At the half-way point of the Sprint Cup regular season, some car owners and star drivers are taking a good look at their under-performance so far in 2011 and deciding some changes are in order.

At Michael Waltrip Racing, the crew chief of the #56 Toyota driven by Martin Truex Jr. has been replaced with immediate effect, after Truex slumped to 21st in the Sprint Cup standings with just three top-ten finishes so far this season.

NASCAR veteran Pat Tryson joined the team at the start of 2010 after switching from Kurt Busch's Penske crew, but Tryson and Truex have never won a race during their partnership and have only one top-five and ten top-tens in 49 races.

Matters came to a head during the April 30 Cup race at Richmond, when friction between driver and pit crew led to Truex yelling "You're all [freaking] fired" over the team radio following a botched final pit stop.

Tryson is replaced by lead engineer Chad Johnson, 31, with Tryson - who has a year still to run on his contract - redeployed elsewhere within the Michael Waltrip organisation. Johnson formerly worked at Gillett Evernham Motorsports and JTG Daugherty Racing, and the drivers with whom he has worked include Kasey Kahne, Marcos Ambrose and Elliott Sadler.

Johnston's first race in his new position will be Sunday's 5-Hour Energy 500 at Pocono Raceway.

"This is a good opportunity for me since this is the direction I have always wanted to go in my NASCAR career," Indiana-born Johnston said in a team release. "My goal is the same goal that this team set at the start of 2011 and that goal is to win races and try to get in the top 10 in points."

"Chad's been a great addition to the NAPA team ever since he joined us last season," Truex said. "He jumped in and really helped with the preparation of our cars. He's shown a lot of leadership in his position and has shown me he really understands a racecar.

Meanwhile, team owner Tony Stewart has also been ringing in the changes with the removal of Bobby Hutchens, Stewart-Haas' director of competition since 2009 and one of the main architects in bringing the team together in the first place, being one of Stewart's first hires from Richard Childress Racing when he decided to set up his own team.

"Bobby played an integral part in building Stewart-Haas Racing into a team that wins races and contends for championships. We're grateful for all his hard work and dedication,|, said Stewart in a statement. "We wish him nothing but the best."

The team said that Hutchens was being released as part of a management shakeup. Matt Borland, a former crew chief who has been the Stewart-Haas technical director, has been promoted to vice president of competition and will cover Hutchens' role on an interim basis.

Stewart ran strongly in his last two races at Charlotte and Kansas only to lose out on fuel mileage. He's currently eighth in the standings while the team's other driver Ryan Newman is tenth in the Cup standings, right on the cusp of making the chase, but the performances of both drivers have been very up and down.