Brad Edwards explains why Alabama is not the No. 1 team in the preseason FPI rankings. (1:56)

Scoot over, Alabama.

The team to beat in 2018 is Clemson, not the defending national champion, according to the Football Power Index's (FPI) initial preseason rankings release.

The reason is simple: returning talent. Though Alabama was the most efficient team in the country last season, it will return only 10 starters. Clemson is bringing back 15, eight of whom are on defense.

In addition to the surprise return of defensive lineman Christian Wilkins, who probably would have been a first-round NFL draft pick had he decided to leave school, the Tigers bring back fellow underclassmen DE Clelin Ferrell, DE Austin Bryant, LB Kendall Joseph, CB Mark Fields and LT Mitch Hyatt. All that explains why FPI projects the Tigers to have the best defense in the nation next season and is the difference between the Tigers and the Crimson Tide.

Incredibly, this is actually the first time the model has put Clemson first, in any week, dating to 2005.

Alabama is currently ranked second, a little over two points per game worse than the Tigers. Of course, Alabama's offense last season mostly functioned with Jalen Hurts at quarterback. Because Hurts will return, the model is mostly basing its projection on the assumption that the same quarterback (with another year of development under his belt) will hold the reins, but there's a possibility national championship hero Tua Tagovailoa is the starter on opening day.

Clemson, meanwhile, also returns its starting quarterback in Kelly Bryant.

What's FPI again?

For the uninitiated, FPI is our predictive rating system for college football teams that we use to fuel our predictions and projections before and during the season. You can read about how FPI works here or watch a short video about it below. But the abridged version is that factors used in the preseason model include:

Team performance over the past four seasons (most emphasis on last season)

Returning starters at quarterback, on offense overall and on defense

Whether a team has a returning head coach

Recruiting rankings over the past four seasons

play 5:02 Breaking down the Football Power Index College football analyst Brad Edwards shows how ESPN uses four seasons of data to rank college football's best teams.

The key thing to remember with FPI is that it is predicting team strength. We'll post projections later in the offseason, but for now, we have rankings and ratings for all 130 FBS teams. If you can't wait to see where your favorite squad landed, scroll to the bottom of this post.

Last year, two of the four teams that eventually made the College Football Playoff were ranked in the preseason top four (Alabama and Oklahoma). If you want an in-depth look at FPI's accuracy last season, you can read about it here.

Big year for the Big Ten?

Last season, two schools from the same conference made the playoff for the first time in the format's history. In that case, it was the SEC, which is a strong candidate again with the second-, third- and seventh-ranked teams in FPI. But don't sleep on the Big Ten, which looks primed to boast a multitude of talented teams in 2018, with five of the top 12 teams in the current rankings.

Ohio State (4) and Penn State (8) are no surprise to be ranked that high, but FPI also has a pretty optimistic view of Michigan State (10) and Michigan (11).

FPI is relatively bullish on the Spartans following their bounce-back campaign in 2017 (10-3) after a disappointing three-win 2016 season. Michigan State is returning a remarkable 19 starters, including quarterback Brian Lewerke, who finished last season ranked 14th in Total QBR.

Over in Ann Arbor, the heat is starting to turn up on Jim Harbaugh, though FPI thinks the fourth-year head coach's team is looking better. The good news for Michigan: This rating does not include the possibility of Shea Patterson playing this season for the Wolverines (Patterson's eligibility is uncertain), though the model will not upgrade its rating if the former Ole Miss quarterback can play because the Wolverines already are returning a starter in Brandon Peters.

On the other hand, we're just talking about Michigan's pure team strength. While our projections aren't out yet, it's pretty clear that the team has an exceptionally hard path to success in 2018, with games against five of the top 12 FBS teams, per FPI.

Speaking of one of Michigan's opponents next season ...

The Irish are back

Notre Dame spiced up the playoff conversation for part of last season before tailing off at the end. Expect the Irish to at least be a tough out this year, with FPI rating them the fifth-best team in preseason.

Notre Dame's rating is mostly about its overall level of play from last season, when the school finished eighth in overall efficiency (which is essentially expected points added per play, adjusted for opponent and down-weighted for garbage time) despite losing two of its final three games of the regular season. It also returns 10 starters from the 10th-most efficient defense in the FBS and has had solid recruiting over the past four seasons.

play 2:17 Breaking down college football's FPI rankings Here is a look at the numbers that built the preseason college football FPI rankings and put Clemson at the top over Alabama.

No repeat here

It's going to be hard for UCF to proclaim back-to-back championships.

Coming off of an undefeated campaign in which the school declared itself the season's winner (might as well take a shot, right?), FPI ranks the Knights ... 53rd in FBS. Their 68th-best defense is not the stuff of champions.

Though they were a legitimately good team last season and bring back starting quarterback McKenzie Milton, the Knights lost head coach Scott Frost and have four-year recruiting rankings in the 70s.

Gators most improved?

No team in FPI's top 25 improved its ranking from the end of last season to the preseason more than Florida, which bumped up 28 spots to No. 22. Florida is bringing back 19 starters and has had four-year recruiting ranks in the teens. The Gators also are bringing back Feleipe Franks at quarterback, though after last season Florida fans are probably hoping that new head coach Dan Mullen finds another option to start instead.