Big thanks to Joe and Mike @ JMS

[2manymustangs]

I forgot to add the important things.

Broken valve

OUCHIE!!! that was going to be an ugly one... Can we see a pic of the face of that valve please??? Just for grins you should measure the overall length and compare to some of the other valves that don't show as much of the melt down/stress...

I would look for a deeper issue/cause... Lean is mean but not tooooo lean... Maybe too much spring for the quality of that particular valve??? Could be a few different things going on... Cool stuff, ty for sharing...

This is from the AERA (engine builders association)

"The two intake valves shown in Figure 2 are both out of the same engine. The one tuliped valve is .220" longer than its original length. This was the result
of an abnormally high combustion chamber operating temperature. This temperature is in the cylinder of the engine, NOT THE ENGINE COOLANT TEMPERATURE. The temperature surrounding the valve got much higher than the valve was ever designed to withstand. That excessive temperature resulted in the valve material softening, allowing the valve head to stretch to create what is called a "tulip" shape.

A normal combustion process operating temperature in the combustion chamber could be considered 2500° F with cylinder pressures between 900-1200 psi. Engine valves will only survive the normal temperatures because they are seated during the combustion process and transfer heat as designed to the head casting.

An abnormal combustion process may increase the operating temperature in the combustion chamber as high as 5000° F with pressures between 3,500 - 5,000 psi. If this "tulip" condition re-occurs after rebuilding of the engine or reconditioning of the cylinder head, the original problem has not been fixed. This condition may affect one or more valves at a time.

Items to check; properly working EGR system, correct ignition timing, ECU operation, vacuum leaks (intake manifold, hoses, etc.), correct air to fuel mixture, correct angles between the valve and the valve seat, excessive valve spring pressure and any other source that affects the combustion process.
The AERA Technical Committee"