The lifters are definitely good for 7000+ RPM. Had them run in a 331 stroker and they are hard to beat.
Your problem, I feel, it with your distributor. At around 6000 RPM you are pushing a lot of air into that chamber, very rapidly. Your distributor will DEFINITELY have a problem if it is the stock piece. Alot of air and fuel is nice but if you can't burn it, it is of no use. It won't be able to accurately advance the timing so aggressively, seeing as the original 85 mustang topped out revs at 5500 rpm.
Another reason why you might not have experienced this problem with the higher compression ratio, is that the higher compression ratio would have helped out igniting the mixture by just compressing the mixture itself.(similar to how a diesel works)
I would consider getting a better distributor, thicker wires, a good cap and rotor, and some top notch plugs that are properly gapped.
Best of luck!
Your post confirms kinda what i was thinking. Thanks for all of your replies! Encouragement is what keeps me around these things sometimes! Keep the ideas coming as i will try them until something gets figured out, i never have been one to quit! I just dropped the distributor off to be recurved and have the vac advance locked out (finally) What you say does make sense about the higher compression!


