That Clemson team was #1 in scoring defense. They picked off a ton of passes, so if you didn't really pass much, that wasn't an issue.
Michael Dean Perry (Browns) and the Fridge made for a tough DL. Jeff Davis was their team MVP at LB.
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A good summary of that team was that they beat ho-hum Tulane 13-5. The next week, they beat #4 UGA 13-3. That was basically the bulk of their season: only score some, allow even less.
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You could say they only played 2 ranked teams before the OB vs Nebraska, and only scored 23 points combined (both wins, obviously), and you'd be right. Maybe Pitt would have whipped them. Maybe SMU could have run on them. Maybe Texas would have shut them out.
But their win over UGA was enough to check the box of 'could they beat an elite team?'. The Dawgs wound up ranked 2nd before the bowls.