Last off-season, three NFL teams made the switch the Buffalo Bills are currently making: from using the 4-3 defensive front as their base package to the 3-4. The Green Bay Packers, Kansas City Chiefs, and Denver Broncos experienced varying degrees of success with the change, and each defense not only improved in points allowed, but also in rushing and passing yards against.
Before I show you the numbers, know that the changes we are talking about can be based on multiple factors. Offensive improvements, coaching changes, and new personnel all play a part in these defenses' improvements from 2008 to 2009. There is a possibility the defensive numbers could go in a negative direction, as happened to the 2007 New York Jets. It's not a predictor of success, just some food for thought.
Notes: "Holdovers" are current starters that were also starters during the previous season. "Promotions" are players that were on the team the previous year, but not as a starter. Half spots were given when time was shared heavily between two players.
2008: 8-8 record, 448 points allowed, 2,337 rushing yards against, 3,656 passing yards against
2009: 8-8 record, 324 points allowed, 2,059 rushing yards against, 2,981 passing yards against
Front seven: 2 holdovers, 2.5 promotions, 2.5 free agents
The only team that made the switch that didn't improve on 2008's overall record was the Denver Broncos. When new head coach Josh McDaniels hired Mike Nolan to run his defense, the Broncos moved to the 3-4. The team jumped from 29th and 30th against in yards and points, respectively, to 7th and 12th in 2009. The Broncos did a very large turnover in personnel in order to achieve this, with only two returning starters in their front seven - Elvis Dumervil switched from DE to OLB and had an All-Pro season, and D.J. Williams moved from 4-3 OLB to 3-4 ILB. Kenny Peterson, a backup in 2008, started 14 games at DE, while fellow backup Mario Haggan stepped into the starting lineup at OLB. Ryan McBean, a practice squad player for the Broncos in 2008, started 14 games at defensive end, splitting time with free agent addition Vonnie Holliday (each get half). They added ILB Andra Davis and NT Ronald Fields in free agency. They only added one front seven player in the draft, when they selected Robert Ayers 18th overall. He is slated to start at OLB for the Broncos in 2010, while Haggan replaces the departed Davis.
Kansas City Chiefs
2008: 2-14 record, 440 points allowed, 2,543 rushing yards against, 3,748 passing yards against
2009: 4-12 record, 424 points allowed, 2,504 rushing yards against, 3,707 passing yards against
Front seven: 3 holdovers, 1 draft pick, 1 promotion, 1 trade, 1 free agent
The Chiefs experienced the most modest improvements in their defense when switching from the 4-3, and also acquired the most new players for use in their 3-4. When Todd Haley was hired as head coach, he installed Clancy Pendergast to run the defense and transition to the 3-4. The Chiefs gave up 0.2 fewer yards per play in 2009 than in 2008, and improved only slightly in yardage allowed. Only three members of the starting front seven were retained as starters, as Glenn Dorsey moved from DT to DE, Tamba Hali took his hand off the ground to move from DE to OLB, and Demorrio Williams moved from OLB to ILB. The Chiefs' first-round selection went to DE Tyson Jackson, and DT Ron Edwards was brought up to the starting lineup at NT. Perennial backup Corey Mays was signed as a free agent and started 13 games at ILB, and the team traded for OLB Mike Vrabel from New England.
Green Bay Packers
2008: 6-10 record, 380 points allowed, 2,105 rushing yards against, 3,244 passing yards against
2009: 11-5 record, 297 points allowed, 1,333 rushing yards against, 3,218 passing yards against
Front seven: 5.5 holdovers, 1.5 draft picks
Dom Capers was hired as defensive coordinator by the Packers to reform their defensive woes. He switched to the 3-4, and the Packers' win-loss record and defensive ranking both took considerable jumps. Aaron Kampman slid back from his DE spot to OLB, Ryan Pickett shifted from DT to NT, Johnny Jolly moved from DT out to DE, Nick Barnett shifted over from MLB to ILB, A.J. Hawk also remained at linebacker, and Cullen Jenkins returned from an injury-shortened 2008 to play DE. First-round pick Clay Matthews started immediately, and their other first-round selection, NT B.J. Raji, started one game at NT, playing in 14 games. Raji and Pickett's stats are similar so I'll give them each a half spot. Seventh-round pick Brad Jones started 7 games for the injured Kampman, while DE Jarius Wynn was also selected in the draft. The Packers' ascent in defensive rankings was even more dramatic than Denver's. Green Bay went from 22 and 20 in points allowed and yards allowed to seventh and second. They had the top rushing defense in the entire NFL one year after ranking 26th. As mentioned earlier, the fact that the Packers' offense was ranked third in the NFL in points scored means several teams were playing catch up by throwing the ball, but the Packers' passing defense still ranked fifth.
2009: 6-10 record, 326 points allowed, 2,501 rushing yards against, 2,948 passing yards against
Expected front seven: 3.5 holdovers, 1 promotion, 2 free agents, 0.5 draft picks
When Chan Gailey was hired, he brought in George Edwards to run the defense and transition to the 3-4. At this point all we have are roster projections, but we can still make educated guesses about how Buffalo's defensive front seven will look in 2010. Kyle Williams will get the most "starts" at nose tackle, with heavy doses of second-round selection Torell Troup. Let's call it half for holdover and half for draft pick. Marcus Stroud will bump out to DE, Paul Posluszny will move from MLB to ILB, and Chris Kelsay will likely start at OLB. The Bills pursued and signed ILB Andra Davis and DE Dwan Edwards to fill two holes, while promoting Aaron Maybin in place of the questionably retired Aaron Schobel. Danny Batten will likely see playing time on run downs at OLB if Schobel officially calls it a career.
For some older data on how teams fared in their first year using the 3-4, check out this post from SB Nation's Chargers blog, Bolts from the Blue, from last off-season previewing the changes underway in Kansas City and Denver.