I have an XE cam in my car. I just switched to a different set of heads with different valve springs and when I’ve taken it out to dial it in, at around 5900 rpm or so, the car feels and sounds as if it’s breaking up a little. I had no issues before with my old heads and springs; everything was smooth up to 6300-6400 (as high as I’ve ever revved it).
6200 is about the best I've ever gotten out of one, and that was a SBC with factory(pretty light weight) stamped rockers.
On the BB motors, they've never made it to 6k.
I did some experimentation on the dyno years ago, playing with different spring loads trying to see if I could get more out of one. Increasing the springs from about 120/310 to 145/370 netted maybe a 100rpm gain..... And when the valvetrain got unhappy with the higher spring loads, it happened even more abruptly.
I ended up swapping to a solid lifter cam, put in springs that were 135/345, and it would rev cleanly to 6500(as high as I was comfortable with on that short block) with not even a hint of a stability problem.
The one time I recall seeing this phenomenon play out in a magazine was in Mopar Muscle 2005 with that EZ headed 446 Dulcich built.
I had already been through what he was experiencing and knew how that was going to turn out.
About 20 years ago, before my friend was running Schubek lifters in his Stocker we were trying to find some upper rpm stability out of his motor.
The cams that showed the most promise just wouldn't rev high enough to be usable.
Around that time we had also played with spring loads, and got basically nothing out of that.
We swapped some solid lifters into it one time to see if it was lifters or spring load.
Set them at .004 lash....... And the motor would go almost 1000rpm higher before getting into some float. The problem was the lifters all along.
The next year he put Schubeks in it...... And that was the end of the rpm problems.
