Wednesday was the type of day at Wrigley Field where Steve Trachsel could make a mistake and get away with it.
"It was a great day to pitch," Cubs manager Jim Riggleman said of the 16 mph winds that kept almost everything in the ballpark.
"Sammy absolutely crushed that ball," Riggleman said. "He had to really hit it to hit it out of here."
Sosa led off the sixth by hitting an 0-1 pitch from Jeremy Powell (0-4) into the left-field bleachers to give the Cubs a 3-0 lead.
Sosa, who has six homers in his last seven games, started the day tied with Mark McGwire for the major league lead with 41 homers each.
McGwire and the St. Louis Cardinals played San Diego later at night.
"The home run race is there," Sosa said. "But last year I was more pumped up because we were in a pennant race. When we had a chance to be in the playoffs, that's something that motivates you."
Expos catcher Michael Barrett called for a slider to Sosa, but Powell shook him off and threw a fastball.
"He'd have hit the slider out, too," Expos manager Felipe Alou said.
Trachsel (4-14) took advantage of the wind currents.
"The wind blowing in gave me the opportunity to challenge guys," he said.
"I couldn't believe my eyes before the game when I saw he was 3-14," Alou said. "I've only seen him oce without good stuff and that was last year. He won that game, too."
Trachsel, who leads the majors in losses, walked three and struck out eight in his third complete game.
"I really wanted (the complete game)," Trachsel said. "When Marty (pitching coach DeMerritt) told me that I had 70 pitches after seven innings, I knew I could make it."
How has he survived this season?
"Knowing I'm better than that," Trachsel said. "You know it's going to turn around sooner or later."
The win was only the Cubs' third in their last 12 games. The Expos had won four in a row.
Powell gave up five runs and six hits and four walks over 5 1-3 innings.
"I just tried to do too much after the Sosa homer," Powell said. "I tried to strike out Henry (Rodriguez in the sixth) and didn't and everything snowballed from there."
Powell is now 1-9 in 12 career starts in two seasons.
"Losing is getting pretty old with me," Powell said. "Errors and not scoring runs will happen and you've just got to roll with it. It's going to even out in time."
Houston hit a sacrifice fly in the fourth to make it 2-0.
After Sosa's leadoff homer in the sixth, Rodriguez walked and Houston singled. Nieves hit a sacrifice fly to make it 4-0 and Benito Santiago added an RBI single.
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