Ok, I re-read your first post. If it runs GOOD, and pulls hard up to 4500, then it might be your tach. Mine pulls hard all the way to redline. Maybe you can borrow someone's aftermarket tach, hook it up and compare it to your factory tach. Maybe you are really pulling to redline, but your factory tach just isn't reading it past 4500. I've seen other postings with tachs that do all sorts of weird ****, including going up to 3000 and sticking or bouncing around. When you hit 4500, does it FEEL normal, or does it feel like it wants to do more but can't? Until you check your tach, I probably wouldn't try to climb past your 4500 reading. Who knows what RPM you're hitting. Be happy it runs good. Or drive it "by ear"... alot of muscle cars did not have factory tachs.