Back in July 2001 we covered how to
use a very sensitive pressure switch to find areas of high pressure at the front
of the car – locations ideal for a cold-air intake. In this article, which is a
revised and expanded version of that original, we take the concept a step
further by using an ultra-sensitive pressure gauge to directly measure the high
and low air pressures around a moving car.
These days, most people know that placing an exposed air-filter under the
bonnet is just asking for the engine to take big breaths of hot air. To avoid
that outcome, it's now common to fit a cold air intake and seal it to the
However, what's generally not realised is that if a positive pressure can be
generated in the cold air intake, the engine will perform better again. Actual
on-road measurements that we have made "Eliminating Negative Boost -
Part 5" have shown that at speed, a good cold air intake can easily have
sufficient air pressure within it to completely cancel the flow restriction of
the aircleaner. So, even though the effect of ram-air on performance isn't huge,
there's a worthwhile gain that can be made.
But getting that pressure build-up in the duct requires that the mouth of the
cold air intake be sited in an area of high aerodynamic pressure.
(Note that the
mouth of the duct is sited in an area of high aerodynamic pressure, it doesn’t even have to be facing forward - something which has real advantages in stopping the inhalation
of insects, pigeons, dust, etc.)
But how do you find these areas of high pressure so good for engine intakes?
As the car moves forward, air is deflected above, below and to each side of
the car. As an example, take the flows occurring along the centreline of a sedan
that has a very good drag coefficient (ie it's slippery) like the pictured early
Lexus LS400. Some of the air is deflected over the top of the bonnet, passing
along the bonnet, flowing up the windscreen, along the roof, down the rear
window and then leaving the car's body at the trailing edge of the boot. (To
keep the flow attached to the body in the described manner, the car must have
gentle changes of angle only - especially from the roof to the rear glass to the
Other air is deflected beneath the car, flowing along an engine under-tray
and then getting mixed with the turbulent flows exiting the engine bay (the air
going thorough the radiator has to get out somewhere!) and the other turbulence
created by the exhaust and suspension, not to mention the spinning wheels.
But back to the front of the car...
At the front there must be a point above which the air goes over the car, and
below which the air goes under the car. This is called the stagnation point, and
it is here that there will be the greatest pressure developed on the front of
the car as it moves forward. This graphic of a Mercedes shows the high-pressure
stagnation area in red, lesser high-pressure areas in green, and the
low-pressure areas in blue. (They're low pressure areas because the airflow is
being accelerated around a curved surface at each of those spots.)
And hey, that's all very well and good - but how do you find where the
stagnation point is on your own car? And what if it's impossible to site the
intake for a cold air duct there anyway? How do you rank the quality of other
It’s possible to directly measure the air pressure at different locations
around the car. What’s needed is a very sensitive pressure gauge, eg one reading
from 0-1 inch of water. The most widely available gauges of this sort are the
superb Magnehelic pressure measuring gauges made by US company,
Inches of Water
Low pressures are often measured in “inches of water”. But what does this
A pressure of (say) two inches of water is that exerted downwards by a
water column 2 inches high. One psi is equal to a pressure of 27.69 inches of
Vertical manometers, which are measuring instruments that actually use
columns of water, can very accurately measure pressures above about 3 inches of
water. For example, flowbenches use manometers to measure their testing
pressures and airflows. Additionally, inclined manometers (like the one shown
here) can be used to measure accurately down to just tenths of an inch of
However, the pressures we’re dealing with here are so low that it is hard to
measure them with a fluid manometer, especially in a moving car.
Magnehelic gauges are available in a very wide variety of sensitivities, from
a full scale of just 0.25 inches of water up to 150+ inches of water. (Note that
sometimes the scale is calibrated in kilopascals or other units.) Because Dwyer
has been making Magnehelic gauges for what seems like forever (well, at least 35
years, anyway), the gauges very often come up second-hand on the web. In fact,
the gauge shown here was bought as New Old Stock (NOS) on eBay for US$25.
Alternatively, new gauges are quite reasonably priced from Dwyer and its
The 0-1 inch of water range of this gauge is excellent as it is sensitive
enough to measure actual aerodynamic pressures without excessive road speeds
The test procedure is simple.
Firstly, buy some small diameter plastic hose that fits tightly over the
Magnehelic gauge’s high pressure hose nipple. I bought 4 metres of 5mm clear
plastic hose from the local hardware store for 70 cents a metre.
Then, making sure that the mouth of the hose is placed at right-angles to the direction of
airflow, place the open end of the hose at the location that you are
investigating. Run the other end of the tube back into the cabin, holding the
tube in place with pieces of good quality masking tape. (Good quality so you
don't harm your car's paint!)
All that you then need to do is to drive the car at constant speed and have
an assistant read off the gauge. Once you have measured the pressure at one
location, move the hose and repeat the process, making sure that you are doing
the same speed each time the measurement is made.
Testing was carried out on a 2003 Lexus RX330 SUV. A Magnehelic 0-1 inch of water
gauge was used and a road speed of 60 km/h worked well.
(inches of water at 60 km/h)
Middle of front numberplate
Front tow-hook blanking plate
Leading edge of front undertray
Middle of headlight
Middle of Lexus badge in grille
Base of windscreen
Below front foglight on bumper
+ 0.05 – 0.1
Front wheel arch
Outer edge of headlight
Top of windscreen
This data is presented in order from the highest measured pressure to the
It can be seen that the numberplate is definitely in the stagnation zone,
with a pressure at 60 km/h much higher than any of the other measured pressures.
It’s obviously pretty hard to site a cold-air intake duct in the middle of the
numberplate, but it can be seen that there’s also relatively high positive
pressure at the leading edge of the front undertray – a much easier place to
locate the mouth of the duct.
But what about at the base of the windscreen? Since time immemorial, the
intake to the cabin ventilation system has been sited here, because this is
generally a high-pressure area. It's also common to place the mouth of a cold
air intake through into this plenum volume - in fact we've covered one such
approach at "Free-Flowing a
Miata MX5". But these measurements show that on the RX330, the available
pressure – while still being positive – is much lower than can be obtained
across the front of the car.
We’ve also seen air intakes placed through to the wheel-arch area, but on the
RX330 the pressure in the wheel-arch is actually lower than atmospheric.... which
wouldn’t be very helpful for pushing air into the engine!
As is also shown above on the Mercedes graphic, where air wraps around bodily
curves, the pressure is low. This can be seen in the fact that at the top of the
windscreen and around the outer edge of the RX330’s headlights, the measured
pressure was much lower than elsewhere.
Incidentally, the Magnehelic gauge can be configured to measure either high
or low pressures by the simple expedient of moving the tube from one sensing
port to the other.
As can be seen here, previous testing of a first-generation Lexus LS400 found
similar patterns of pressure distribution across the front of the car.
Once the location has been found that looks good to site the cold-air intake,
testing can be carried out at a variety of speeds. This step is needed in case
the pressure distribution across the front of the car changes as other
aerodynamic effects occur.
In the case of the Lexus RX330 SUV, the 0-1 inch of water gauge was too sensitive to
measure the pressure build-up at a variety of speeds, so this 0 – 150 inches of
water gauge was used instead. The chosen measuring point was near to the towhook
blanking plate on the front bumper.
The measurement showed that the pressure rose smoothly with speed – from 0.4
inches at 60 km/h, to 3 inches of water at 120 km/, to 10 inches of water at 150
It’s easy to make some measurements and so get a clear picture of the
aerodynamic pressures acting on the front of the car. Doing so will mean that
you can locate the engine intake to make the most of the free power often going
to waste. You'll certainly avoid making some of the glaring errors that we
occasionally see... like drawing air straight from the wheel arch!
But keep in mind that once you've found the best position for the mouth of
the intake, seal it all the way back to the filter. You don't want that precious
pressure just dissipating itself in the engine bay....
It’s possible to use a very sensitive pressure switch and a variety of road
speeds to get an indication of the pressure differences around the car. For more
on this, go to Siting Cold Air Intakes.
The 0-1 inch Magnehelic gauge is amazingly sensitive. When making the
measurements on the Lexus RX330, the presence or otherwise of other traffic made
a quite clear difference to the readings. For example, when there was a truck in
an adjoining lane, the turbulence of the truck’s aero wake could be clearly seen
in the flickering of the gauge needle. For this reason, try to do the testing on
an empty road.
Additionally, wind will make a difference to the reading – if you have to do
the testing on a windy day, always test in the one direction (eg always into the