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Ultimate DIY Automotive Modification Tool-Kit, Part 3

More low cost aerodynamic testing techniques

by Julian Edgar

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At a glance...

  • Measuring aerodynamic pressures to find...
  • The best place for the engine air intake and bonnet vents
  • And the effectiveness of...
  • Scoops and undertrays
  • Measuring changes in drag
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Last week in Part 2 of this series we showed how the airflow pattern over a car can be revealed by zero-cost wool-tuft testing. As described in that story, by looking at the airflow pattern revealed by the wool tufting, you can often get a good idea as to where the pressures on the car’s body are going to be low and high. 

But you can do better than that – you can directly measure them!

Measuring Aerodynamic Pressures

As with the flow testing of intake systems (covered in Part 1 of this series), the small pressure variations occurring on a car’s body can be measured with either a water manometer (near zero cost to make) or a Dwyer Magnehelic gauge. Magnehelic gauges are designed to measure both positive and negative pressures, and so have two measuring ports. By using both ports simultaneously it’s easy to measure pressure differentials – just what is wanted in many applications. 

Magnehelic gauges can be bought new from Dwyer, or alternatively, secondhand. eBay is a good way of buying these gauges very cheaply – expect to pay anything from $10 – 40 for one.

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When buying a Magnehelic gauge for this application, select a gauge that measures up to a maximum of about 3 inches of water. (The 3-inch gauge lets you use it in other ways as well. If you intend using it purely for aerodynamic work, buy a 0-1 inch gauge like the one shown here.)

In this application I recommend the Magnehelic gauge rather than the water manometer, but if you’re on a really tight budget, Undertrays, Spoilers & Bonnet Vents, Part 1 shows you how to make a sensitive water manometer.

The test procedure is simple. 

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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. 

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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 masking tape. 

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.

The sensing of pressures can be done anywhere on the car – under the bonnet, under the car, on the rear window, on the grille, and so on.

Engine Intake

Like wool tuft testing, pressure measurement is a brilliant technique because it takes away the guesswork normally associated with aerodynamics. For example, let’s say that you want to position the engine’s air intake in an area of high pressure. I first did this on an Audi S4 I owned. 

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On the Audi a large front-facing duct fed an oil cooler. (This view is with the bumper cover removed.) It seemed reasonable to suppose that in front of the oil cooler a high pressure would be developed, and so if a new intake duct was fed from this location to the engine’s airbox, some ram air effect would occur. 

But was the air pressure in front of the oil cooler actually high? 

Testing with a Magnehelic gauge showed that at 80 km/h there was a positive pressure of 2 inches of water, and at 100 km/h this had risen to 3 inches of water. Yes, it was a good place from which to pick up intake air.

(Referring back to Part 1 of this series, you’ll remember that the full load pressure drop across an air filter is typically in the range of 1 -2 inches of water. Therefore, in the case of the Audi, by picking up air from the high pressure location, I could more than make up for the pressure drop through the air filter! More on this idea in a moment.)

It’s easy to move the hose around and so check out a heap of different potential locations for an air pick-up. For example, some people suggest that the area at the base of the windscreen is a high pressure area, so one where it’s good to place the engine’s intake. (This can be achieved by placing a duct into the plenum area used for the cabin ventilation intake.) In fact, this is a high pressure area – but other areas are better.

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I did some testing on a 2003 Lexus RX330 SUV. A Magnehelic 0-1 inches of water gauge was used and a road speed of 60 km/h worked well. The results are shown in the picture and, in more detail, in the table below.

Probe Location

Measured Pressure

(inches of water at 60 km/h)

Middle of front numberplate

+ 0.55

Front tow-hook blanking plate

+ 0.40

Leading edge of front undertray

+ 0.30

Middle of headlight

+ 0.20

Middle of Lexus badge in grille

+ 0.20

Base of windscreen

+ 0.10

Below front foglight on bumper

+ 0.05 – 0.1

Front wheel arch

- 0.10

Outer edge of headlight

- 0.45

Top of windscreen 

- 0.60

So in the case of the Lexus, the middle of the number plate was the highest pressure area! The next lot of testing would be to check locations behind the grille and in front of the radiator – an easier place than the centre of the number plate to locate an engine intake duct...

Setting a Record

In Part 1 of this series I described the performance of the standard Honda Insight’s intake as the best I’ve ever measured , with a max pressure drop of 4 inches of water. 

But could it be improved even further?

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The standard car looks like this...

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...and the revised intake, placed in the high pressure area in front of the radiator, looks like this. 

After the new snorkel had been installed, the maximum recorded pressure drop of the complete intake system – snorkel, airbox and filter – decreased to just 2 inches of water! 

And, even better, in any constant throttle cruise over 40 km/h, there was in fact a positive pressure on the throttle side of the airfilter. This pressure was typically about half an inch of water. 

To put this another way, in cruise conditions the intake system is now posing less than zero restriction, and even at maximum flow, the throttle is seeing 99.5 per cent of atmospheric pressure.

Bonnet Vents

Pressure testing can also be used to identify the best location for bonnet scoops and vents. 

For an effective bonnet vent (ie one where air travels out from under the bonnet to the atmosphere) the pressure under the bonnet needs to be higher than the pressure above the bonnet, at the point at which the vent is mounted. 

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I carried out some testing on a mid-Eighties Nissan Maxima turbo. This pic shows the measured pressures on the surface of the bonnet at 80 km/h. As can be seen, the lowest pressures are near the front and the highest pressures are towards the windscreen. 

But what about the pressure differences? Measurement was then made of the pressures under the bonnet. The following table shows the pressures above, below and then the difference.

Leading Edge of Bonnet

Front Third of Bonnet

Midpoint of Bonnet

Rear of Bonnet

Above bonnet pressure





Below bonnet pressure










As the table shows, the greatest difference between the underbonnet and overbonnet pressures was at the very leading edge of the bonnet, where it was 0.6 inches of water. (However, in fact it was very hard to site a vent here and moving backwards a little to the front third of the bonnet still gave 0.4 inches of water pressure difference at the chosen vent site.)


Direct pressure measurement also works well when evaluating scoops – for example a scoop that feeds air to a top-mount intercooler.

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I carried out testing on a Peugeot 405 SRDT turbo diesel, a car that instead of using a bonnet scoop has a duct integrated into the underbonnet sound insulator to feed the top-mount intercooler. 

Exactly where on the Peugeot the duct picks up air from isn’t all that clear, but it appears to gather air from the gap between the bonnet and its locking platform. However, as indicated above with regard to vents, it’s the pressure difference that again matters.  So even if the feed duct was pretty poor, perhaps the engine bay was optimised to create a low pressure below the intercooler?

The pressure probes were in turn placed in the middle of the top and bottom faces of the intercooler. At a test speed of 100 km/h, there was a pressure on the top surface of the intercooler of positive 0.4 – 0.5 inches of water. (The 0.1 fluctuation being caused by wind gusts and the presence of other vehicles.) Under the intercooler, at the same speed and in the same conditions, there was a pressure of 0.4 inches of water. (The under-bonnet pressure fluctuates less as wind gusts and the presence of other vehicles have less impact.)

That meant the difference in pressure above and below the intercooler was just zero to 0.1 inches of water! It doesn’t sound like much of a pressure difference - and it sure isn’t. As a comparison, the pressure difference across the Peugeot’s radiator / air con condenser at the same speed was a relatively constant 0.25 inches of water – 2.5 times as much.

To put this another way: at speed, the airflow through the underbonnet intercooler was terrible. Therefore, installing larger intercooler core would have achieved little – in this car, the first step in improving intercooling efficiency would need to be the creation of a greater pressure on top of the core (eg by an external bonnet scoop) or the reduction in pressure under the core (eg by experimenting with different shaped undertrays). 

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Using a different undertray to enhance intercooler flow was in fact done on another of my cars. The car was the Nissan Maxima referred to above, a vehicle that had an underbonnet intercooler installed. It was fed air by a large scoop. 

The desire to make some aerodynamic changes to the front of the car came about because when doing some other road testing, I’d had the standard undertray off the car. With the undertray removed, the measured intake air temp rose, indicating that the intercooler was performing more poorly. It therefore seemed that some tweaking of the undertray had the potential to dramatically improve intercooler efficiency.

Pressure measurement showed that, despite the bonnet scoop, the pressure under the intercooler was higher than the pressure on top – that is, the airflow was going from the engine bay, through the intercooler and then out the ‘scoop’ (that was really a vent)! In fact, the pressure differential across the intercooler was minus 0.1 inches of water.

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However, by fitting this undertray, the pressure differential across the intercooler core was increased from minus 0.1 inch of water to plus 0.3 inches. Measured intake air temps in cruise on a 30 degree C day dropped from 65 to 47 degrees C. 

Testing Drag

The easiest way of testing for changes in aerodynamic drag is to measure fuel economy in repeated tests undertaken over a test loop driven at as high a consistent speed as is legally possible.

I fitted an extensive rear undertray to a Honda Insight. 

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In standard form the rear underside of the car looks like this..

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...and the trial undertray (made from coreflute) looked like this.

Testing was undertaken on a freeway. A 64-kilometre loop was driven, half heading in one direction and the other half in the other direction. The car was driven at 105 km/h for about half the route and at 100 km/h for the other half. The same driving style was used for each test and the traffic was such that very similar runs could be made. 

No change in fuel economy could be measured – so I took the trial undertray off.

However, a front undertray I fitted to a NHW10 Toyota Prius was more effective. 

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Standard the underside of the car looked like this...

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..and the new undertray looked like this.

The measured open-road fuel economy improved by a maximum of 10 per cent.

Testing Downforce

If you can log an electrical variable like voltage, it’s possible to measure changes in lift and downforce by measuring suspension ride height, using a pot as a sensor. However, you’ll need good logging equipment- and that doesn’t fit in with the rest of this series’ very low buck approach. See Real World Spoiler Development for more.


By using wool tuft testing, pressure measurement and fuel economy tests, you can see the airflow pattern over the car, test the effectiveness of modifications, find the best location for bonnet vents and scoops, test different undertrays and evaluate changes in drag. 

Next week we’ll look at testing vehicle performance – your very own zero cost dyno...

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