LONG BEFORE RICHARD PETTY WAS NICKNAMED “THE KING” OF STOCK CAR RACING, his cars were built in a dirt-floor building that might remind some of the barn depicted in “Days of Thunder,” a barn that still stands in Mooresville, N.C.

But those days are long past.

These days a NASCAR Sprint Cup car is crafted in a building that looks more like a laboratory. Its measurements are precise, it’s calibrated with lasers, and multiple sensors detect air flow and many other details.

Engineers are more likely to hook the car up to a computer and analyze a screen readout than they are to open its hood and check its plugs.

One of those engineers is Dr. H.A. Mergen, who knows something about the feel of a race car’s operations on the track, having gotten behind the wheel of a quarter-midget car when he was a Sacramento 3-year-old anxious to be 5 so he could take the car into competition.

Now Mergen works in the research and development department of Hendrick Motorsports.

Hendrick’s four drivers are big names on the track: Jimmie Johnson, the El Cajon driver who has been the Sprint Cup champion six times — five of them in succession; Kasey Kahne, who joined Hendrick in 2012 after driving for Richard Petty Motorsports; Dale Earnhardt Jr., ranked as the most popular of the current drivers and this year’s winner of the Daytona 500.; and Jeff Gordon, who started racing in his home town of Vallejo and who has been with the team the longest.

Mergen, like Gordon, comes from a family with a passion for motorsports. “I didn’t have any choice!” he said. Nor did he mind. He progressed through several racing series, including late models, which he drove at Roseville until that track dropped that series, and he switched to other Northern California tracks.

But he realized he needed “to get serious about college.” He earned his first two degrees at Sacramento State University, then earned his doctorate in mechanical engineering at the University of California-Davis, graduating with a 4.0 average.

His next step in motorsports took him behind the scenes rather than back behind the wheel.

“I was at a conference in Detroit and met the folks at Petty Enterprises,” Mergen said. He learned that a new race team wanted an engineer for its new, young driver — Kerry Earnhardt, the eldest son of late NASCAR racing legend Dale Earnhardt.

“I interviewed in North Carolina and got hired,” he said.

That got his foot in the door, and he’s been in the industry ever since, joining Hendrick Motorsports in 2009, and finding himself matched up with another Earnhardt — Dale Jr. — as the race engineer for the No. 88 Chevrolet.

Mergen originally traveled from race to race with the teams. Now he gets to stay home more often, because he’s moved into research and development.

He’s liked the move, saying that much of the Hendrick operation is large, but the R-and-D department is a small, close-knit group.

His work shows the contrast between the early Petty era and what racing is today. Race cars are covered in sensors for testing in wind tunnels. Mergen knows how air flows around every inch of the cars.

At Sonoma Raceway last week for an intended round of tests that was spoiled by rain, he pointed to a Hendrick car painted not in bright race-day colors, but in marbled gray. Lasers and other sensors were mounted on its wheels in preparation for the wheel force testing.

“Three years ago, that was a cutting-edge thing,” he said. “Now, all the teams have it.”

Geometry and coefficients play a big role in making sure a car is ready for race day, and Wednesday, five Hendrick employees were working on calculations, “going as fast as we can to get better.”

And the calculations change anew when Goodyear introduces a different type of tire with a different compound. That means the team will need new tire maps for the weekend.

“You used to just show up at the track,” he said. “Now, you must be certified. You’re in such a tight box.”

The engineering job Mergen has makes him proud. He often wears his Hendrick-emblemed clothing, as he did early last week when he took a break away from work to return home to Sacramento spend some time with one of his passions — trains — by visiting the California State Railroad Museum.

But he also realizes that few fans realize the work that goes on before the start of the show, the actual race.

“They have no idea what goes on behind it. I’m an engineer. I like technology. That’s what drives me. But that doesn’t add to the product the fans watch. They know Jeff Gordon ran good.” That’s the end product, he said, “to put on a good show.”

What some fans also don’t realize is that some of the technology developed by NASCAR racing teams has been incorporated in the cars they drove to the track to see the race.

Mergen said Rick Hendrick worked with General Motors on improving the materials that are used in automobile manufacturing. Some of the sensors used in racing now are installed in street cars, too.

At one time, race cars were built of aluminum and steel. Now carbon and other compounds are used instead, improving the car’s weight and strength.

On the other hand, sometimes racing comes late to incorporating some of the technology used by daily driver vehicles — fuel injection, for instance. Though fuel injection had been around for years, NASCAR’s founder banned its use in 1957.

It wouldn’t be used in regular Cup competition until 2012. And that was a near disaster — the cars wouldn’t start.

“We struggled with the first fuel injection test,” Mergen said. Engineers had nightmares, anticipating the order for the 43 drivers lined up on pit road to start their engines, with no one able to comply.

“It was a huge accomplishment when they ran the Daytona 500 and the cars started,” Mergen recalled. “You have no idea how significant an accomplishment that was. Twelve months earlier, we couldn’t get them to start.”

Visiting the railroad museum made him appreciate the skill of the engineers of the 1800s, who worked with blacksmith tools and built “an iron beast that would burn wood and boil water and hurtle at the unheard-of speed of 50 mph.

“They figured it out with what they had then.”

Race cars are built to be identical, but they still can operate differently, Mergen said. Some are really good, he said, “and some are evil — although there are a lot more good ones.”

He recalled a particular car belonging to a previous employer.

At Dale Earnhardt Incorporated, where Mergen worked before moving to Hendrick, “We had a Pennzoil DEI car that was the best speedway car. We couldn’t figure it out,” he said.

Ironically, it never won, unlike the team’s No. 8 and No. 15 car. Invariably, the Pennzoil car tested the best, “and we never knew why.”

Besides its own four race teams, Hendrick builds engines for several other teams. Among them are the drivers of Stewart-Haas — Tony Stewart, Danica Patrick, Kurt Busch and Bakersfield native Kevin Harvick — and Chip Ganassi’s Sprint Cup racers Jamie McMurray and the rookie Kyle Larson.

At one time, engine manufacturers were suspected of keeping “the good ones” for themselves and selling inferior versions to other teams. Mergen said that doesn’t happen anymore.

“There used to be a day (when) the ports were hand-ported, everything was done by hand. It was like baking a cake,” he said. But with modern machinery, “we don’t build a bad one. They’re all good ones.

Goals of the designs have changed through the years, Mergen said.

He said a long-time Jeff Gordon fan looking at footage of his early Cup races would notice the nose of the No. 24 Chevrolet rise and fall as he negotiated the track.

“Today, you wouldn’t consider that,” Mergen said. “Hendrick was the first to figure that out.”

Some of that change has been produced by the simulators, which have a two-fold role.

One is the dynamic simulation that Mergen said keeps the car traveling right — and low.

“You don’t want to kill the track with the splitter,” he said. “The goal is to keep the car as low as possible, but not kill the track.”

After all, a week ago drivers were entering Turn 1 at the Fontana track going 200 mph. “If the splitter hits, you’ve got a problem,” Mergen said.

The second role of the simulator is to address key pieces in getting the car right. In particular, he said, is the crossweight, from one back wheel to the diagonally opposite front wheel.

Each driver’s engineer has different specifications, because of the driver’s preferences, and each one wants to get it right. And with a nod to the popularity of stock car racing, Mergen said, “When we miss on Friday, the whole world can tune in and find out!”

The importance of simulation has grown as the opportunities to test and practice have been reduced.

“When I started, we’d take a new car and test it and work out the bugs. Today, the first lap is the first lap. You practice in a race,” he said.

Teams not only once had unlimited practice, they also had unlimited testing, traveling to New Smyrna Beach, Fla., for short track tests, or to Sonoma to experiment on a road course, or to Charlotte, N.C., for a larger oval.

Now teams are limited by the number of vouchers they get — in Hendrick’s case, four for the whole team. That puts even more pressure on the team’s engineers to get it right the first time.

Mergen said all of NASCAR’s race team engineers face the same circumstances. Every driver says the cars are loose coming into the corners, won’t turn in the center, and loose coming off.

“If we could fix that, we would win every time,” he said.

He told a story of two drivers — neither driving for Hendrick Motorsports — who were talking about that very problem, with the younger expressing his frustration. The elder driver said, “They have something to fix that.”

The younger asked what that was.

“Retirement.”

Hendrick Motorsports and other teams have different cars for different tracks, though NASCAR has been striving to have a single car that can run on the half mile, the mid-sized and the super speedways as well as the road courses, such as Sonoma Raceway.

While Hendrick makes cars for flat tracks, short tracks, high banks, intermediate-sized, road races and speedways, Mergen said the differences between those customized cars is less than it used to be.

“My experience is if you get a good car, it’s good anywhere.”

But what if that good car wins the Daytona 500? Rather than getting a chance to compete again, that car is put on display at the Daytona International Speedway gift shop for a year.

That is what happened this year to Hendrick Motorsports’ No. 88 Chevrolet after Dale Earnhardt Jr. crossed the finish line ahead of the pack Feb. 23.

Teams don’t mind the loss, Mergen said.

“Steve Bergie — you saw him in Victory Lane, the one Junior hugged the longest — he’s the one who wanted it there the most,” Mergen said. “He said he can make another race car.”