how much lift can stock valve springs handle?

[505challenger]

ok heres the situation:  i bought this car from a buddy who had just installed a cam in the 340 with x-heads, i finally got the car restored and now it wont rev past 5000 rpms without losing power and breaking up.  i got a few sugestions from members on this forum that the valve springs might be old.  so i asked my buddy what valve springs are in the heads and he said they are new, but they are stock!.  so my uestion is what size cam are stock x-head springs good for?  the cam s a lunati voodoo
.494in   .513exh.
duration .050 226in.  234 exh.
it also has 1.6 rockers

[moper]

If that is the case I'm surprised it runs at all. You may have some tweaked pushrods too as the spring should be going into coil bind or very close. It's crutial you either run the springs that Lunati recommends or your shop finds some that are similar spec but a single w/dampener type rather than duals... Dual springs need some machining on the heads to work.