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Negative Boost Revisited, Part 2

How to accurately measure intake restrictions

by Julian Edgar

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At a glance...

  • Finding where the intake is restrictive
  • Simple, cheap measurements on the road
  • Forget guesssing!
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Last week in Negative Boost Revisited, Part 1 I introduced you to those sexually deviant, loud-mouthed yokels called negative boosts. Sometimes referred to as pressure drops - and by the uncouth as vacuums - these ugly little critters can cost you heaps of power. They can be loud and domineering - or subtle and sly. But whatever shape they take, they're obstructions to intake flow that result in a loss of power.

But before you can fix 'em, you've gotta find 'em.

Click for larger image

Sorry - just shot across to a Falcon 3.9/4.0 in-line six cylinder forum ‘sticky’.

Poster 1: Here is my take on inline 6 modifications for EA-EL Falcons. Induction: EA to EB Falcon Multipoint can have an XH airbox snorkel fitted to increase airflow. EF-EL can fit an AUII XR8 Snorkel for the same effect. These are around $30 from Ford.

Poster: 28: You say that you can fit an AU XR8 snorkel to the EF/EL 6cyl Falcon but I tend to disagree because I went out and sticky taped one so it would sit in the position it would screw down on. I then gently closed the bonnet slowly and to my surprise the rubber seal on top of the bonnet covers the top part of the mouth of the snorkel, therefore rendering the snorkel useless with about the same airflow as the stock one. I went back into Ford and exchanged it for an EL GT snorkel which in my opinion is larger and better flowing plus it fits to the original screw points.

Poster 93: I think that the apparent issues with the AU XR8 snorkel being blocked by the rubber on EF/EL Falcons should be addressed as a yay or nay. I've read a lot about this and it seems about 50/50 (or maybe its just the same ppl writing the same thing in many threads). Or is the ELGT snorkel the way to go, and if so, is it worth the extra money???

Well, guys, there’s simply no need for guessing. In five minutes of road testing and for zero cost you can accurately find out which flows best.

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The way that you find pressure drops is by using a Pressure Drop Measurer. Well, some people don't call them that - instead they're known to most as manometers. A manometer is a device for measuring pressures below atmospheric. (Its cousin is the pictured blood pressure measuring sphygmomanometer, a device full of dangerous mercury.) But a manometer used in a negative boost hunting mode doesn’t use mercury; instead just ordinary tap water is fine.

You can use a single column manometer, or a U-shaped manometer, or you can use something that isn't a manometer at all. In fact, it's a nice, round analog meter called a Dwyer Magnehelic Differential Pressure Gauge, which is such a mouthful I'll abbreviate it to DMDPG - which will confuse everyone who comes across DMDPG and who hasn't read this para first. Hah - death to those who skip text!

The DMDPG is a fantastic instrument that can often be picked up on eBay for twenty bucks or so – see the box at the end of this article for more details.

There's another positive to taking the negative pressure measurement approach. As well as saving money by not needing to make random replacements, retaining as much as possible of the standard intake system optimises the resonant intake tuning. Take away all of the duct work, all of the filter box (etc) and you risk losing substantially in bottom-end torque through destroying the factory tuned system. Instead, measuring pressure drops lets you tweak selectively with care - usually, most of the factory system stays in place.

If you can’t afford – or can’t find – a DMDPG, you can make your own for nearly nothing. Here’s how:

Making a Manometer

This'll take you about 5 minutes.

  • You need: a plastic soft drink bottle (1.25 - 2 litres), a metre of so of dowel (any small bit of wood will do), sticky tape, 4 metres of small-bore flexible clear plastic tube.

  • Attach wood to bottle, so when bottle is sitting upright, wood protrudes vertically above it.

  • Attach plastic tube along length of wood, one end of the tube going to the bottom of the bottle. They'll be lots left over at the top.

  • Three-quarters fill bottle with water (put some food colouring in it if you want).

  • Wherever the top surface of the water ends up, place a texta line on the bottle. Every inch above that line (working your way up the stick) place another line. Write some numbers on the wood to mark off 5 inches, 10 inches etc, counting upwards from the surface of the water.

You've finished.

Hard wasn't it? The way this manometer works is similar to last week's analogy of sucking water up a straw. 'Cept, this time the pressure drop in the intake does the sucking for you.

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Don't underestimate what you've just made. When it's held upright by a passenger, it's prob the most sensitive instrument in the car. After all, it'll accurately, clearly and repeatably read off pressure drops of a few inches of water - and that's a pressure of less than one-tenth of one psi! And of course, it only needs to be in the car for the 30 minutes or so it takes you to map the complete pattern of pressure drops through your car's intake. Yep, that shorta time.

Using the Manometer

To measure what sort of pressure drops exist in the standard intake, run the free end of manometer tube to the bit that's of interest.

Say, you want to know what the restriction of the whole intake system (snorkel, airbox, airfilter and airflow meter) actually is. Connect the tube from the manometer to a section of the intake system after all of these bits - but before the throttle (or turbo). In this way you'll be able to see how much pressure drop is actually occurring in the complete pre-throttle intake system during real life driving.

Note: real life!

Note: actual!

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The measuring system automatically takes into account the aerodynamics of the front of the car, any ram-air pressure build-up that might occur, sudden engine air demands - the whole bit. (And keep in mind that on a dyno most of these things can’t be measured – there’s not enough airflow past the car.) What you read off - on the incredibly sensitive instrument - as you drive along is exactly what's happening. With the sensor hose in this position, you've just measured the total pressure drop minus the drop across the throttle - so you've got nearly the worse news you're gonna get.

Want to see how much of that restriction was being caused by the airflow meter? Position the sensing tube on the other side of the meter and see what the difference is. Simple as that.

Next week we'll start looking at a real car, but if you wanna get in early, you need to know two things. (1) You have to drive the car at full load if you want to see the problems at their worst. (2) Don't let the engine suck the water out of the manometer - possible if the intake system is really bad. If the manometer water level is shooting up in an outa control ballistic OTT way, get your assistant to yell at you to get off the gas. Then the pressure drop will instantly go away again.

Key Points:

  • People who make random changes to the intake system are often wasting their money

  • Dyno testing air intake systems ignores the affects of aerodynamics

  • A manometer is an instrument for measuring pressure drops (ie flow restrictions)

  • You can make a manometer for nearly nothing

  • Using a manometer is easy

  • DMDPG is a vital abbreviation and an even better tool

Next week: getting down and dirty in the Falcon

The Dwyer Magnehelic Differential Pressure Gauge

Click for larger image

It's a mouthful - but it's a pretty damn good instrument too.

First up, what's this 'Magnehelic' stuff? A registered trademark of Dwyer, the Magnehelic principle transmits the movement of a diaphragm to a pointer without the use of gears or other direct mechanical linkages. This has some significant advantages. Firstly the use of a large diaphragm means that the gauge can be much more sensitive than one using a tradition Bourden tube. And another reason that the sensitivity of the gauge can be so high is that the diaphragm movement is transmitted to the gauge pointer magnetically, avoiding physical contact that can also cause hysteresis (backlash) and jerkiness.

The 'differential' bit of the title means that the gauge has both high and low pressure ports. If you want to measure a pressure drop to below atmospheric, leave the 'high' pressure port exposed to the atmosphere and connect the sensing tube to the 'low' port. If you want to measure a pressure above atmospheric, swap the hoses. And if you want to measure the pressure difference (say across a throttle butterfly), connect a port to each side of what you are measuring eg the low-pressure port to the downstream side of the butterfly and the high-pressure port to the upstream side.

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And the 'pressure' bit? As indicated in the main body of the text, these gauges are extraordinarily sensitive. The one that is being used here has a full-scale deflection (FSD) of 10kPa (1.4 psi) and it is ideal for measuring the pressure drop through most intake systems. However, the gauges are available with quite incredible sensitivities - one model has a FSD of just 1 kPa! Centre-zero gauges are also available eg 10-0-10 kPA which allows easy positive and negative pressure measurement without swapping hoses.

Note: if you’re buying a Magnehelic gauge off eBay, the easiest way to see the range is to look at the picture and carefully check the units. Some people advertise the gauges as being ’15 psi’ because that is the maximum pressure the gauge can be subjected to – it’s not the reading of full-scale deflection, which is almost always vastly less.

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