over 5 years ago that we first dragged the squealing hairy horribleness of
negative pressures out of their dingy caves. That series of articles revealed to
many for the first time a dark and dirty secret, the sort of which they’d never
imagined except in the nightmares caused by too much alcohol and too much
adrenaline and too much sex. Negative boosts could be found hiding – crouched,
dirty and sneering – around each apparently innocuous bend in the intake tract,
waiting to take away power... Well, now it’s time to force them from their lairs
again, this time on a Ford Falcon.
If turbo boost means power, what's negative boost
then? A power loss, that's what. It doesn’t matter if your car hasn’t a turbo or
a blower, negative boosts occur right through the intake system. And hunting
down and eliminating those negative boosts is a very cheap and effective way of
In this series we'll take you step-by-step through
completely sorting the intake system on a 4-litre six cylinder EF Ford Falcon -
what negative boosts are, how to find them, and how to fix them. But it doesn’t
matter what the car is – the techniques are common to literally all cars. 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....
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
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.)
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 (where fitted), 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 4-litre
six cylinder has an individual cylinder capacity of 667cc (4000cc / 6 cylinders
= 667cc). So, at peak torque, it's quite likely that each cylinder will be
filled with nearly 0.7 litres 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.
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
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'.
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
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
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, bar - 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. 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 above the
surface level of the fluid. You're then creating a negative pressure of 2 inches
of water. This is a really, really important point. The water rises up the straw
because of the negative pressure you’re applying, so the height the water
reaches is an accurate measurement of how much negative pressure you’re
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).
Boost and Negative Boost
In this story we’ve talked about boost in positive
and negative terms. But you might be thinking: WTF?...none of this matters to me
cos I don’t run a turbo or blown car. But just remember that positive boost is
good and negative boost is bad. That’s all you need to know.
And, to take it the next step, the more negative
boost you have in the intake system, the slower your car will go.
You already know that strapping on a turbo or a
blower will give you positive boost and make your car go faster. And so it
stands to reason that negative boost – decreasing the pressure in your intake
system – will make your car go slower. So get rid of those deviant negative
boosts, and enjoy the released power!
We are all subjected to atmospheric pressure
It is atmospheric pressure that pushes the air
into an intake duct
When the airflow is less than ideal, a pressure
below atmospheric can be detected following that restriction
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
Did you enjoy this article?
Please consider supporting AutoSpeed with a small contribution. More Info...