After Busch set the pace early with a blazing first-lap time that was the best for nearly two hours, Dillon finished with the best time of Friday afternoon.
Driving the No. 3, Dillon turned a lap time of 40.728 seconds, a speed of 176.783 mph. It capped a furious finish to the practice that saw Kyle Larson post the best time with about two minutes to go, before Dillon topped Larson's mark 20 seconds later.
Larson settled for second place, with his best time of 40.960 seconds (175.781) coming on his final of 52 laps.
That sequence of events knocked Elliott Sadler down a peg, although the No. 11 driver still finished third (175.683 mph, 40.983 seconds).
Busch, who sat atop the leaderboard for most of the day, posted the sixth-best time after running 38 laps.
Busch has won five of the past seven Nationwide Series races at Auto Club Speedway; Joey Logano won the other two, including last year's race, but he's not in this year's Nationwide Series event.
The practice lasted for more than two hours in advance of Saturday's Coors Light Pole Qualifying, scheduled for 1:30 p.m. ET.
The Royal Purple 300, to be televised on ESPN, is Saturday at 5 p.m.
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FULL SERIES COVERAGE
- Motor Racing
- Sports & Recreation
- Austin Dillon
- NASCAR Nationwide Series