Bill Elliott won his eighth Grand National race of the season by seven lengths over Neil Bonnett yesterday in the Pocono 500 at Long Pond, Pa.

Elliott has won 10 of the last 14 stock car races run on superspeedways. His victory at the Pocono Raceway tri-oval was accomplished by taking the lead on the 191st lap and turning back challenges by three other drivers during the 200-lap race.

Bonnett's teammate, Darrell Waltrip, scraping fenders with Geoff Bodine over the last half-mile, managed to grab third place by inches at the finish.

There were four lead changes over the last 20 laps. Elliott, averaging 134.008 mph, had a crowd of about 40,000 screaming encouragement at the end as he took the lead for good on the 191st lap. He had led from lap No. 116 to 166, dropping command temporarily on a caution yellow . . .

Alain Prost won the British Grand Prix in Silverstone, England, for his third victory of the Formula 1 auto racing season.

Prost, who finished a lap ahead of the field in a McLaren, earned the 19th victory of his career and moved into second place in the world championship standings with 35 points. Michele Alboreto, second in his Ferrari, retained the lead with 37 . . .

Auto racing's world governing body, FISA, announced plans to impose regulations that would reduce the speed of Formula 1 cars. The announcement came one day after Keke Rosberg of Finland became the fastest Formula 1 driver of all time when he clocked an average 160.925 mph for one lap in taking the pole for the British Grand Prix.

Jean Marie Balestre, FISA president, said the organization wants to reduce the size of engines from 1500cc to 1200cc in 1987 -- a year earlier than planned -- and that other technical regulations would be imposed on qualifying engines and turbo-charged engines . . .

Paul Reiffle of Pittsburgh overcame two late-race spinouts on the No. 5 hairpin turn to win the 40-mile professional sprint event for Formula Fords at the two-mile, 10-turn Summit Point (W.Va.) Raceway. Bob Tupper of Woodbridge defeated four Formula Vee contenders to win his 30-mile sprint road race.