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
- just shot across to a Falcon 3.9/4.0 in-line six cylinder forum ‘sticky’.
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.
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
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.
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
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.
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.
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
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.
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
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!
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.
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
Next week: getting down and dirty in the Falcon
Dwyer Magnehelic Differential Pressure Gauge
a mouthful - but it's a pretty damn good instrument too.
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.
'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.
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.
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.