There are two areas that causes vapor lock in our old Fords. I have done several years of research and found #1 the carb boiling is easy to pretty much eliminate with a 1/2" thick ventilated spacer between the intake manifold and the carb. Bob showman makes a high quality one that worked on the cars I installed them on.
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VL #2 is the other problem. This causes the engine to start starving for gas in warm weather when the fuel pump temperature gets to about 135 degrees. This occurs below 55 MPH. Above 55 mph more gas is flowing through the pump and cools the pump to where there is no VL. Get below 55 and into town with a few stop lights and stop signs at very slow speed and the engine cuts of. The fuel pump needs to be cooled to get restarted.
I just finished putting a simple 3/16" steel brake line fuel return from after the fuel pump back to the filler neck in my 39. I have a fuel pressure gauge in the pump to carb line after a "T" with a needle valve from where the return line runs right after the pump. I can look in the filler neck and see the fuel returning. I can also watch the pressure gauge and see the effect of pressure to the carb as I open the needle valve returning more fuel. It looks like I want to see about 1/2 lb of pressure drop to the carb to get about the amount of return fuel I THINK I need.
Two considerations, first enough fuel at 75 MPH and second enough fuel returned to cool the pump without starving the engine. I only need a little more fuel returned than the extra fuel used between 55 and 75 MPH. I have about a 1/2 tank of the old type fuel in the tank which I have to run out before filling up with the new stuff. I know this will work and I will stick my neck out and say I can tell if it works by the outside temperature of the pump. The pump on this car runs at 135 when it's hot out and the engine gets to 180, this is about where VL starts below 55 MPH. I'm pretty sure from all the testing I have done that the pump will run down in the 125 range which is below the boiling point of the new gas.
I can tell even with the old type gas by the temperature of the pump if it's going to solve the problem with the new gas. It's going to be 85 tomorrow so we will take it for the same 30 mile run for lunch we did all the other tests on. I have a few pictures I will post over the weekend. G.M.