Hi,
The TR4 and later definitely need the shroud in place to keep air flow from "escaping" around the rad. Without it, the cars are far more likely to overheat. Due to the shape of the air intake, lack of a shroud is a little less critical on TR2/3 (but still helpful). Beginning with TR3A "wide mouth" design, it got more important.
TR4 used a one-piece shroud that attaches via 3 spring-type screw clips (I dunno exactly what they are called) across the top of the valance and two more on either side of the "mouth".
Later cars used slightly different arrangement. I think your 4A has a 3 piece shroud. By the TR6 the shroud was much wider and more sharply angled (But can be made to work on TR4, etc. In fact, some years ago Vicky Brit sent me a TR6 shroud marked "TR4". I used it with some slight mods, see below.)
Original shrouds were "fiberboard", with a black finish. These disintegrate pretty quickly.
There are plastic replacements now available that are much better, although not quite original in appearance.
Or, make your own out of sheet aluminum - or sheet plastic, if you can find it locally. I riveted together a homemade aluminum one and used it for many years. The original, tired fiberboard one was used as a template. The aluminum shroud worked fine, but eventually cracked and got dented up. Because it was more rigid, it was more difficult to install and remove, too.
I ended up reinforcing the plastic "TR6" replacement with an internal, aluminum brace. Without that, it sagged all on it's own, blocking an inch or two at the top/front of the radiator. I can't imagine how much of the rad would have been blocked by it eventually, when the plastic got hot under the hood and sagged even more! I also riveted two additional alu brackets on the sides, so that it can be attached to the front valance. There was no provision to attach the sides on this particular shroud (I don't have a TR6 to check, maybe that's is normal).
The later shrouds aren't as deep (valance-to-radiator) as the early TR4 ones: about 1-1/2" less. This is actually useful, if a thicker custom radiator is being used, or if the radiator is moved forward to make way for a puller fan.
No, in fact the smaller 12" pusher/auxiliary electric fans that most vendors offer will fit just fine inside the shroud, where it will be pretty well hidden so the engine compartment looking relatively original. However, before installing one, see if the shroud alone doesn't cure any tendency to overheat. A 12" fan is generally more than enough, if used in this configuration. Keep in mind that a pusher fan like that blocks some air flow all the time, just isn't as efficient as a puller fan (which might be larger, I've got 14", some have installed 16"). There is some redundancy with the original fan on the engine, which might be good. But the electric fan motor is more exposed to the elements, out in front of the rad, and the original crankshaft-mounted fan puts some drag on the engine all the time.
Anyone having overheating tendencies probably should not consider an electric fan as a solution, by itself. If there are other problems, IMHO they need to be addressed first, then the elec. fan is added "for extra insurance".
My TR4 simply never overheated on me. That was with the stock, 4-blade fan, alu shroud in place and original "long neck" rad (without the crank starter hole). That was at high altitude and with increased compression, headers, etc. Even had the very "wrong" non-sleeved thermostat installed. Summers in Colo. were often 90+, sometimes 100+ degrees, too (it's a "dry heat").
Hope this helps!
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