DEATH VALLEY, Calif. — As if torture-testing cars in the nation's hottest place wasn't enough, Lee Foster had to worry about spies.

Lots of them.

Foster was leading a team of engineers from South Korean automaker Kia putting disguised cars through the most grueling tests imaginable. All the while, they had to fend off a legion of car paparazzi hell-bent on grabbing shots of vehicles that hadn't yet been shown publicly.

The cat-and-mouse game plays out all summer in Death Valley National Park, where the temperature was the highest in the United States on all but three days last month. The destination's scorching conditions draw automotive teams from around the world to see whether their latest engines, transmissions and air conditioners can take the heat.

"Death Valley is the mecca for car testers," Foster says.

To see firsthand what the cars — and engineers — endure, Kia granted USA Today rare access to a Death Valley testing excursion on a typical 117-degree day in August.

The peek into this usually veiled part of producing cars and trucks showed that the engineers' work is sweaty and tedious. Unrelenting heat takes its toll on team members, who stay at a $51-a-night hotel, catch lunch on the run, and put in 12-hour workdays for four or five days at a stretch. All the while, they dodge the prying cameras of spy photographers, who are paid to get pictures of vehicles still under wraps.

It also showed the amount of effort automakers put into testing new cars under the most rigorous conditions to improve performance and ensure reliability.

From nearly dawn to dusk, Kia's engineers and contractors tinkered with engines, recorded data and gained insight on improving performance. The test data can prove invaluable. Any breakthrough to enhance performance or endurance can be an edge in the hyper-competitive car business. "From our perspective, if we can beat the goal, that's something else (Kia) can advertise," says David Peterson, power-train evaluation engineer.

For the Kia team members, on their third day of the trip, the morning began early at their desert outpost, the Hyundai/Kia Technical Center, a grand name for an unmarked, tan stucco double-wide garage behind a no-trespassing sign in Beatty, Nev. It's a few minutes' drive from the rundown hotel-casino where they spent the night and about a half-hour from the Death Valley park entrance across the state line in California.

Foster called the nine-man team together, reviewed the previous day's test results and outlined what they would work on that day.

The team included four members from South Korea. Also on the team was Tony Vespa, a former top cooling engineer at Chrysler, who was upbeat despite the heat. "This is fun. I never got to turn wrenches at my other company," he says.

This day, Kia's crew was focused on three prototypes: a sporty crossover and two seven-passenger SUVs. The automaker plans to sell the sportster in the United States, but hasn't yet said whether it will offer the SUVs.

The prototypes are costly, running from $200,000 to $500,000.

The engineers were testing a new engine calibration in the SUVs to see if it helped with cooling. And the sports crossover was put through a test meant to simulate steamy stop-and-go city driving to judge the output of its air conditioning.

Whatever the tests, secrecy was an obsession. The vehicles were masked by yards of black vinyl with foam inserts over the hoods, sides and rear to shroud design details. Even door handles were swaddled, lest they be photographed and a competitor get a surreptitious peek.

Every hint of what a new car looks like can prove helpful to the spy photographers, who come from all over the world to trail the teams around Death Valley.

They sell their photos to auto magazines and Web sites, many of which post them within hours. A single picture of a desirable new model can generally fetch $500 to $900, says veteran spy photographer Brenda Priddy.