Yet another heat shield

The MG TD XPAG, with modern fuel is rather prone to vapor lock phenomenon, especially in the sunny summer days of southern France where my TD lives! The reader may find many articles about the symptoms of vapor lock and how to cure the defect from various sources. Curious readers may even find theoretical explanation related to the use of modern fuel in our ancient engines in previous editions of this magazine. Search for “Manchester project” in the archive. Also, you can read the well documented book of Paul Ireland Classic Engines, Modern Fuel: https://classicenginesmodernfuel.org.uk/

The purpose of this article is to describe one way to implement one of the solutions. Installing a heat shield between exhaust manifold and carburetters prevents overheating of fuel mixture in bowls and channels between bowls and carburetters.

Standard heat shields are available from after-market usual sources but I decided to design my own for three reasons.

– Standard heat shields are metal sheets inserted between the carburetter flanges and the manifold. This fixing method introduces an additional weak point for air leak in the inlet manifold.

– Standard heat shields may present an obstacle for carburetter access, leading to difficulties for maintenance and tuning.

– Standard heat shields only protect carburetters from exhaust manifold radiated heat. An important heat source in MGTD engine compartment is also the warm air flow blowing out of the radiator. I realized that the front bowl was hotter than the rear one.

First, a bit of theory about thermal propagation.

Basic rules: The first is that heat always propagates from the hottest part to the coolest: the second is that heat uses three ways for travelling.

1) Conduction. When the hot part touches the cool part. It is the more efficient way of propagation. When your finger (25°C) touches the soldering iron (250°C) it is burnt through conductive mode. If you are just millimeters away, you feel the heat but your finger is not burnt immediately.

2)  Convection. It is a sort of conductive mode but through the flow of a gas. The flow of fresh air (25°C) continuously evacuates (dissipates) the heat from the radiator of our MG (80°C).

3) Radiation. It is the way we receive the heat from the sun (5000°C). Neither contact nor gas between the sun and the earth.  It is a sort of wave. This propagation mode is very efficient at high temperatures.

Now let’s go back to our concern. The diagram at Figure 1 illustrates the heat transfers and the principles of the proposed heatshield.

Exhaust manifold is approximately 300°C whilst carburetter should not be more than 50°C to avoid fuel boiling. We can really feel the difficulty as they are only separated by a few inches.

The radiative mode is the most important mode between manifold and carbs. When driving, convection is evacuated (dissipated) to the rear of the engine bay and then behind the car. To prevent radiative mode is very easy, we just have to put a screen between them. That screen will capture all the heat from the exhaust. But this shield will get hotter and hotter and it will then radiate to our carbs on the other side. The shield now needs to be cooled also. In my design, it is cooled thanks to convection mode. The air coming from the radiator is directed to the back face of heat shield and carries the heat behind the car. This energy is now outside of the car.

The shield also protects the front bowl from radiator air flow. This heating mode is totally different. When the engine runs, dominant propagation mode is forced convection due to the fan blades. To prevent this, we need to deflect the hot air flow off the bowl.

The design is very efficient because it covers the entire exhaust manifold, from bottom outlet flange up to head ports. The drawback is that it is a rather tricky process to install it. However, once installed, access to the carburetters is as easy as if the heat shield wasn’t there.

The shield is only fixed at its bottom part with the existing outer bolt of the exhaust manifold flange and an 8 mm bolt in the centre of the inlet manifold bracket for the top. Too many fixing points and the shield could itself be heated by conduction from the hot manifold. My car has no supporting stud for the air cleaner in this bracket because of the carb insulating spacers (another trick against vapor lock). I guess, it should be easy to clamp the shield with the air cleaner supporting stud, if any. Figure 2 shows the drawing of the metal sheet.

My shield is made from a 1.8 mm thick steel sheet. Thinner material may produce vibrations at the edges of the device. The red dotted lines indicate folded flaps. Do not cut the sheet along these lines but fold it. About 45 ° away from the manifold for front flap and right angle toward the manifold for the bottom fixing part. The front flaps, enhance the air flow for cooling of the manifold and the shield itself. The three upper flaps have to be bent toward horizontal. They aim at covering the exhaust ports at head level. Adjust the upper fixing slot on the car for proper distance between the shield and the manifold. The right-hand side of the drawing is the front bowl carburetter shield. Bend and adjust around the bowl avoiding any contact. 2 cm is a convenient distance.

Figure 1

Figure 2

During several trials I realized that covering the metal with a self-adhesive insulation fabric was a good bonus. More than this, both sides are now covered with that self-adhesive protection that you can find in any car tuning shop.

How can we explain this result?  My hypothesis is that at low speed, the convection mode between manifold and shield is not negligible. Other hypothesis is that this insulated fabric is very reflective for infra-red as I don’t know what is it made of.   Any other explanation is welcome.

Fitting the shield is not a pleasant moment. Be patient. You will need to remove the carburetters partially. Final adjustment of flaps may be necessary when fitted. They should cover but not contact the exhaust manifold. Take care also of the throttle shaft and the command link that should not interfere with the shield. A little tweaking may be necessary for this at installation. You might even get angry when fitting the bottom bolt.  Be prepared!                                 

 Laurent Castel

Ed’s note: I have done my best to ensure that nothing is lost in the translation.

Some pictures follow:

Referring back to the drawing at Figure 2, this is the upper middle flap
(the one measuring 30mm with the 8mmm gap in the middle).

The picture shows the front flap covering the exhaust manifold.

This is the front bowl carburetter shield
(see the right-hand side of the drawing, as mentioned in the text).

One thought on “Yet another heat shield

  1. MGTF1500 Ardeche France says:

    Dear friend, your heatshield is not bad!
    It’s the first one I’ve seen that doesn’t fit on the carburetors and it’s a very good idea. I have to make one this winter for my TF and I think I’ll use your idea.
    Congratulations, Thierry from Ardèche

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