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Notes about Matra Murena aerodynamics

These are some notes I have assembled about aerodynamics of the Murena. The Murena has a very sleek body and low drag.

First a disclaimer: The reason I have written these notes is purely academic: I am exploiting the theory and current established knowledge about car aerodynamics as I have learnt it from various sources (see below). In fact, I don't have an opinion on whether Murena deserves aerodynamic improvements or not. A previous version of this page caused a bit of a stir because some readers wrongly assumed that I was raising critique of the aerodynamic properties of the car. Perhaps I was, at least implicitly, but with this rewrite I hope to have cleared any misunderstanding. I have no reason to raise critique of the Murena as it is.

But Murena is for various reasons an interesting subject for aerodynamic studies. For me personally, it is a question about an enthusiasm that I have for the car, but in general also: Murena was revolutionary by the time it appeared in the market. Never before had the market seen a car like it, and never has it been copied. Various other mid engined sports cars of the same class exist, but none with such beautiful, sleek, and aerodynamic lines as the Murena. Murena is the mending of the French tradition for extremely low-drag cars with race car properties in one beautifully combined package.

Still, there may be some reasons why one would want to study and perhaps optimise some aerodynamic properties of the Murena:

  • Engine converted cars with higher power may need improved high speed stability (note how the 2.2S was given a rear spoiler that the standard 2.2 did not have)
  • Race cars will need as much down force as they can get - as long as it's legal per the regulations of the series.

Aerodynamic properties of the Murena body

The shape of the Murena pretty much follows the lines of any hatch back car. Air flows over the front bonnet, creating some turbulence as it reaches the wind screen where some of the air is pushed over the car, and some is pushed to the sides. Air flowing around the A-pillars is seperated (turbolent) along the side of the car. The air that flows over the roof reaches the rear window where it stays attached. As this will be a low pressure area, some air will be dragged in from the sides of the car over the rear window, where it will create some turbolence on the sides of the rear window. The sharply cut off rear will also be a turbulent area. Air flowing under the car will be seperated from the body as the underside is not smooth. The tapered rear underside will drag air out under the car and in from the sides.

Airflow over the Matra Murena
Simplified diagram of imagined airflow over the Murena. The dashed flow line indicates air flow through the radiator.

As we know, drag of the Murena body is quite low, with Cd values around 0.31 being quoted. This is comparable to cars like Porsche 924 (same Cd value) and Renault Fuego (Cd = 0.34 - 0.37) [See Katz appendix 1, pp 298,299]. Don't forget that Cd is a cooefficient - the actual resistance factor is multiplied by the frontal area.

We don't know the Murena's lift-coefficient, but it is likely to be positive: Murena will lift itself from the ground when going faster. This is confirmed by Murena owners reporting that their car becomes "tip toing" when approaching 200 km/h. This lift is most likely generated primarily over the rear window and by cooling air travelling under the car.

Some lift is acceptible or even desirable in a road car. First, unlike race cars where the ability to do high speed cornering is very critical, a road car does not have that necessity since cornering usually takes place at lower speeds. Second, uplift can add to the drivers' percieved level of comfort since a lighter touch at the ground will make the tyres better at absorbing small bumps in the road. High downforce race cars can be very tirering to drive since they are pushed so hard towards the ground that every little bump is transmitted into the chassis and steering.

Aerodynamic improvements?

Some The following sections describe some suggested changes that can be made to the Murena body changing the cars' aerodynamic properties.

Front spoiler/lip

The easiest way to gain downforce at the front is to mount a lower front lip. There are body kits available for the Murena that has that. E.g. one from Politecnic or the Lamm front spoiler.

Murena 2.2 with Lamm front spoiler
Lamm front spoiler on Matra-Hans' Murena 2.2

But fitting any of the available kits change the body of the car considerably, so for me neither of the solutions are really acceptible. Perhaps a small (black) rubber or plastic underlip of the type mounted on some sports cars (called a 'splitter' on race cars because it splits the air going over the car from that going under it) could be used without distorting the look of the Murena too much.

Front spoiler of the Ford GT
This front spoiler under lip is found on the Ford GT - it was not on the original GT40. Note the horizontal part of the lip; it probably increases the size of the spoiler as seen by the air being pushed downwards by the original front bumper design of the GT40. Note how it resembles the original spoiler design of the Murena! This is an obvious upgrade to the origianl GT40 design. Picture by Lennart Sorth. Original is available here.

Biggest problem with lower front spoiler (apart from lower ride height) is that it will block some of the airflow under the car. This can become a problem since the air intake for the engine comes from under the car, and oil cooling also takes place this way. The entire engine compartment is ventilated with air being sucked out under the car and out through the two ventilation openings at the rear. So a front spoiler should not be mounted without considering other ways to improve airflow to and around the engine. I'll discuss ways to do that below.

Higher air pressure on top of the car

The long angeled front of the Murena means that the entire upperside of the front hood is a high pressure area. It is difficult to enhance that, but one possibility is to change the flow of the cooling air from going down under the car into something that goes over the car.

The spare wheel is in the way of an upwards going airflow, so it will have to be replaced by a tyre repair kit, and its weight replaced by extra ballast.

The battery is also somewhat in the way, but since its rather small, air can be directed over it. The airbox will have to be mounted on the underside of the hood to ensure access to the battery.

Changing the airflow will lower pressure under the car - and provied that sufficient air exits can be made in the hood, it will increase pressure on the upper side. The air exits can be made as a single or pair of large openings of the type employed in e.g. Ford GT, or as sets of luvres. The best solution from an aerodynamic point of view is the large openings because they will work as a reverse venturi, slowing the exit air down thereby increasing the pressure of the air exiting the radiator.

But it will very dramatically change the look of the Murena. I would not consider that acceptible.

Ford GT cooling airflow
The airflow through the radiator on the Ford GT. Would this solution be acceptible on a Murena? No, I don't think so. Picture by Lennart Sorth. Original is here

I see another solution, however, where the radiator air is directed all the way under the hood to exit in front of the wind screen. There are several problems with this:

  • It will be difficult (if not impossible) to make the air exit area bigger than the intake area - this means that pressure of the air exiting the radiator will be lower than the intake air. Even though the air will quickly slow down again once it starts passing over the wind screen, this will inevitably reduce the pressure over the wind screen, thus limiting the decrease in uplift by changing the air exit altogether.
  • Modifications will have to be made to the chassis to allow the air to exit under the hood.
  • The cabin air intake is at this same point, so it will be difficult to get cold air into the car - the radiator will work as a supplementary heater! Care must be taken to split the cabin air intake and the radiator air exit so that it will still be possible to get cool air into the cabin, or it may be possible to take cabin air through a tube from an opening in the front spoiler (where pressure is high), but this will have to be evaluated.

A different solution, which will not add to the pressure on top of the hood, but will still direct the air exiting the radiator away from the underside of the car is to lead it into the front wheel wells from where it can exit to the side of the car. This will also increase the airflow around the brakes, assisting cooling of these. Large open alloy wheels may allow air passage directly through the rims. The air will be warm, however (up to 50-80 C), so the cooling effect on the brakes will not be optimal, but may still help to keep the temperature somewhat down.

In all cases, a larger or additional radiator fan may be necessary to ensure that the flow through the radiator is big enough at low vehicle speeds, since the air exit in the above mentioned solutions is made a great deal smaller.

A smooth undertray and rear diffuser

Race cars have smooth undersides unlike most production cars. A smooth undertray allows for better airflow under the car and it usually ends with some kind of diffuser in the rear working on the vacuum behind the car to ensure that the car's underside is kept at low pressure.

I beleive that the Murena would benefit from a smooth undertray covering chassis sections, cooling pipes, part of the suspension etc. This will lower drag and allow the rear suction to work better on the underside of the car.

As discussed above, front downforce is my primary concern, so the undertray should have channels to ensure that air is sucked in at the front and not at the sides.

The 2.2S side sills are necessary to ensure this; fortunately they cannot be said to alter the looks of the car since they are part of a factory upgrade. Further back, the undertray must ensure a smooth airflow to the engine room where at least part of the air is directed. If air channels can be made around the engine, then a rear diffuser may enhance the front downforce, but with the engine sitting sideways, the situation is somewhat more complex than in LMP-cars where the engine is longitudally mounted and therefore sits less in the way of the air flow (and this is why Porsche switched from flat-six engines to V6 back in the 1980's: The V shape is more narrow at the bottom allowing for a lower mounting position of the engine without distorting the airflow).

A potential benefit from a smooth undertray is that the air flow to the engine can be kept, even if most of the air is directed under the engine to the diffuser further back.

So the diffuser must be made as an extension of the undertray mentioned above. The undertray is actually very important as the effectiveness of the diffuser will depend entirely on having an unblocked airflow to it.

The pressure distribution along a diffuser is such that the lowest pressure (where there is most suction) is at the point where the flat underbody meets the angled diffuser. How low the pressure will be in this point actually depends on how sharp the edge can be made. This point should obviously be as far forward as possible so that a longer, less sloping diffuser will be preferred to a higher sloping one.

Diffuser looking rear on
an Audi A4 Quattro
For two reasons, this diffuser probably has very little effect: There seems to be no unblocked airflow to the diffuser, and it is very short. It seems to be a trend to to put aerodynamic trickery on road vehicles to add to the 'sporty' look.

Further, a diffuser will change the rear of the Murena, so it may not be acceptible. On a two/four pipe exhaust, the diffuser can be located between the exhaust pipes perhaps making it visually less apparant, but this will have to tried.

It will also be a challenge to move the exhaust silencer out of the way.

( this Ferrari-rear Murena could have had a diffuser was it not for the odd looking cooler under the rear. But it may give us a hint of how a rear with deepersides would look. Personally I think it changes the look more than acceptible.)

But the biggest problem will be to get airflow under the engine. This picture shows how the sump potrudes into the airflow to aid cooling of the engine and how it will be difficult to obtain a symmetric airflow under it.

Note that the diffusers' effect on the airflow through the two ventilation channels under the boot should be evaluated.

Rear spoilers and wings

The 2.2S was equipped with a rear spoiler ensuring better downforce and more suction on the rear of the car.

Murena S rear spoiler
Picture of the Murena S, notice the rear spoiler and the side sills

The spoiler must also have increased drag somewhat (air no longer stays attached to the rear window, I have been told that raindrops simply stay on the window on Murenas with the spoiler on, whereas on other Murena's they are quickly pulled off the window), but with the added engine power of the S this was probably acceped and a new Cd value was not quoted so we don't know the effect of it.

Alternative rear spoiler for the Murena by Politecnic
Alternative rear spoiler as offered by Politecnic

Another version of the spoiler is available as a body kit, this is a wing shaped spoiler which probably generates a similar amount of downforce, but probably with less drag penalty.

For increased airflow under the car, the S-spoiler is interesting however, since it will probably be good at directing the air upwards in a laminar flow instead of ending in a turbulence behind the car. If a rear diffuser can "fill" the suction created by the spoiler, we may actually be able to lower drag in this way.

Concluding remarks

I beleive it is possible to enhance the downforce of the Murena without distorting its visual apperance.

I'm not sure what the best solution is yet, and not even if it is necessary within the speeds one will encounter on road or on track (up to around 200 km/h).

But changing the airflow from the radiator to exit in front of the wind screen (pass over the car instead of under) seems interesting. Perhaps it can be combined with a solution for improved airflow into the wheel wells where some of the air otherwise passing through the radiator is directed to the front brake discs.

A slightly deeper front spoiler and a smooth undertray with appropriate tunnels to direct air where it does the most good also seems interesting. Further, with a smooth undertray and a rear diffuser, I think it will be possible to generate a good level of additional rear downforce.

But any change in aerodynamics may switch the balance of the car and thus has the potential to make things worse. It will be necessary to try out changes in a way so that their effects can be evaluated - for example on a circuit with appropriate measurement devices fitted on the car (air pressure and suspension forces).

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Page updated: 2007-04-16 08:44:14