half of the airbox, the filter, and the snorkel.

The last pressure tap was in the airbox on the atmosphere side of the filter. This one measures half of the airbox flow losses and the restriction caused by the snorkel. Again, I placed a short length of hose on the fitting.

Oh yeah, doing all of this took about half an hour.

Road Testing

With the Dwyer Magnehelic gauge in the car (could have been a zero cost manometer, remember), and an assistant in place to read the instrument and write down the results, I went driving. I decided to get the worst news first, so connected the gauge to the 'Total' pressure measuring port.

Click for larger image

One of the first things that you learn when doing this type of stuff is that in cruise conditions, the air being drawn into the engine is almost zilch. It's full throttle where the action happens. What action? - shuddup, you're about to find out. All testing was done in second gear, full throttle, on (the standard) 1 Bar boost and on a 12 degree C day. Where there was any doubt about the readings, the runs were repeated.

So what did the Total pressure drop look like? Like this, that's what.

RPM Total Pressure Drop
(inches of water)
2000 2
3000 10
4000 16
5000 26
6000 32
7000 29.6

As you can see, peak pressure drop in the intake system was 32 inches of water at 6000 rpm. That's over 1 psi pressure drop - heaps and heaps. Somewhere in that intake system a big smelly, hairy, dirty critter called a negative pressure was hiding. Or mebbe there was more than one? Perhaps there were in fact a couple! Ooooooooo the bastards! But I'll find them - I'll dig 'em out... How? By moving the pressure tapping and seeing what happens to the figures.

RPM Total Pressure Drop After the Filter
2000 2 1
3000 10 2.8
4000 16 6
5000 26 8
6000 32 10
7000 29.6 9.6

The pressure drops After the Filter are radically lower than the Total pressure drop. So what's going on here? What's causing the big flow restriction between the After the Filter tap and the Total tapping point? What's in between? - the airflow meter! Hell - of that 32 inches total max pressure drop, the airflow meter's causing no less than 63 per cent of it! Aaaaah, but be careful. Be real careful. Let's be precise - 63 per cent of the total peak pressure drop is being caused by the second half of the airbox, by the exit duct from the airbox, by the airflow meter, and/or by the section of curved rubber duct that connects the airflow meter to the turbo compressor. OK - that's precise enuf.

And what about that classical victim - the airfilter? Blamed by all and sundry as the negative pressure monsta, it doesn't look like it's got much room to be bad, does it? After all, we've already found where nearly two-thirds of the restriction is - and it ain't the filter. Anyways, let's have a look at the results when we place the pressure tap on the atmosphere side of the filter.

RPM Total Pressure Drop After the Filter Before the Filter
2000 2 1 0.8
3000 10 2.8 3
4000 16 6 4
5000 26 8 8
6000 32 10 9
7000 29.6 9.6 8.4
Click for larger image

Hmm, so how much loss is there through the filter? Very nearly nothing at all! And guess what? That is the case on nearly every car - the standard filter poses very little restriction at all in the system. Look at the Before Filter and After Filter lines - don't get much closer than that, do ya? In the Audi's case, the filter makes up 1 inch of water pressure drop out of the 32 inches total pressure drop that is present. In other words, 97 per cent of the flow restriction of the intake is not the filter. And to hammer home the point, when you make actual on-car measurements, it's pretty well always like this. The airfilter as a halitosis-suffering, smelly, dirty, hairy negative pressure is a total frame-up created by those with vested interests in selling drop-in aftermarket replacement filters.

But back to the figures. The intake snorkel and airbox front half (plus a trivial amount caused by the filter) is making up about one-third of the total flow restriction of the standard intake. That's not minor, but just what the hell is so bad in the other part of the system - the bit between the filter and the turbo?

Is it just that airflow meter?

Key Points:

  1. This car has a major pressure drop through its standard intake system.
  2. Therefore, this car has an intake system that is restrictive to flow.
  3. Most of the restriction occurs after the airfilter.
  4. As is the case with most cars, the filter element itself poses little restriction.

Stay tuned as next week we hunt out those nasty negative boosts - in fact we go on a bloodily successful extermination raid!

Eliminating Negative Boost - Part 1
Eliminating Negative Boost - Part 2
Eliminating Negative Boost - Part 4
Eliminating Negative Boost - Part 5


Other Cars

Over the years I've measured pressure drops through the intake systems of a number of cars. For comparative purposes, here are some of those figures. All are for the car in pretty well standard form and are expressed in inches of water at full throttle.

Subaru Liberty RS

Engine RPM Before Filter After Filter After Airflow Meter and Resonant Chamber
3000 5 6 7
4000 12 17 20
5000 19 26 31
6000 26 31+* 31+*
7000 31 31+* 31+*

*a 31-inch high manometer was being used - so any pressure drop greater than that could not be measured.

Modifications to the intake system - a new intake duct to airbox (replacing a resonant box in the guard), oiled foam filter, replacement of resonant box after the airflow meter - caused about a 40 per cent reduction in pressure drop. Most of this came from the removal of the in-guard box.

Holden JE Camira

Engine RPM Before Filter After Filter
2000 1.2 2.4
3000 2 3.2
4000 3.2 5.6
5000 5.6 8
6000 6.4 9.6

Modifications to the system - new intake duct to airbox and a new bellmouth within airbox - decreased the pressure drop by a max of 42 per cent.

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