there is no way 3.91 will hurt its time it will only help it. it helped the 1le it will help the z/28.
the origianl z/28 sucked at the quarter mile becaue it had a 302. this one has a 427.
I think if the 1le can do 12.7 or so the z/28 whould have no problem with 12.2-12.3 at minimum.
the power difference betten the LS3 and the LS7 is night and day. the LS7 smashes it everywhere low end, top end, more rpm range. I have owned both. stock vs stock it has some much more power.
2015 Hellcat - being built
2013 ZL1 -sold
2013 Megacab 8" lift 38"mt's