B16A2 Head B20 Block

 

Jumped timing?

 

What cams? Could be many things. Mechanical over rev (accidentally mis shifted), valves floated, along with many other things could cause the problem. What happens a lot of time is a valve will float due to revving past the valve spring's limit or due to mechanical over rev. The valve from the opposite side opens under the floating valve. When the valve tries to close, the other valve holds it open and the piston takes those 2 out. After that, things just plain get crazy. Kind of like those "Will it Blend" videos but inside your engine....