My friend just bought a 94 Z28. At WOT at around 5,200 rpms the car has a back fire right before the 1-2 shift. What could be the culprit for this?

 

plugs, wires, opti are all possibilities. Good place to start.

 

Plugs, wire, and opti are all new. Previous owner said somebody told him it might be valvesprings? That a valve might be getting stuck open or something?

 

That is also possible. Might want to take your valve covers off at least look at your valve springs. I did that one time and found two broken valve springs on the driver's side. Luckily, the valves had not yet dropped.