Through six weeks, the San Francisco 49ers have been very good at defending against the pass. They rank second in the NFL in yards allowed through the air at 183.2 per game (Dallas has allowed only 181.6 ypg) and first in yards allowed per attempt (6.0). They’re also ninth in opponents’ QB rating (77.8) and tied for seventh in terms of fewest touchdown passes allowed (7).

However, there’s a number some are worried about where the 49ers aren’t even close to the league’s top 10 — sacks. The 49ers recorded 42 sacks in 2011, tying them for seventh in the NFL. This season, only four teams are sacking the quarterback less often than San Francisco — the 49ers have only nine sacks, which means they’re on pace for only 24. With Aldon Smith now playing full-time and players like Ahmad Brooks and Justin Smith alongside him, those numbers are surprising and worrisome to some.

Not Vic Fangio, however.

“We don’t have the sack numbers, which a lot of people want to sink their teeth into, fantasy players want to sink their teeth into sacks. That’s not the only measure of good pass rush. And right now to this point, I haven’t seen pass rush as being a weakness for us,” the 49ers’ defensive coordinator said during his weekly press conference on Tuesday afternoon.

Fangio is right, sacks aren’t the be-all, end-all when it comes to how one quantifies a good pass rush. There are also quarterback hits and hurries. Just from watching Sunday’s loss to New York, it appeared that Giants quarterback Eli Manning was left relatively untouched — especially compared to the NFC Championship Game when Manning was sacked seven times, hit seven other times, and hurried on a mind-boggling 30 occasions, according to Pro Football Focus.

In Sunday’s game, Manning wasn’t sacked once and was only hurried 12 times. PFF says that Manning was only hit one time, but the 49ers’ defensive coordinator took issue with that figure.

“That’s an incorrect stat,” Fangio said. “I can remember at least three.”

Manning also attempted only 28 passes a couple days ago, whereas he threw the ball 58 times in the NFC Title Game. However, that number was so much lower on Sunday in part because the Giants were so successful in all phases of the game and didn’t hand the ball to the 49ers. As Fangio himself noted today, most turnovers occur during pass plays due to interceptions caused by quarterback pressure, fumbles on sacks or receivers getting stripped.

No pass rush, no turnovers.

49ers pass rush by the (PFF) numbers

According to PFF, the 49ers had the second best pass rush in football last year behind the Philadelphia Eagles. The Eagles’ cumulative score was 104.7, while the 49ers checked in at 82.3, with Dallas coming in third at 46.7. Keep in mind that a score of zero is the median, and 17 NFL teams finished with a negative score in regards to pass rush last year.

It’s safe to say the 49ers were elite in this category in 2011.

This year, the 49ers are one of the 16 teams on the negative side, with a cumulative pass rush score of -5.1. The Seattle Seahawks come into San Francisco on Thursday night with the best pass rush in the league according to PFF, with a score of 44.4 (thanks mostly to Chris Clemons, Jason Jones and rookie Bruce Irvin).

Potential pass rush problems

When looking at those lofty PFF scores from last year, the two guys that jump out immediately are Justin Smith and Aldon Smith. Justin had an astonishing score of 40.9, while Aldon was able to score 29.9 as a situational pass rusher. While last year they combined for 70.0 of the team’s 82.3 points in that area, this year Justin is at -2.5 while Aldon is -0.6.

Perhaps this is just an early season anomaly, or maybe Justin is showing his age while Aldon is having trouble adjusting to his expanded role. The loss of Parys Haralson could be a factor, as both Smiths might be required to protect against the run defense more than last season and unable to “pin his ears back” and rush the passer, as they say.

Bright spots

Teams are attacking the 49ers differently this season.

“At times they’ll be chipping us, keeping extra guys in, or throwing it quicker. I think it’s on their mind. And again, I think that plays into the pass defense, which is what we’re all concerned about here, being pretty good because when they do that, they limit themselves also a little bit,” Fangio said.

How quickly opponents are getting the ball out of their quarterbacks’ hands could also have a lot to do with how few sacks the 49ers have recorded as well as the low yards per attempt number.

Three more silver linings for the red and gold:

1. Ahmad Brooks’ PFF score has risen from -4.4 last year (worst on the team) to 5.2 so far this season (best on the team).

2. In two games against Seattle last year, the 49ers recorded nine sacks, 12 hits and 24 hurries. With the 49ers smarting from Sunday’s loss, the pass rush could come alive on Thursday night.

3. They have Fangio on their side. In the video below he answers several questions about rushing the passer (excuse all the construction noise, which was especially loud today).