Having removed the top cooling pipe I decided today would be a good time to run a test in a high temperature. So took her out for a 55 min, 45 mile drive.
As expected she passed 80° C within 6 mins rather than the usual 20 mins so a very quick warm up now the top of the rad is not being fed. But again, as expected, she began to run a lot hotter and in fact she remained firmly pegged at 90° C for the remainder of the trip. But that was because the fan was running continuously. I don't suppose it matters, but it is a bit of a power hog and the alternator voltage dropped from my normal 13.4V to 13.2V.
So do I trade a fast warmup time for more running of the fan ? Not sure yet.
Still might go down the thermostatically controlled water valve inserted in the top pipe so it remains shut until the engine reaches temperature..