Well, maybe we should say the J8 is based on both Mig23 &
Mig21s design, J8 is dramatically different from the J7 (
Mig21).
1st, its powered by 2 engines instead of mig21 1.
2nd, its typical for high speed, high altitude intercepting role
with speed exceeding Mach2.2 and ceiling over 20000M
BTW, its long time rumored that Egypt secretly exported 1 of its
Mig23 to china in exchanging for more J7 fighters in 19070s or
1980s.