ddie Murray hit a grand-slam home run and his teammates chipped in with one-run, two-run and three-run homers as the Orioles routed the Kansas City Royals, 13-5, in a brisk American League season opener at Memorial Stadium today.

A sellout crowd of 52,034, the largest regular-season throng in Orioles history, ignored pessimistic forecasts of rain and snow and was treated instead to sunshine and a most optimistic show of power. The four homers and 13 runs were opening-day Oriole records.

It was a day in which every question mark in the Oriole lineup was eradicated, at least for the moment, and after which Manager Earl Weaver, who intends to retire after the season, said, "If they got four homers a day for the next 161 games, I might decide to change my mind."

Every Oriole batter scored, and the only batter without a hit was the perennial heart of the lineup, Ken Singleton, who managed to get on base four of five times, anyway, on three walks and an error.

Starting pitcher Dennis Martinez was the only Oriole who left the game on less than a lustrous note, and even he could be said to have done his job. Martinez gave up four runs and six hits before leaving with none out in the fifth and the Orioles ahead, 7-4.

Martinez' wife Luz had given birth to their third child Sunday, and it made sense for him to get home early, particularly since Sammy Stewart lay waiting in the bullpen. Stewart pitched the final five innings to gain the victory, giving up one run and two hits.

Said Stewart afterwards: "What a day. What a day!" He said he never felt quite right on the mound. "I didn't have all that much. I couldn't get the curve over for a strike. But with our hitters, you don't need to."

Murray's grand slam put the Orioles up, 6-1, in the third inning. His feat upstaged Cal Ripken's first major league homer, a two-run shot in the second that gave the Orioles a 2-1 lead.

Both home runs landed within an eyelash of the Orioles' bullpen in left, as did the two Oriole homers (by Gary Roenicke and Dan Ford) that followed, both Royals' homers and a ground-rule double. A moderate breeze blew that way all afternoon, and with balls flying hither and yon, Oriole reliever Tippy Martinez said some of his mates were reluctant to warm up.

Weaver said if the breeze had been blowing in, as it was with a vengeance on Sunday, none of the homers might have gone over the fence, and "This one might've been 3-2, or even 3-1."

As it turned out, the miseries of the preceding day, when both teams suffered through inhospitable workouts, fell away to a fine baseball day. "I was dreading coming out here after yesterday," said Murray. "But it was really nice, a beautiful playing day." That was an overstatement, he decided. "Well, not beautiful. I'll take 80 degrees. But it was nice."

Left fielder Roenicke had four hits, including his home run, which followed Murray's. Newcomer Ford, who made two strong throws from the outfield, had the fourth homer, a three-run job in the seventh, and two singles.

Roenicke knocked both starter Dennis Leonard and reliever Paul Splittorff out of the game, loser Leonard in the third inning and Splittorff in the seventh. Roenicke said it was the "best opening day I've ever had in my life," and said it was particularly important to him, since it followed the best spring he'd ever had, "and I wanted it to carry over."

Roenicke is an important part of the Orioles' plans. He had 25 home runs in 1979, the pennant year, but fell to 10 in 1980 after he broke his wrist. He had only three last year after he underwent elbow surgery.

The Royals' homers were a three-run shot by second baseman Frank White and a solo by third baseman George Brett, who also doubled.

The closest the Royals came to making a game of it was at 7-5, after Brett's homer in the seventh, but the Orioles sent 10 hitters to bat in their half, scoring the final six runs and putting the game out of reach.

Stewart said at that point he had to concentrate and remind himself to stay aggressive, "because you start to lay back with a big lead." Evidently it worked. He didn't allow another hit.

Stewart added that the offensive output was a little grander than he'd envisioned. "Before the game me and (Mike) Flanagan tried to guess how many runs we'd score. He figured seven. I guessed 10."