Hot inlet air device

[Anders Dinsen]

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.)


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.


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

[macaroni]

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.

[Anders Dinsen]

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

[krede]

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   

[Anders Dinsen]

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  

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

[Lennart Sorth]

GLOBAL WARMING will take care of this issue for you..

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

[roy4matra]

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)

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 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?

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

[Anders Dinsen]

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

[roy4matra]

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.

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.

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.

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.

Would you remember which Renault's they used to be fitted on?

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

Roy

[Anders Dinsen]

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

[roy4matra]

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.

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

[Anders Dinsen]

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.

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.

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

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!!!

- Anders

[michaltalbot]

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

 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...

[Anders Dinsen]

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

 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

[michaltalbot]

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