BMW Z3 A better air intake
Article reproduced with the kind permission of Mike Fishwick

I have a feeling that the original design of the Z3 air intake system was dictated more by the space available, rather than engineering considerations. For example, the air filter is smaller than even that of the 8-valve Volkswagen Golf GTI, which is known to be obstructive on even a 1.8 litre engine. In 16-valve form the Golf filter is enlarged to almost double the size of that used on the 2.8 or 3 litre Z3 and the 3.2 litre M Roadster.

While the potential rate of flow can be increased by using a ‘high flow’ filter element such as the K&N or Pipercross types, I found that they made virtually no difference to performance. This made me suspect that even with such a small filter the air supply was the limiting factor, as it must find its way around the back of the left headlight unit, meeting sundry bends and obstructions.

Even to find this torturous path the air must flow through the left kidney grille, then move sharply to the right, and pass through the narrow gap between the inside of the bonnet and the upper part of the radiator inlet duct.

Engine air intake path – seen from below with front apron removed.

Although this will provide a sufficient supply at low speeds and small throttle openings, at higher speeds this will not be the case, as the air will not obligingly make a sharp turn after leaving the grille.

Air, as with any other fluid, increases its inertia with speed, and so resists any attempt to change its direction. At higher speeds it therefore prefers to travel in straight lines – and its direction is further stabilised by the deep vanes of the kidney grille en route for the radiator. This high-speed air stream will tend to attract air from adjacent areas, producing low-pressure zones. One such zone is the area immediately to the right of the grille, where the intake gap is sited, so feeding the engine with a stream of confused low pressure air.

Original air intake paths

This intake path appears to have been designed in opposition to the known behaviour of high-speed air flow. To reach the intake gap the air is expected to pass through the grille, then immediately reverse its direction to reach the inside front of the bonnet, and then bend around the front of the radiator duct to reverse its direction again, into the intake path. These contortions must be performed while travelling in the opposite direction to an adjacent stream of high-velocity air from the left-hand kidney grille. Such behaviour is virtually impossible – perhaps BMW thought they could ignore the laws of physics!

Most of the air which does manage to enter the intake path is actually provided from the main intake below the bumper, flowing through a small triangular hole at the end of the plastic liner below the headlamp unit. A large proportion of this must, however, be immediately drawn through the intake gap by the low-pressure zone formed outside it. The narrow gaps between the headlamp unit and the bonnet also make a contribution.

Whatever is left then meets several obstacles as it wends its way around the back of the headlight unit, each setting up turbulence in the airflow, and reducing the quantity of air supplied to the filter still further.

This can only result in the supply of a lower amount of air than would be the case if the intake path were larger, of uniform section, and was presented directly to the air stream. At high speeds, or in low air density such as at high temperatures and altitudes, this effect will be even more marked. All things considered, it is a very poor design, and seems calculated to reduce the engine’s breathing ability as much as possible.

Even if the entire filter housing were replaced by a large conical filter, such as the K&N 571 range, the flow supplied to it would still be subject to the limitations of the standard air supply path. I had seen the occasional Z3 where a conical filter was provided with an additional air supply in the form of a large-bore flexible pipe, which passed through the left radiator compartment panel. This appeared to be a very sensible modification, which I resolved to incorporate.

None of these cars, however, were of identical specification to mine. The presence of the air conditioning condenser in front of the radiator meant that space for an additional intake was very limited, and as an October 1998 (i.e. a ’99 model) the Red Zed already had one cold air intake on the left side of the radiator compartment panel, for alternator cooling.

The M Roadster however is provided with an additional air supply, in the form of a scoop behind the left lower ‘mouth’ of the front apron. This discharges into the filter intake through an aperture behind the left headlight housing. On the non-‘M’ models this tubular intake aperture is present, but sealed by a plastic blanking plate. The use of this additional supply aperture on any Z3 will certainly improve the air flow to the filter, and if connected in such a way as to directly face the incoming air flow it will also add a small ‘ram’ effect.

On all the non-‘M’ models however, the left fog lamp prevents use of the ‘M’ air scoop. While it would be possible to remove the fog lamps, the resulting hole would spoil the aesthetic appeal of the car.

I therefore decided to install an additional intake which would retain the fog lamps while increasing the air flow to the filter, and also provide a small ram effect. The intake end would obviously have to be positioned at the left of the lower intake ‘mouth,’ avoiding the fog lamp and climbing to the auxiliary intake tube, the diameter of which readily lends itself to being extended by use of standard 68 mm PVC rainwater pipe. The left radiator compartment panel provided just enough space to accommodate a 68 mm hole, if it were cut across a 35 mm deep step in the panel, above the alternator air intake.

New air intake trunking – made from B&Q drainpipe

The first step was to remove the front wheel arch liners, using 8 and 10 mm sockets. This was followed by the bumper and apron – an easy task, requiring only disconnection of the fog and parking lamps, then use of a 10 mm socket and T50 Torx key. This enabled a pair of semicircles to be marked on the panel, one over each of the adjacent surfaces to be cut. After cutting, these were combined to produce a neat hole, which made a tight fit around a 67 degree ‘double female’ plastic rain water elbow, positioned to just clear the power steering oil cooler. Due to the limited space available for its insertion, this elbow would not be glued to the adjacent section of pipe, but secured after installation by a small self-tapping screw.

New intake trunking fitted

Two sections of pipe were cut, joined by another 67 degree double female elbow to couple onto the auxiliary intake tube by means of a male-female elbow. The various components were then installed and marked for alignment, before being removed and glued. The completed intake trunking was then sprayed matt black before being secured to the radiator compartment panel with black silicone rubber, and to the auxiliary intake tube by a self-tapping screw, before sealing it with silicone.

The elbow at the lower (intake) end of the trunking faces forwards, mostly below the bumper, at the top of the main air intake grille. It is in an area of high pressure air, which will be subject to a ram effect as speed increases, but is sufficiently above the road to avoid all but the very deepest water.

The intake faces forward towards the incoming air

The finished result is efficient, having a smooth and uniform cross section, with its intake in a sensible position, and looks as if it were an original item. Although I would have preferred to use pipe of the same diameter as the filter inlet (approximately 90 mm) this would not have been practical, due to the limited space available. Space considerations also prevented the entire area of the intake elbow from facing the air stream, but even so it is a great improvement, with a cross sectional area more than three times that of the original intake gap.

View from the rear

These modifications would probably be even more effective if coupled to the use of a large conical filter, but a well-fitting partition is necessary to prevent them from ingesting hot air from the radiator. They also excite one’s insurance company, increase intake noise, often reduce low-speed torque, and – if it is important to you – will be marked down by Concours d’Elegance judges as a deviation from standard.

Another problem with these oil-dampened filters is that oil particles will gradually coat the hot-wire air flow sensor, insulating it from the air flow and so weakening the mixture. For this reason many specialists refuse to fit this type of filter to cars using hot-wire sensors. The wire can, however, be carefully cleaned with a fine artists’ brush and pure alcohol, a procedure which should be followed a few thousand miles after the filter is cleaned and re-oiled.

As a bonus I decided to also improve the original intake, in order to increase the overall amount of high-pressure air available to the engine. The original intake gap was sealed by attaching a short piece of pipe lagging to butt against the inside of the bonnet. The intake path around the headlamp was then connected to the main intake area by cutting away a small area of the plastic lining at the side of the headlamp.

This provides a path for air to flow into the original intake path from the high pressure area below the bumper, so equalising the pressure in the intake path. As well as providing an additional intake supply, this prevents the air heading for the radiator from ‘stealing’ air through the original intake gap, and so setting up a reverse flow which would attract some of the air provided by the new intake trunking.

Modified air intake paths

It is interesting to note that on the last Z3 models the air filter was provided with an integral intake duct which led directly to the area alongside the headlamp, so avoiding the convoluted path around the back of the headlight unit. The plastic lining below the headlight unit was also modified in much the same manner as I had thought necessary, obviously with the same goal in mind.

My doubts regarding the original intake arrangements were therefore confirmed by BMW, and I was able to easily improve matters to those achieved by the manufacturer, and beyond.

So – these modifications seemed sensible, were easy to incorporate, and only cost £10, but do they do anything?

The short answer is “Yes,” for at all speeds the engine seems crisper, and runs to peak revs with even more eagerness than before. Acceleration times over each 20 mph increment between 30 and 90 mph have improved by around a second, and fuel consumption in all conditions has improved by at least 1 mpg. These improvements are indicative of additional torque, which is directly linked to an engine’s breathing capacity. It just makes you wonder why BMW originally produced such a poor design, and took so long to realise it!