I had hesitation in my 91 5.0L over 4,500RPM. I pulled out Autolite plugs, which looked worn out to a whopping .072 gap!!! I installed Motorcraft factory spec plugs, and I gapped them at .048 since my car has 145K on it. No more hesitation, and I noticed a big difference in performance. I can now squeal tires in 2nd gear!!!