NASCAR Daytona 500: Stenhouse and McDowell lead practice sessions

NASCAR's showpiece race will switch from the old single-car qualifying format to the Formula 1-style knockout system for the first time this year, with Sunday's session broken down into a series of five-minute segments to decide the front row for the 500 and the grids for Thursday's two qualifying heats.

The new system raises the likelihood of an unusual pole winner, as cars will run in large drafting packs in the brief qualifying periods.

Saturday's two practice sessions - the first full-field running of the 2015 season after Friday's practice for Sprint Unlimited drivers only - further hinted at that prospect as one-off laps from slipstreaming groups set the order.

Roush Fenway driver Stenhouse was the quickest man of the day, lapping in 44.413 seconds and 202.643mph in the opening session.

That put him ahead of Cup returnee Sam Hornish Jr in his debut for Richard Petty Motorsports and shock 2011 Daytona winner Trevor Bayne - who is starting his first full Cup season with Roush.

In the slower second session, McDowell led the way for the small Leavine Family team on a 45.153s, 0.002s ahead of six-time champion Jimmie Johnson. Ty Dillon completed the top three.

Newman's drama was the afternoon's main incident. His Richard Childress Chevrolet suffered an engine failure and will require a new motor, which means he must drop to the back of the field for the start of his Duel race on Thursday regardless of where he qualifies.

Martin Truex Jr's Furniture Row Chevrolet parked with smoke from the rear at the same time as Newman, but could be repaired without an engine change.

Saturday's schedule concludes with the non-championship Unlimited race at 8.15pm local time.

Daytona 500 pole qualifying is the only Cup track action on Sunday.