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Eliminating Negative Boost - Part 1

The hairy, horrible nakedness of power-robbing negative pressures.

by Julian Edgar

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If boost means power, what's negative boost then? A power loss, that's what. And hunting down and eliminating those negative boosts is a very cheap and effective way of gaining power. In this series we'll take you step-by-step through completely sorting the intake system - what negative boosts are, how to find them - and how to fix them. It's one of the few areas where, at home and with limited tools, you can have a powerful lot of success. You just need the stomach to confront the smelly, ugly, aggressive, loud-mouthed, obnoxious, arrogant, argumentative critters that are negative boosts....

Under Pressure

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I probably don't have to tell you what a boost gauge is - but I will anyway cos unless you understand this basic stuff you're gonna be totally rooted in all of the next parts of this series, which stretch on infinitely if I can drag out each sentence like I have this one. A boost gauge measures how much above atmospheric the pressure is in the intake manifold. So when someone says, "I'm running 10 psi", they don't mean that the pressure in the intake is 10 pounds per square inch above zero. They mean that it's 10 psi above atmospheric pressure. So what's this atmospheric pressure thingy?

We live under a thick layer of atmosphere - in fact we're at the bottom of an invisible blanket that wraps the earth. Isn't that nice, we're all wrapped (rapt?) in an invisible blanket.... But since air has mass, it literally weighs down on the earth's surface, exerting a pressure of 14.7 pounds for every square inch of surface. So why aren't we all staggering around, crushed by this heavyweight burden down to the height of malnourished, dwarf ants working in a production line environment where there is no spark, no life, no excitement because the boss is an olde world disciple of Marx - Karl not Groucho. Two reasons - (1) the pressure inside and outside our bodies is equal, so the two pressures cancel each other out, and (2) when air pressure does become visible (eg the action of a suction cap) we just shrug and accept it - not thinking of what's actually going on, nonchalant in our blithe dismissal of one of the wonders of nature... (Are there suction caps in nature? - hmmmm.)

The air pressure acting on the surface of the earth varies a little bit (up and down a max of about 10 per cent) due to air circulations we refer to as the weather. Y'know, those big H and L symbols all over synoptic charts. Also, as you increase in altitude, the thinner layer that's left above pushes down less heavily - air pressure decreases as you go upwards. However, apart from these variations - which I'll ignore cos they'd make the confusing style in which this article is written even more confusing - atmospheric air pressure is 14.7 psi, or close enough to 1 Bar. (One Bar is 100 kilopascals - we'll come back to this in a minute.)


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But back to engines. When the piston descends in the cylinder, it creates an area of low pressure. Like a hypodermic syringe being drawn back, it's making a hole in the earth's atmosphere. Air is a fluid - it can flow - and so it rushes in to fill the void being created. After all, it's got the whole of the earth's atmospheric pressure pushing behind it. The air flows in through the airbox snorkel, blasts its way through the airfilter, charges through the airflow meter, tumbles through the throttle body and into the plenum chamber, squeezes down the individual intake runners, ducks past the intake valves, and - whew! - finally fills the cylinder. After which it throws up because it gets real bad travel sickness.

At some particular rpm, that process (the flow not the throw) will probably completely fill the cylinder. For example, a 3-litre six cylinder has an individual cylinder capacity of 500cc (3000cc / 6 cylinders = 500cc). So, at peak torque, it's quite likely that each cylinder will be filled with half a litre of air - it's called having 100 per cent volumetric efficiency. And that's good - cos there are no flow losses anywhere! But the news is usually a lot worse than that. Let's take a few steps back up the chain to see some of the reasons why.

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Remember the beginning? The air that rushes into the airbox snorkel? Well, it's pretty likely that some air that should have made its way into the mouth of the duct, gets lost and doesn't. Doesn't what? Doesn't get in the duct, that's what! If the duct is as full of air as the atmosphere is full of air, then the air pressures will be the same. If it's 14.7 psi in the McDonalds Drive Thru, and it's 14.7 psi inside your intake air duct as you do a full-load burn-out through the drive-thru, then that intake duct is as perfect as can be. More perfect - even - than the Big Mac you're picking up.

Aaah, but what about if the air pressure inside the duct during that burn-out is only, say, 8 psi absolute? In other words, what if it's 6.7 psi less than atmospheric? Then you have an intake duct that is working godawfully. In fact, much too godawfully to be real - basically, it's just a silly example I made up. But what if the pressure inside that duct is half a psi less than atmospheric? (And this is much more realistic.) Then you've just found an example of negative boost....

Negative Boost

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Positive boost makes cars go hard. Positive boost churns the road into smoke, causes horizons to fast-forward towards you, gives dry mouths and clenched knuckles... followed afterwards by hysterical, bubbling stomachs of joy.

Negative boost? Well, that's a cause of slowness, of bad fuel economy, of decreased power. Of boredom and despair. 'Ban Negative Boost' should read the placard-waving protestors outside the airfilter box. 'Negative Boost - We Don't Want You'. And - from the confused - 'Same Sex Marriages For Negative Boost'.

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Any restriction in the intake system can be measured as negative boost - it's a drop in air pressure to below atmospheric. (If you're into electronics, it's very similar to resistors in series. There's a voltage drop across each resistor, and the total voltage drop at the end of the string can be found by adding up the individual drops.) So, by measuring pressure drops (negative boosts), you can see exactly how well the intake duct to the airbox flows, how well the airbox flows, how restrictive the airfilter is (in both clean and dirty states!), if the airflow meter is restrictive - and so on.

Ahh. So it must be pretty expensive to have someone measure all this stuff for you? Nope - you do it yourself. Ooooh, so I need to buy expensive tools to do it? You can, but it's pretty easy to make a measuring tool yourself... for nearly nothing. Oh, so I need a dyno? Nope - you definitely don't want a dyno - instead you need just a companion and an emptyish road.

Negative Boost Units

The most common negative boost measurer is called a vacuum gauge. (The unadventurous know negative boost by the boring name - vacuum.) But a vacuum gauge isn't a sensitive enough instrument for measuring pressure drops in intake systems. Well, not unless the system is so unbelievably bloody atrocious that the car can barely stagger above idle, anyway.

But one thing that vacuum gauges are good for is showing the different units used. Inches of mercury, mm of mercury, negative kilopascals - aaaah! What happened to positive boost in psi and negative boost in negative psi? Dunno, but you never see it. So y'know all of those examples above about how many psi pressure drop there was in the intake? Forgotten them already? That's OK - we're not using those units again.

Instead, we'll use two lots of units (Yes - two. No, not one set only - even though that'd be easier. I SAID TWO LOTS OF UNITS!! Hmmmph. Everyone seems to have left the room.) I'll give you the easy lot first - kilopascals. Positive kilopascals refer to boost pressure, and negative kilopascals refer to negative pressure. Easy, huh? So 100 kPa boost is about 14.5 psi boost. Minus 50 kPa means that you're running about 7 psi less than atmospheric pressure. And a pressure drop of, say, 10 kPa means that you're losing a fair bit of flow that you'd like to have.

Sometimes, though, we need even more sensitive units. Instead of doing the logical and splitting up each kilopascal, we'll use totally different units. These are called inches of water. Say you're sucking on a straw placed in a drink. You suck just enough to lift the water level in the straw two inches. You've then created a negative pressure of 2 inches of water. Measuring pressure drops in inches of water allows very precise measurement because 4.01 inches of water = 1 kPa (and it takes no less than 27.68 inches of water to equal just 1 psi).

Key Points:

  1. We are all subjected to atmospheric pressure.
  2. It is atmospheric pressure that pushes the air into an intake duct.
  3. When the airflow is less than ideal, a pressure below atmospheric can be detected following that restriction.
  4. These negative pressures are measured in minus kilopascals or inches of water.

Next week: we pull back the covers to reveal the sexual depravity of negative pressures in all their hairy, horrible nakedness.

Eliminating Negative Boost - Part 2
Eliminating Negative Boost - Part 3
Eliminating Negative Boost - Part 4
Eliminating Negative Boost - Part 5

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