SANTA CLARA — Isaac Sopoaga has always impressed the San Francisco 49ers with his power and physical presence. Now he’s doing it with performance.

The fourth-year defensive tackle has produced two of the best efforts of his career during exhibition games the past two weeks, holding the point of attack in San Francisco’s new 3-4 defensive scheme and solidifying his role in the line rotation for the upcoming season.

Sopoaga was dominant during Saturday’s preseason loss at Chicago. Even though his primary role at nose tackle is to absorb blockers, Sopoaga had two tackles at the line of scrimmage that went for no gain and another for a loss in the Chicago backfield.

It was indicative of the way Sopoaga has played throughout the summer in place of projected starter Aubrayo Franklin, a free-agent acquisition who suffered a knee injury early in training camp and won’t play until the team’s Sept. 10 season opener against the Arizona Cardinals.

“Isaac is a very athletic, very strong, powerful man, and when he plays that way you’ve got something good,” 49ers coach Mike Nolan said. “He is playing stronger at the line of scrimmage, keeping guys off the linebackers, and every once in a while when you turn him loose, he’s made plays in the backfield. He’s a better player than he was last year and hopefully he continues to stay on track like he has.”

Sopoaga’s career has been marked by inconsistency since he joined the 49ers as a fourth-round draft pick in 2004. Noted for his tremendous strength, Sopoaga set a record at the NFL Combine that year by bench-pressing 225 pounds 42 times. He also is athletic enough to have played safety during his prep career in Pago Pago, American Samoa.

But Sopoaga didn’t begin playing football until his final years of high school, and his technique was raw when he entered the NFL after two standout seasons at the University of Hawaii.

Despite his imposing size — 332 pounds spread over a thick 6-foot-2 frame — Sopoaga has made only sporadic contributions during the past two seasons, when he started only three games. That convinced the 49ers to bring in Franklin during the offseason and make him the starting anchor in the team’s conversion from a standard 4-3 defensive set.

But that seemed to light a fire underneath Sopoaga, who has been carrying out his assignments with discipline while also displaying some play-making ability.

“There’s more improvement in my game because I’m reading the offense now,” Sopoaga said. “I’m at another level of learning the other team’s offense and their game plan on the field. That’s the biggest thing I needed to work on. I’ve been working hard doing the things that need to be done in order to help our football team win.”

After the 49ers signed Franklin to a $6 million deal in March, then selected Florida defensive tackle Joe Cohen in the fourth round of the April draft, some observers believed Sopoaga would be battling this summer just to keep his spot on the San Francisco roster.

But now, as the 49ers prepare for their preseason finale Thursday at San Diego, Sopoaga appears to be battling for a starting job. He leads all San Francisco defensive linemen with nine preseason tackles.

“We all knew what Isaac was capable of doing,” said 14th-year end Bryant Young, the 49ers’ defensive leader. “It was just a matter of him really going out there and making his mind up to do it. You can tell he’s really putting forth the effort and playing with a lot of confidence, and now he’s putting it together on a consistent basis. He’s making the plays that everybody thought he’d be making.”

• Offensive Objectives

Quarterback Alex Smith will see limited action against the Chargers on Thursday, but he enters that game with a clear goal in mind. “It’s a running joke,” Smith said Tuesday. “I’m three points behind (Trent Dilfer) on the quarterback rating. The goal is to get above 106.”

Smith, who has displayed steady progress in his third year with the team, has compiled a passer rating of 103.9 through the first three exhibition games. Dilfer, the 14th-year veteran who serves as Smith’s backup, has a preseason passer rating of 106.6.

Dilfer never played last season when Smith became the first quarterback in the 49ers’ 61-year history to take every offensive snap in a season. Dilfer again will get the bulk of the action behind center Thursday, as Nolan said Smith and the other starters will see no more than one or two series of action.

• Extra Points

TE Billy Bajema (ankle), CB Donald Strickland (chest contusion) and S Vickiel Vaughn (shoulder) did not practice and won’t play against the Chargers. LB Brandon Moore (thigh contusion) and WR Darrell Jackson (hamstring) also didn’t practice and are questionable for Thursday’s game. Young (back) and RB Frank Gore (broken hand) have returned to full practice drills but will be held out against San Diego. Neither has played in the preseason.

• Rookie RT Staley named starter, Harris demoted

Nolan announced late Tuesday that rookie Joe Staley is the new starter at right tackle.

Staley, the No. 28 overall selection in the April draft, unseats veteran Kwame Harris. The two have been battling throughout the summer for the job.

Harris, the team’s 2003 first-round draft pick, has started each of San Francisco’s 32 games the past two seasons at right tackle and has a streak of 37 consecutive starts. Harris started the final five games of the 2004 season at left tackle.

Harris started each of the team’s first three exhibition games this summer, with Staley alternating in with the first unit. Staley will make his starting debut Thursday.

“Joe was more consistent,” Nolan said. “Joe earned the starting position based on his production on the football field both in practice and in the games.”

Staley progressed quickly during the summer and performed well in pass protection, which has always been an area of inconsistency for Harris.