A. J. Foyt chased down leader Gordon Johncock, took the lead with 16 laps to go and won an unprecedented fourth Indianapolis 500-mile auto race today.

Foyt and Johncock, winner of the tragedy-ridden 1973 race, waged a 460-mile duel that ended when Johncock radioed his crew, "It (the engine) just blew up. It just blew up." As he coasted down the homestretch to a stop below the first turn, Foyt flashed past and into a 30-second lead he held to the finish.

Foyt's speed was 161.33 miles per hour, just off Mark Donohue's 1972 record of 162.96 m.p.h. Today's race was slowed five times for caution laps at 80 miles per hour, but was without serious mishaps except for Lloyd Ruby's spin into the second-turn wall at the 90-mile mark. The 49-year old driver, making his 18th start here, was uninjured.

Tom Sneva, winner of the pole position, placed second. One lap behind Foyt, was two-time Indianapolis champion Al Unser in third and Wally Dallenbach in fourth. Fifth, six laps behind, was JOhnnie Parsons, sons of the 1950 winner. Completing the top 10 were Tom Bigelow, sixth; Lee Kunzman, seventh; Roger McCluskey, eighth; Steve Krisiloff, ninth, and Jerry Sneva, Tom's younger brother, 10th.

Foyt, auto racing's one-man team, designs and builds his cars and engines, and serves as his own chief mechanic with help from his father. His earlier 500 victories were in 1961, 1964 and 1967. This year he earned an estimated $250,000 of the more than $1 million purse. Official figures will be announced Monday.

The field got off to a remarkably even start after a warm, hazy sky. The 21 rows maintained their 100-foot separation throughout the pace laps before moving into single file withing three racing laps around the 2.5-mile, four-cornered track.

Al Unser in the American Racing Special jumped Sneva, in the Norton Spirit, coming down for the green flag and took the early lead. Johncock, driving one of the three STP entries, was next with Foyt in his Gilmore-Foyt third as they opened a three-second margin over the field.

Defending champion Johnny Rutherford, charging up from his sixth-row starting position, turned into the pits after 30 miles and parked his car, a victim of gear box problems.

Janet Guthrien the first woman to qualify for the race, had been steadily losing ground and made the first of many stops at 45 miles. Her crew said fuel was not feeding to the engine.

Johncock took the lead at 45 miles as many of the cars made their first pit stops for fuel. He gave up the lead to Foyt as pit stops were being made again 20 miles later.

Two caution flags, one for a harmless spin by Eldon Rasmussen and the other for Ruby's crash, then slowed the field. As the green flag came out to signal racing speeds. Johncock passed Foyt at 125 miles. Forty miles later, the two pitied at the same time and Johncock beat Foyt back to the track.

At this time, it appeared Johncock could run faster than Foyt any time he wished. Tom Sneva, Al Unser and Bobby Unser, driving the Cobre Tire entry, were racing with the two leaders but never appeared to have the speed needed for a serious challenge.

Johncock was cruising comfortably ahead of Foyt when, at 230 miles, Foyt rolled slowly into the pit lane. It turned out he had run out of gas on the backstretch.

"It was the only error we made all day," Foyt said later. "I managed to coast around to the pits when everything on the car just stopped."

The mistake gave Johncock a 30-second lead with half the race completed. Only 19 cars were still running with Tom Sneva in third, Al Unser fourth and Johncock's teammate, Wally Dallenbach, fifth. All the caution laps had slowed the average speed to 150 m.p.h. Johncock was hitting 188 steadily when at speed, however.

With cars forced to stop for fuel every 50 or 60 miles, no lead was safe, and at the 300-mile-mark the blackboard used to flash signals to Johncock indicated he was losing two to three seconds a lap to Foyt. When he pitted at 350 miles, Foyt raced by and slashed the leader's margin to seven seconds, then to one second 25 miles later.

It now was a two-car with the leaders just lengths apart. At 395 miles a caution flag came out as two trailing cars eased off the track. Foyt had pitted just before the caution, losing ground because Johncock was at racing speed.

Surprisingly, Johncock did not pit under the caution flag. He ignored a "Pit" signal from his crew and continued to circle the track at 80 m.p.h. until the green falg reappeared and it seem he had lost a chance to fuel without losing time to Foyt.

Two laps after the green was shown, the yellow caution signal came out again. The track crews had not fully cleaned up after the two disabled cars. This time, Johncock pitted immediately for fuel and two right tires. Foyt also came in to add fuel. When they left the pits together, Johncock had a 16-second lead.

Again, Johncock's signal board told of his evaporating margin. By 430 miles Foyt was seven seconds behind. Johncock then stopped for fuel, allowing Foyt to move into first with a 44-second edge.

When Foyt stopped for fuel and right tires at 455 miles, Johncock again took the lead. Two laps later, his car stopped, and with it his chance for victory.

It was then just a matter of Foyt running out the race. This was the 42-year-old Texan's 20th Indianapolis start and his 58th victory in U.S. Auto Club championship races since he began racing in 1953. His record includes wins inn the Le Mans 24-hour road and countless firsts in sprinters and midget racers over dirt ovals.