Vapor Lock

[tartan18]

A few days ago I had several incidents of vapor lock. My car always runs on the cool side -- never higher than 185 degrees and the radiator fan works just fine. The only thing different is that this time I refueled with regular gas -- I usually use Non Ethanol Premium (92 octane) but this time I used Regular (87) on my return from a 120 mile freeway drive. The car then sat in the garage for a half hour (heat soak) before I ran a few local errands which is when the vapor lock happened. Poring cool water on the intake manifold cured the problem.

Here are my questions ... I have heard that the higher the octane level the lower the volatility of the gas. Is this correct? And if it is, then does lower volatility lessen the chance of vapor lock re-occurring?

I have a stock 1756 engine, an EMPI 32/36 DFEV carb and a mechanical fuel pump. I would prefer not to go with an electric fuel pump if I don't have to.