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Eliminating Negative Boost - Part 2

Finding negative boosts lets you see exactly which parts of your intake system require improvement - and without spending more than the price of a pizza to do it!

by Julian Edgar

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Last week 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.

Sorry - just been interrupted at the keyboard. Small obnoxious dweeb is on the other end of the ICQ:

"It's Simon here. Say, whyda we have to do all of the crap of measuring pressure drops with fancy instruments when all we gotta do is put on a cold air intake and whack in a K&N? And if ya don't like that, why not just rip out the whole airbox and put in a rampod? That's what I'm gonna do - and I'll put in a bigger airflow meter at the same time. That'll fix it."

"Ah yes; Simon. What is it that you're fixing?"

'Well - you know," blusters Simon. "The intake, it's restrictive. Like, it's harming power my good buddy. I'll let you into a secret. I can just tell by lookin' at it that it's no good."

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"But Simon - which parts are bad? The airbox? The filter element? The airflow meter? The intake duct to the box?"

"Shit I don't know - who cares? I'm fixing the lot!"

"No sweat my good ol' buddy," I say soothingly. "It's just that when I modify cars I can't afford to replace things on a random basis, hoping that they'll make a difference. If this airflow meter's OK - I don't want to spend the time and money 'fixing' it. If this filter's flowing fine, I don't want to money on an alternative that works no better.... But hell, if you've got resources to burn - fine."

Ah - OK, back again.

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 blood pressure measuring sphygmomanometer, a device full of mercury which easily gushes out of instruments bought from secondhand shops and dismantled in the middle of the lounge room.... Ooops.) But a manometer used in a negative boost hunting mode isn't full of mercury, instead just ordinary water is fine.

There's another positive to taking this 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.

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, I'd guess, a megabuck instrument - unless you're like me and buy a 0-10kPa one from a secondhand store (not the same one selling mercury sphygmometers) for just $20 cos someone doesn't know what the gauge does...

But stop the scrolling! Hey you! Over here! Pssssht!

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Er, I've just rung Dwyer Instruments (in Australia 02 9756 5355) and found that a Series 2000 10 kPa Magnehelic 4-inch gauge is just $147 + 10 per cent, brand new! Wow - what a helluva bargain that is..... In fact it's so good that I've added a breakout box at the end of this article to explain a bit more about the instrument.

But if you can't afford that, do this:

Making a Manometer

This'll take you about 5 minutes.

  1. 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 plastic tube.
  1. Attach wood to bottle, so when bottle is sitting upright, wood protrudes vertically above it.
  1. 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.
  1. Three-quarters fill bottle with water (put some food colouring in it if you want).
  1. 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.
  1. 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

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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 a turbo car, often there's a crankcase breather than feeds to the intake just in front of the turbo. If you temporarily pull this out and seal (just use electrical tape) the manometer tube to the intake fitting, you'll be able to see how much pressure drop is actually occurring during real life driving.

Note: 'real life'.

Note: 'actual'.

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. 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 - so you've got 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 take you thru a step by step on 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:

  1. People who make random changes to the intake system are often wasting their money.
  2. A manometer is an instrument for measuring pressure drops (ie flow restrictions).
  3. You can make a manometer for nearly nothing.
  4. Using a manometer is easy.
  5. DMDPG is a vital abbreviation and an even better tool.

Next week: getting down and dirty in a real car.

Eliminating Negative Boost - Part 1
Eliminating Negative Boost - Part 3
Eliminating Negative Boost - Part 4
Eliminating Negative Boost - Part 5

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.

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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.

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.

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While I have previously used water manometers in cars, this is the first time that I have used the Dwyer gauge - I found it enormously useful. In fact, had I realised how cheap these gauges were new, I'd have bought one long ago....and not waited until I found one secondhand!

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