strength of stock valvetrain

are stock valvetrains only reliable up to so much hp over stock? i have a d16z6, does anyone know how much hp the stock internals can take without risking something breaking? i'm thinking of anywhere from 160-250 hp range. valves and springs in particular. and about valves, can they actually snap if there is too much power or is it usually the springs that give out?

On a naturally aspirated engine with the stock cam, it's rpm that usually does in the valvetrain, rather than power output of the engine. If you're keeping the cam the same, and staying with natural aspiration, I'm not sure how you'll get the big performance gains that you mention, but if you somehow manage to then the valvetrain should be able to handle it.

Some things to worry about for various changes that you might make:

More aggressive cam:
with a more aggressive cam (higher lift, higher acceleration, or shorter duration), you have several things to worry about for durability (and probably some that I'll forget to mention):
* increased acceleration levels can result in stress levels that are too high at the cam-follower interface
* increased acceleration levels can result in excessive valvetrain vibration, and loss of dynamic control (valve float, seat recession, etc)
* increased lift will increase the working stress range of the spring, perhaps beyond the levels that the spring can tolerate
* increased lift can potentially "coil bind" the spring if you don't have enough solid clearance

Forced induction:
* situations can sometimes arise where the inlet pressure exceeds the in-cylinder pressure, which can pop the valves open if the spring fitted force is not high enough. This is probably not a common problem on small car engines, but perhaps someone else can point out where I'm wrong?
* ditto for the exhaust side
* the cylinder pressure at EVO will be higher than normal, resulting in a larger force that the valvetrain must work against to open the valves

I assume you're not planning on installing an exhaust brake, so I'll skip that part of the lecture...

my brother and i are doing a mild rebuild to the extra d16z6 i have. we are going to put in ZC pistons to raise the pressure and other things like micropolishing the crank, put in a block guard, port and polish the exhaust ports, maybe get the cam reground, ect. i want a strong engine by it self and want to be able to give it a 50-75 shot of nitrous. i didn't know if the stock valvetrain was strong enough for this or if it will need to be done over.

what is EVO? what is an exhaust brake?