Mike Rapoport wrote:

They become worthless. The problem isn't just octane it is also vapor
pressure. The octane problem can be solved but there doesn't seem to be an
economical solution to the vapor pressure problem. The pressurized piston
twins are probably toast if TEL becomes unavailible.

There is a guy (in NC I believe) cutting the engines off (3' on each
wing) the Barons and sticking a turboprop on the nose. Working on his
second conversion now...

Higher fuel per hour burn, but flys faster and higher so it actually
burns less...