Red Sox starter Tim Wakefield releases a pitch in the first inning against the Padres in their game last night at Petco Park.

AP / Lenny Ignelzi Lenny Ignelzi

SAN DIEGO — On Friday night, Red Sox general manager Theo Epstein was seated in the dugout at Petco Park, chatting with his former mentor, Kevin Towers, the GM of the San Diego Padres.

Epstein told Towers that the series between their two clubs that went into Friday’s games with the best record in their respective leagues would likely be determined by which bullpen was more effective over the weekend.

The Sox edged San Diego, 2-1, in Friday’s opener, with each bullpen holding the opposition scoreless — three scoreless innings from Boston’s relievers and two from the Padres’ — thus creating a draw.

Still, Esptein’s larger point can’t be argued — that a team’s success can’t be separated from that of a bullpen. It was no coincidence that the Padres came into the weekend with the lowest bullpen ERA (2.39) in the game, with the Sox right behind at 2.90.

San Diego’s success can’t be considered much of a surprise. Towers long ago earned a reputation as one of the best executives in the game when it comes to putting together effective relief staffs. It doesn’t hurt, of course, that the Padres play in spacious Petco, where runs are hard to come by, and in the N.L. West, which houses a number of pitching-friendly parks in a league that doesn’t feature the DH.

Having success in the American League can be trickier, as Epstein has found. In Epstein’s five years at the helm of the Sox, the club has experimented with a number of closers, before settling on Jonathan Papelbon.

Papelbon closed out Friday’s win with his 17th save in 18 save opportunities, reaffirming the wisdom of returning Papelbon to that role in spring training. But it’s the rest of the bullpen that been such a pleasant surprise.

“We’ve had some tough luck in the past,” said Epstein before last night’s middle game with the Padres. “We had some guys who had pitched well elsewhere, didn’t pitch well for us, then went to pitch well again elsewhere.”

Such a list might include Rudy Seanez, Bob Howry and the recently released J.C. Romero.

But this year, the Sox have fared far better with the rest of their pen. Javier Lopez has blossomed into an effective lefty. Kyle Snyder, is finally healthy after a series of surgeries and setbacks. Brendan Donnelly, cast aside by the Los Angeles Angels of Anaheim, was enjoying a comeback season before being sidelined earlier this month with soreness in his forearm.

Most surprising of all, though, has been the work of Hideki Okajima, who went into last night with sparkling 0.98 ERA, having allowed just four earned runs in 36 2/3 innings.

“His emergence,” said Epstein, “has allowed us to treat games like seven-inning [contests]. It’s been a big advantage for the manager.”

Okajima’s work in the eighth has allowed the Sox to construct the bullpen from back-to-front, with eye-opening results. Going into last night, the bullpen had allowed just three runs over the previous 24 innings and had limited hitters to a .231 batting average for the season.

Why has finding the right bullpen combination been so difficult? Simple mathematics, suggested Epstein.

“Relievers throw fewer innings,” he said. “Starters can throw 200 innings, but relievers are more likely to be around 50, so it’s harder to evaluate them. They’re more prone to random variation. Also, a lot of relievers are two-pitch guys, compared to starters, who might have four. So if a reliever has trouble with, say, his breaking pitch for a few months, he becomes a one-pitch guy.”

Epstein further credits the team’s revamped “pitching infrastructure, led by pitching coach John Farrell, and more detailed advanced scouting as additional reasons for the bullpen’s improved performance.

To put the bullpens’ work into historical context, only once in the last half-century have the Sox’ relievers finished a season with an ERA under 3.00. That was in the pennant-winning campaign of 1967, when the Impossible Dream Red Sox posted a 2.90 ERA.

And now?

“We picked the right guys, said Epstein, “at the right time.”

smcadam@projo.com