Pen has been mighty for White Sox all season

With a 3.67 ERA through Sunday, the White Sox are in the upper echelon of the American League. They’re third behind the Astros’ 3.39 and the Rays’ 3.63 and stand 40 points better than the league average.

But there’s an extreme split on the South Side. Starting pitchers have a 4.32 ERA, good for 11th in the AL. The bullpen has been lights out with a 2.48 ERA, second behind the Indians’ 2.19.

It’s no surprise closer David Robertson has been solid (3-1, 2.41 ERA, seven saves, 26 strikeouts in 182/3 innings). Many have looked at Robertson, 32, as a valuable commodity when contenders are looking for bullpen help at the trade deadline. He has the track record, with a career 2.95 ERA, 11.9 strikeouts per nine innings and a three-year run of 39, 34 and 37 saves.

But Anthony Swarzak and Tommy Kahnle have been bigger surprises. Let’s check them out by the numbers, with the caution that sample sizes are always small for relievers with limited innings.

Swarzak is 31 and has a 1.16 ERA in 231/3 innings. His career ERA is 4.36, and the only time he has been below 3.00 in a season with more than 14 innings was 2013, when he had a 2.91 ERA in 96 innings for the Twins.

He has allowed 3.9 hits per nine innings, well below his career average of 9.5. Some of that is chance, with Swarzak allowing a .184 batting average on balls in play, compared with .300 in his career.

But Swarzak also is striking out 10 batters per nine innings, compared with six for his career. Hitters are swinging at 41.4 percent of his pitches outside the strike zone against his career 29 percent. The Blue Jays’ Roberto Osuna (44.2 percent) is only AL pitcher with a higher percentage.

With 21.7 percent of batted balls being flies on the infield and a low home-run rate of 0.4 per nine innings, Swarzak’s 1.99 FIP suggests he’s pitching nearly as well as his 1.16 ERA.

Kahnle, 27, has pitched 20 innings and has allowed only three earned runs for a 1.35 ERA after 4.19 and 4.86 ERAs for the Rockies in 2014 and 2015 and 2.63 for the Sox last season.

His FIP is right with his ERA at 1.37, and the key to that is an exploding strikeout rate. With 37 strikeouts, Kahnle is at a whopping 16.7 per nine innings after seasons of 8.3, 10.5 and 8.2.

Like Swarzak, he has been good at getting hitters to swing at pitches outside the zone. His 34.8 percent rate ranks 28th in the AL.

Among them, Robertson, Swarzak and Kahnle have given the Sox one of the best bullpens in baseball. How many of them remain in Sox uniforms at trade-deadline time remains to be seen. At current performance levels, there’s more than one valuable piece in the pen.

Follow me on Twitter @GrochowskiJ.

Sale has no regrets as he prepares to face White Sox for first time

Cabrera stays hot with homer, four RBI; Shields nears rehab start