BROOKLYN, Mich. — NASCAR is making aerodynamic changes, again, hoping to create more competitive racing.

“The rules package is what is on everyone’s mind,” Brad Keselowski said Friday after a practice session at Michigan International Speedway’s two-mile oval.

Part of the intrigue revolves around the fact that no one — including the drivers, engineers and NASCAR officials — is sure how the changes will shake out during the FireKeepers Casino 400 on Sunday, when Joey Logano will be on the pole.

NASCAR is reducing sideforce to cars by setting the rear toe to zero. It is also shortening the spoiler from 3.5 inches to 2.5 inches, reducing the splitter located under the bumper to 2 inches and resizing the deck fin to match the spoiler. After the aero package makes its debut at Michigan, it is expected to be in place again for next month’s race at Kentucky.

“The car, as it goes through the air, it casts this aerodynamic shadow,” said Gene Stefanyshyn, NASCAR’s vice president of innovation and racing development. “And, we’re trying to make this shadow as small as possible so the other cars around it don’t feel the effects of the shadow.”

At least one driver, Aric Almirola, is confident the results will be good for a series that has occasionally great racing but also clunkers.

“At least with the cars behind, they won’t be affected as bad with the dirty air we talk about all the time,” Almirola said.

With all the changes, the start of qualifying was delayed by 15 minutes Friday. Keselowski’s car was among the final ones cleared to enter the track.