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Our Peugeot Diesel - Part 2 - The Intake

Another dirt-cheap but effective modification

by Julian Edgar

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Last time in Our Peugeot Diesel – Part 1 – The Exhaust we covered the selection and fitting of the new rear muffler. As the rest of the exhaust pipework looked good and was free from flow impediments, it was a simple, quick, cheap and easy upgrade. Oh yes, and effective too!

If only the intake was so simple...


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The standard 4065 SRDT diesel Peugeot intake is certainly not bad. Air is picked up from the high pressure area ahead of the radiator and then is funnelled through a winding, contorted tube that joins to the base of the airbox. Inside that tube is a whirler and a water drain – the whirler spinning the air and causing the water to be separated, to then drain through the bottom-facing vent. (More on water later.)

The airbox, an oddly long and narrow design, connects to the turbo through a long - and again, bendy – tube that runs right across the front of the engine bay before joining to the hose going to the turbo intake.

Even a glance at the engine bay shows the airbox is on the wrong side – it should be where the ABS unit is positioned, on the other side of the engine bay. However, many different engines were fitted to the 405 so I assume that this design was the best compromise.

The intake tube size is generally pretty good, being about 2½ inches. However, the top half of the airbox is mostly filled with an internal resonant chamber built into the lid, something which on a turbo engine is of no help to either engine efficiency or intake quietening.

The breathing opening above the radiator is also rather small and the number of bends in the intake system high.

I performed a full-load pressure drop measurement to find that the maximum intake restriction was 20 inches of water. As the following table shows, this isn’t woeful but it also isn’t particularly good.


Max Pressure Drop

(inches of water)

Honda Insight 1-litre 3-cylinder


Toyota Prius 1.5-litre 4-cylinder


Holden Camira l.8-litre 4-cylinder


Ford Falcon 4-litre 6-cylinder


Peugeot 405 SRDT 1.9 litre 4-cylinder turbo diesel


Toyota Crown 2-litre 6-cylinder supercharger


Audi S4 2.2-litre 5-cylinder turbo


Subaru Liberty RS 2-litre 4-cylinder turbo


New Airbox

The first step I took after measuring the maximum restriction was to visit a wrecking yard, dimensions of the standard airbox written down and a tape measure in hand. By selecting an airbox from a more powerful car, you’re almost guaranteed lower intake restriction.

However, in addition to considering engine power, you should also take into account these points:

  • Many airboxes follow the shape of the inner guard of the donor vehicle – that is, they have a curved bottom (and also often side) so they nestle into place. Unfortunately, though, that makes them suitable for use in a different car only if: (a), the box is going to be placed on the same side of the engine bay, and (b), the shape of the inner guard of the new car is much the same as the donor car. This is one aspect that means a box that looks like it will easily fit oftentimes turns out to be no where near the right shape to fit!

  • Once you have found an airbox shape that will fit, the next aspect to look at is the sizing and positioning of the inlet and outlet ducts. Again, these come in a huge variety. Sometimes the inlet is at the bottom and the outlet is at the top – and other times it’s the other way around. Sometimes the inlet and outlet are round tubes, while other times the inlet might be rectangular in shape.

  • Does the outlet pipe direction match the direction the pipe actually needs to go in to connect to the airflow meter, throttle, turbo or supercharger? Is the inlet pipe going to pick up cold air from a new location or is the inlet going to be from the same location as the standard car?

  • While most people want airbox plumbing that’s as big as possible, it makes more sense to look at the diameter of the bits and pieces that you’re trying to connect the airbox to. For example, in airflow meter cars, what is the diameter of the airflow meter? If you can connect the airbox outlet straight to the airflow meter without a change in diameter, life becomes easier. In many cases the inlet pipe to the airbox is able to be enlarged, so the standard diameter of the inlet pipe on the new airbox isn’t so important.

  • Generally, the more curved and smoother flowing the box, the better it will flow. So specifically, the outlet tube should be integrated into the curved shape of that part of the box, rather than being just a tube butted up against a flat wall. If the box that you’re looking it doesn’t have flowing curves, carefully look inside the box where the exit tube is. At minimum, it should have a radius’d outlet, or even a separate bellmouth.

  • To pick a filter with best flow it makes sense to measure the area of the filter - but the thickness of the filter is also as important. This is because thicker filters use deeper pleats which – when spread out - add up to more filter area. So when selecting an airbox, you want to pick one that houses the widest, longest and thickest factory filter you can find. However, at the same time it also makes sense to pick an airbox that uses a fairly common filter – otherwise you might end up paying heaps for a new factory element each time you change the filter. If you pick an airbox that’s come from a popular car, you’ll usually find that its filters are cheap.

Those criteria (drawn from a previous AutoSpeed article on selecting air boxes) are all well and good – but do you think I could find an airbox for the Peugeot which was better than standard? I reckon I spent a solid two hours in the wrecking yard, looking, measuring, going on to the next model, looking, measuring... A major problem was the long, narrow space that the standard Pug airbox fits into, and the other problem was the fact that both the exit and entrance ducts need to face the centreline of the car. Most airboxes have the entrance duct facing away from the centreline - to connect to a duct hidden inside the guard (fender), for example. And you just can’t turn around the lid – airboxes are always designed so that the lid will clip or screw on in only the one orientation.

At the end of my search I had two potentials – Ford Mondeo (it uses a long narrow filter box) but the inlet was in the wrong place and the strange lower half shape meant it couldn’t be relocated; and Holden (Euro) Astra, that uses a superb airbox (and the Vectra is excellent too) but it was too wide to fit in the space under the bonnet of the Pug.

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So I went back to the standard airbox. Using a red-hot knife I cut out the inner resonant chamber and then opened-up the entrance and exit ducts. But after spending a few hours on it, I decided I wasn’t happy. Chief amongst my concerns was the fact that getting rid of every tiny “whisker” of plastic from the upper half of the airbox was extremely difficult – what with the internal ribs and hard-to-access corners. And that left the potential for a small piece of plastic to fly through the turbo at a later date...

So it was back to the wrecker yard. This time, with my standards set a little lower, I found the square airbox that first appeared in the VL Commodore (or was it the VK injected 3.3?). Both the top and bottom halves of this airbox design went through different iterations in subsequent Commodore models: I got the upper half that uses a rolled steel bellmouth and comes with a bolted-on plastic bend. Oh yes, and I didn’t forget to go through the cars until I found a secondhand filter element!

I also picked up the 3-inch-reducing-to-2 inch plastic pipe from the intake of an EL Ford Falcon, and two sections of bendy couplings from the same car.

The total cost of this lot was $30.


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The first step in the install was to increase the size of the oval-shaped opening through the above-radiator support panel. This opening is well positioned to pick up high pressure air (ie it connects to the area in front of the radiator) but is rather small in cross-sectional area. The panel is made of fibreglass and a file was used to increase the opening’s dimensions. The original duct and the enlarged hole are shown here.

I then heated a piece of 80mm diameter PVC plastic pipe until it was soft and then pushed it through the opening. Further heating allowed the formation of a semi-curved bellmouth on its leading edge and a round pipe shape on the engine bay side. (A full bellmouth wasn’t formed in the leading edge because this duct connects to the plastic in-fill panel which has a guide opening to direct air into the intake.)

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From the intake duct a spiral-wound 3-inch flexible tube was used to connect the outside air with the airbox. While commonly used in cold-air intakes, I’ve never before used spiral-wound duct, preferring plastic pipe and fittings with a smooth inner wall. However, in this case, spiral duct was used for two reason – it has much better high temperature handling capabilities (and this duct is fully in the engine bay) and secondly, the required shape was hard to achieve with plastic pipe and bends.

The Commodore airbox was modified to take the new duct. The original side opening was blanked and a new opening made in the end of the box.

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The standard outlet of the airbox was connected to a section of EL Falcon intake, then to a long length of newly bought 3-inch steel tube that runs across the engine bay, then to another section of bendy EL Falcon intake. This connects to another ex-EL Falcon part, a slightly curved plastic tube that smoothly reduces in diameter from 3 to 2.5 inches. This connects to the original turbo intake rubber tube via a short adaptor comprising 2.5 inch steel tube and a very short length of rubber hose. The long length of 3 inch steel tube is held in location by two steel brackets, while another bracket is used where the new system joins the old.


Using the secondhand filter bought with the airbox, the new intake system gave a peak measured intake pressure drop of 15 inches of water. That’s a 25 per cent improvement over standard but less of a gain than I was expecting. A new filter improved this a little to 14 inches of water.

One interesting test result was the lack of a pressure build-up in the intake system, even at low power outputs. Compared with a petrol engine (that throttles airflow to reduce power), the diesel always has plenty of airflow (and changes power output by altering fuel addition). On the Pug sufficient air was being drawn through the intake that more air was always being taken than was being rammed-in by forward movement.

With the new exhaust fitted (covered in Part 1 of this series), the turbo developed boost much earlier in the rev range. With the new intake, there is a very slight improvement in the revs at which appreciable boost is achieved. However, more noticeable is the way the engine now more freely revs with power. From 3500 – 4500 rpm the power output feels noticeably sharper. (Above 4500 rpm the fuel-cutting rev limiter starts to make its presence sharply felt.)


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Including paint and a new airfilter, the total cost of the intake modification was $40. (The exhaust shop gave me the 47cm of 3-inch pipe for nothing!) While the on-road gain is considerably less than achieved with the new muffler, the engine feels sharper at mid/high revs. And the Commodore air filter will also be a lot cheaper to replace than a Peugeot one.


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As indicated in the main text, the standard Peugeot intake uses the water separator (pictured here) in its intake system. All reciprocating engines are susceptible to damage through the ingestion of water. Water doesn’t compress and so when the piston reaches top dead centre, any water in the combustion chamber has no-where to go and so the con-roads get bent. (If the water has evaporated and so is in the form of vapour, that’s fine.) Because of their high compression ratios, diesels have lesser clearance volumes and so a smaller volume of water can do damage.

The modified Peugeot intake doesn’t use a water separator. So aren’t I worried about water ingestion? Well, to get into the engine, the water has to pass through both the filter and the turbo. The filter will let only very small drops through (very, very small!) and the turbo will tend to mix these up with the air, especially in the small diameter compressor outlet. (I think that there’d be a lot of turbulence in this pipe, so separation wouldn’t occur.) The base of the airbox also has holes in it to allow any water captured by the filter to drain out.

However, these theories were put to the test very quickly. Driving the car the very night I’d finished the intake, I ran into a Queensland-typical tropical rain storm. Lightning, thunder – and a downwards deluge. After being in the very heavy rain for 10 minutes, I found somewhere under cover and lifted the airbox lid for an inspection. The underside of the filter was slightly moist but the spiral-wound intake duct and the bottom of the airbox were both dry.

However, if fitting a new intake to a diesel, you should certainly keep the danger of water ingestion firmly in mind.

Air Filters

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As described above, when selecting an airbox the cross-sectional area and depth of the filter give a good guide to the filter’s flow capabilities. And as you can see here, the original Peugeot filter (rear) is not only greater in area but also much deeper than the new replacement Holden filter. In short, the area of paper through which the intake air can pass is much larger in the original than the replacement.

So why have I taken this path? As described above, I could not find another airbox that took such a large filter and would still fit into the car. And the standard airbox, while using a great filter, needed substantial modifications if it was to perform well. The air box intake needed to be enlarged, the exit enlarged, and a bellmouth fitted to the exit. That lot stacked up to being a difficult exercise indeed.

As the pressure drop measurements show, the modified intake is much better than the standard intake. But it’s likely that a different airbox, one that takes a larger filter, would perform better again...

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