My friends 5.0 with SD and a comp cam has a problem.
He cant rev past 4500 rpms before the car starts acting up and backfiring out the intake.
I told him it could be timing, but I wonder what other things it could be making this happen.
Thanks for your help, hate to make this my first post but I been lurking for a while and had not needed to post until now.

 

It could be valve float. The stock springs are marginal at best, even for the stock cam. Throw in a higher lift cam and you're asking for trouble. I'd still check the timing first, but if that checks out I'd think about putting valve springs on it.