MatraSport Forum

My 2.2 has the device that mixes hot and cold inlet air disconnected, it was probably disconnected when the car was converted to S-spec in the early 80's (since the S doesn't have it), but never removed from the car (original S's have a straight tube instead). (My car has later been converted back to the single 34CIC carburettor, but has had other tunings that make it still comparable to a true S.)

(http://tmp.dinsen.net/murena/inletairswitch.jpg)
In this photo, the air filter connection is on the left. The hot air inlet is on the back of the device. Cold air is drawn in from the end (over/behind the gearbox).

I know this device is there to allow the carburettor to breath hot air when the ambient air temperature is low to prevent or reduce carburettor ice, and to keep inlet air temperature close to the optimum for fuel mixture in the carburettor. There's a thermostatic air valve to be fitted in the inlet which opens when the air is cold. The membrane device on the side of the tube is connected to inlet vacuum through this air valve, and the engine vacuum thus switches the inlet air from cold to hot. The hot air inlet is taken through a tube from near the exhaust manifold.

This kind of device is fairly standard, I think, on many carburettor equipped cars.

The valve in the inlet is missing on my car, so I'm not connecting it up until I have that, but I'd like to know what others think of this device. Is it necessary? Will it make a difference? During the winter, I did have some experiences with carb ice. The vacuum membrane does appear to work and to be able to hold the valve in place.

(http://tmp.dinsen.net/murena/inletairswitchlookdown.jpg)
A look down the device from the air filter connection. On the left is the hot air input. I'm keeping it open with a finger.

Cheers,
- Anders 8)

My twin Dellortos breath through pancake filters about 1" thick!
I guess carb icing is more of a problem in Denmark than the UK, so if it were mine, I'd lose it.

The typical sign of carb ice is when the engine idles at 2000 rpm after a quick run in cold weather. There must be so much ice on the walls of the carb that the throttle valve can't close! That might still happen with the device connected, but once the vacuum returns (idle), the device woult open and send in some very hot air to get the ice out quickly.

- Anders

Don't worry Anders... If we are to believe the egg-heads on television, GLOBAL WARMING will take care of this issue for you.. Long before you get everything sorted out   ;D ;D ;D

Thanks for the encuragement... but I expect to fit this device back on the car quite soon!

hehe - and I have already invested in some sea-side resorts in Iceland, waiting (for global warming to kick in) to turn them into watersports resorts, with jetski and kite-surfing - the lot :)

Actually a true 'S' does have one of these but like the Prep 142 it is not connected to vacuum, and is only used to pick up cool air from low down


I would say yes, but it will be minor to some extent.  It all depends on how much you value a truely correct system.  By this I mean that a manufacturer spends a great deal of time and money getting the driveability of the car to the highest standard possible in all circumstances.  You would be very unhappy if any vehicle you bought from a manufacturer ran rough or was jerky say, under certain conditions - you expect it to be smooth with no flat spots etc.

The main reason for these valves is to help speed up the warm up phase and allow smooth running during the cold and warm up phase, as well as providing cool air once it is thoroughly warmed.  The thermostatic valve in the intake elbow, allows inlet vacuum to control the intake flap so that the intake air is kept to a certain temp. by mixing hot air from around the exhaust manifold with cool air during the warm up phase.  Obviously it starts with only air from the hot side, then mixes hot and cold in varying amounts, until eventually it only feeds cold air when the engine is hot.

The warmed air helps speed up the warming of the metal in the intake, which in turn helps keep the fuel emulsified, which burns easier and means you can use less throttle, saving fuel too.  When fuel/air mixtures hit cold metal they tend to separate and the mixture arriving in the cylinder does not burn as well, producing carbon rather than power.

The thermostatic valve used to be common on carburettor cars - in fact some Renaults had just the same unit, but they are more difficult to get now, which is why I had not fitted one in your car Anders.  I could not get one, before I brought the car.  Also since the Murena is mid-engined, the warm up period is usually very short, compared to a front engined car, so even without this set up working they are not normally too bad.  In fact I would say that on a Murena it is more important that the carbs. are fed with cool air from low down when hot, since you spend more time hot, and hot inlet air is bad for an engine.  This is why I keep saying that all these 'pancake' filters that people fit on their twin side draught carbs. should not be used.  You are picking up really hot air from the top of the engine bay.  This is not only reducing your power, since hot air contains less oxygen, but it heats the internals of the engine, so when people complain their engines tend to run hot, or have overheated or blown gaskets, if they are using pancake filters, they are partly causing the problem.

Roy

Thanks a lot for your response, Roy :) So you are saying that I should not expect it to help carburettor ice at all? That was really my main interest in inquiring about the device.

I have noticed that Simon does not list a p/n for the valve in his catalogue, so I was already aware that this part was probably a bit hard to get. I haven't asked him about it, though. Would you remember which Renault's they used to be fitted on? I might be able to source one from a breaker or from my usual Renault parts source. I do quite a bit of short driving in the Murena, so I would like to see the effect of this for the winter.

Even my fuel injected 1996 Clio had a hot-air intake from the exhaust manifold, but since it had mono-point injection it would also suffer from the same problems with petrol condensating on the sides of the inlet manifold as the carburettor equipped car, which is probably why they chose to keep it. Even the early fuel injected Espace's had multipoint injection, though, and has never had this device - but the carburettor equipped ones did. They had the device integrated in the airbox as far as I can see from the handbooks I have around.

Cheers,
Anders 8)

Oh no, it will help reduce or eliminate carb. icing too.  It's just that that is not normally a problem here, but if you have had it, then I would recommend getting it linked up again.


My guess is that it came with the plastic elbow originally.  And maybe it was the same with other cars which was why I couldn't find one easily.


I think I remember seeing it on the early Megane, but I'll have another look.

Roy

I got the vacuum thermostat from Simon today! He didn't have it new, but the used part looks excellent and his price was reasonable @ 10 EUR.

He forgot to send me the T-piece needed to attach the vacuum hose to the manifold, though :( I hoped to use that to find out which connection on the manifold to use. There are two: The 10 mm hose to the brake booster and the 12 mm hose to the headlights. The latter is reduced in the stud to about 1-2 mm hole, so the pressure must be (numerically) lower in that hose than in the brake booster hose.

Using some bits I have lying around I can make a T-piece to connect to the 12 mm hose, but I doubt if the pressure will be enough to work the hot air switching device.

I have checked the workshop manual of course (my neighbor got very interested when I was reading two manuals at the same time, while having the engine hood off!), but it does not show the hose connections :( The carburettor section of the Tagora manual only shows that it's connected to a certain stud on the manifold.

I'm therefore afraid I'll need the 10-5 mm T-piece. It's really an odd size! But then again, so was the 6-16 mm water hose connection which I broke some time ago. Simon couldn't even supply that, but I now have a brass thing there which my regular hose specialist had "lying around".

But if someone here can confirm positively where to connect the thing, I'd be grateful.

Cheers,
- Anders

The connection for this is neither of those large hose take offs.  You only need a small connection and it should be the same size as the distributor vacuum advance.  The distributor vacuum advance is taken from the carburettor of course and the vacuum varies depending on throttle position, where as the vacuum to the air inlet temp. valve must be constant inlet manifold vacuum.  The other side of the temp. valve then goes to the air intake flap.

Pressure or vacuum in a system is uniform no matter what the hole size, it is only the quantity that varies.

Roy

Thanks, Roy. That will make it easy since it's all 5 mm takeoffs. I did think of this but didn't want to tap into that point worrying that it might affect the ignition timing when the valve opens and closes, but if that's the original location, then it is probably no problem at all.


Yes, Roy, I'm aware of that, but when air starts actually flowing in the system, pressure after a reduction will be lower than it is before. Similar to the way water speed reduces on a river that widens out, but with gasses it causes a pressure drop. You know that of course. In the static case, pressure is equal everywhere, but I wasn't sure if that was safe to assume?

In the case of the headlights system, there must be a significant air flow when cold starting the car. Maybe that's why they fitted the reduction in the manifold for that connection?

Every time I stumble over something, I wanna understand it!!! :D

- Anders 8)

 Could You please specify where is the vacuum pipe from hot/cold air valve connected to the intake manifold or carburetors? Cos I bought my true S demounted and I plugged all pipes by my imagine and with help of few pictures of 2,2 S engine bay...

What Roy is saying (and the spares catalogue can confirm), the S doesn't have the switching capability. The trouble is that the thermostat isn't fitted in the airbox, if it was then you could wire it up, but as it is, you don't have any option except to leave the airswitching device unconnected. Or remove it completely, and replace it with a piece of straight tubing from the airfilter.

I just received some T-pieces, so I'm going to connect it up as soon as I get some time... I'll probably take vacuum from the brake servo hose since that will fit a 10-6-10 piece that I got.

- Anders

My bad english... I understood that ...true 'S' does have one of these, but Prep 142 it has not connected...
Sorry for stupid question  ::)  ;)

No.  Both the Prep 142 and 'S' have the original air filter which is connected to the cold/hot air intake device, but the air flap diaphragm is not connected to anywhere, so it stays permanently open to cool air only.  It never changes to the hot intake side.

On standard 2.2 Murena, the plastic elbow bolted to the top of the 34CIC down-draught carburettor has a small temperature sensitive device inside it, which is connected to vacuum (inlet manifold) one side and the other side is connected to the air flap.  When the intake air is very cold, the temp. device opens and directs the vacuum to the air-flap diaphragm pulling it so that the intake air is drawn from the hot side.  When the intake air is warm enough the temp valve closes and the vacuum is blocked, so the air-flap returns under spring pressure to its closed position and only cool air is drawn in.

The 'S' has a different device mounted under the manifold which the Prep 142 does not have.  I think you are confusing this device with the regulated air intake device.

Roy

I got the device and vacuum installed yesterday. Finding T-pieces in different sizes wasn't straight easy, but I managed to find a 10-6-10 piece that I installed in the vacuum hose to the manifold base coming off the engine breathing filter.

A photo showing the complete installation:
(http://tmp.dinsen.net/murena/warmairdeviceinstalled.jpg)

In case anyone wonders what the vacuum thermostat looks like, here's also a photo of that. Blowing through it before installing it showed me that it seems to switch at around 20 degrees, which makes sense as I have read somewhere that the carburettor is design for 30 degrees air.
(http://tmp.dinsen.net/murena/warmairthermostat.jpg)

I'll report my experiences driving with this installed over the winter.

Uhoh, just realised that I have connected to the only vacuum connection on the manifold where there isn't vacuum at all: The other end is connected straight to the inlet elbow through the three way filter! I'll move it later today.

I fixed the vacuum, tapping into the hose to the headlight lifters.

There is a noticable difference on the engine in practical driving with the hot air device fitted. Cold starting is easier and it runs a bit better cold. This is probably due to the reduced amount of condensation of fuel in the manifold as Roy mentioned. Warm up time is also improved - the engine is now noticable quicker to warmup. Also predicted by Roy. It wasn't bad before, but now it is even quicker. City driving in low rev's and idling is also improved with a more stable and slightly better sounding idle.

Hot air burns faster than cold, so by allowing the engine to breath warm air, a similar effect as advancing the ignition may be accomplished. As I do a lot of city, low rev, low throttle driving, I expect a slightly improved fuel economy because of this.

Some might think the hot air will reduce performance. True, cold air contains more oxygen per volume than hot, but first of all, the temperature isn't that much higher (e.g. a temperature increase of 20 degrees results in only about 7% more molecules per volume) and besides, in full load situations, the device switches over to cold air anyway. This is because at full throttle, vacuum in the manifold is reduced and even if was still breathing from the hot air inlet, the increased airspeed around the exhaust will mean less temperature increase.

Winter will tell the difference regarding carburettor ice, but so far, I can say that if you have the device, don't consider it insignificant! I have learnt something interesting by running without and now with the device :)

Time for an update:

No carb ice so far, excellent cold running - this thing is very recommended unless you *only* use your car in the hot.

I thought I had carb ice the other day though. After 70 km on motorway, the idle settled at 2000 rpm and only slowly climbed down. This was what I had been experiencing last year post-acceleration in cold weather, but when I opened the hatch just to check, I found that the little lever operating the accelerator pump had jumped off the lifter on the throttle - I had been experiencing some dead spots lately - and with that back on the idle was back "spot-on", it was just the arm that had kept the throttle slightly open...