what causes a valveto "drop" ??

[cbassgsx]

ok here goes
i bought a dead parts cart that smoked bad
when i pulled the head i found a gapping hole about 2" around.i flipped the head over and found this...
anyways the story goes the kid went to a "hop up" shopbought an mbc, proceeded to tighten it all the way down or to god knows what level goes about a mile thinking "wow that made my car fast" and before he knew it there was a thick cloud of smoke following him.the head was new 2 weeks proir to him buying the car. no other valves are bent but they are very blackened. too much boost? mis shift over rev? what really happened
thanks for any help or answers
chris

 

[TurboJunky]

When a valve "drops" into the cylinder, it's usually because a spring retainer has broken, allowing the spring to fly off somewhere under the valve cover and thus allowing the valve to fall freely into the combustion chamber, causing bad things to happen.

 

[cbassgsx]

this valve was snapped off right at the place where it starts to go larger

 

[Defiant]

In the old days, when valves were commonly flat to the piston tops, a "dropped" valve would be one that'd lost it retainer and fallen into the chamber. With luck, it'd just pogo up and down until repaired. However, with canted valves, it usually snaps the valve head off the first or second time it gets kissed. In DSMs, this is more often seen when the timing belt goes. Why just one went on an engine would take more investigation.