SU Carburetor Heat Shields
Why use heat shields?
If heated sufficiently, fuel in your carburetor's fuel bowls can boil and cause poor running, or even shutdown your engine. Such heating is possible if the carburetors, particularly the fuel bowls, have a direct line-of-sight to the exhaust manifold or header on your Spitfire. Heat shields block this line-of-site and keep your carburetors and fuel sufficiently cool to operate properly.
Heat moves in three ways—by conduction, convection and radiation. Because much of the heating of carburetors on the Spitfire is due to radiant heat transfer from the hot exhaust parts that are in close proximity, heat shields are a very effective solution. Almost anything will do that blocks the carburetors' view of the exhaust hardware to dramatically reduce and virtually eliminate this radiative heat transfer, but simple sheet aluminum is a good choice. Aluminum is lightweight, won't burn at the subject temperatures, and it reflects at least 20 times more infrared radiation (heat) than it absorbs. Also, because sheet aluminum has a high ratio of surface area to mass, it can cool-off effectively by convection. Simple 0.050" or a little thicker sheet aluminum works fine. Additional insulation on the shields is not necessary because the aluminum blocks the direct line-of-sight of the carbs to the hot exhaust parts, and for the reasons just stated, sheet aluminum is not a significant re-radiator of the intense heat of the exhaust parts. To demonstrate, put your hand between a heat shield and its carb, or just hold a piece of aluminum between your carbs and exhaust parts with the engine running, and notice the reduction in radiant heat. Also notice that the heat shield or aluminum sheet will be a great deal cooler than the exhaust manifold (you won't want to hold onto the exhaust manifold!).
Apparently some Spitfires have carburetor heat shields and others don't. It seems that the early cars did not have them from the factory, but the late ones do. Heat shields are good idea on a 1500 engine using twin SU carbs (e.g., HS2s or HS4s). I can attest to that; I had fuel boiling problems without heat shields, and have never had a problem after installing heat shields. In the case of twin SUs, the heat shield can be one piece that spans both carbs or it can be two pieces—one for each carb. The two-piece approach ensures there is no interference between the shields and the throttle linkage, regardless of which linkage is used. The objective is to block the carburetors' view (particularly the view that the float bowls have) of the hot exhaust parts.
Heat shields for the Spitfire
Here is a FULL-SCALE TEMPLATE for individual HS4 carb heat shields that I use on my 1978 Spitfire. You can print out the template on a sheet of paper and use it to guide the cutting and drilling and bending of aluminum sheet, but I have included critical dimensions too. One heat shield is the mirror image of the other, so simply flip the template over to make the second shield. It can be adapted for HS2's as well, using a smaller hole to match the HS2's smaller throat size and different holes for mounting to the intake manifold.
Here is a photo of my heat shields on my 1978 Spitfire with HS4 carbs:
I mounted my heat shields between thin gaskets at the intake manifold and a non-metallic/insulating spacer, followed by the carbs (so working from the intake manifold outward, it goes: thin gaskets, shields, insulating spacer, then carbs). The choice of surface finish--either polished or brushed--makes little difference in performance.