Domestic automakers, despite shortages of 1983 models, increased new car sales by 21.1 percent in the first 10 days of September, selling 132,333 cars versus 109,278 sold in the same period last year.

The relatively strong closeout of the 1983 model year, which ends this month, is being helped by the infusion of 1984 models, many of which have been pumped into the market before formal introduction.

The hot sales pace, which seems even hotter in comparison with last year's dismal performance, prompted predictions of an even better U.S. auto sales year in 1984.

The upcoming model year will be "the first 10-million-plus car year in five years," said General Motors Corp. Chairman Roger B. Smith.

"Strong gains in industrial production and employment show that the economy is expanding rapidly; and extremely low inventory levels indicate we should have further gains in both ouput and employment," Smith said.

Most major domestic automakers already are recalling laid-off workers to meet expanding production needs, although officials at all domestic companies say they do not expect a full recovery and auto employment.

Ford Motor Co., the nation's second largest automaker after GM, turned in the best performance for the period--selling 32,865 new cars, a 28.9 percent increase over 25,491 sold in early September 1982.

American Motors Corp., which has yet to turn a profit this year, largely because of its relatively heavy product investments, nonetheless posted a 25.2 percent sales gain for the period. The company sold 4,200 cars, compared with 3,355 last year.

Chrysler Corp., which needs strong sales to support a new $1 billion labor agreement and cash outlays in excess of $1.5 billion to settle debts and buy back stock, increased its sales by 19.4 percent. The company sold 76,835 cars, compared with 65,883 in the year-ago period.

GM sold 76,835 new cars, a 16.6 percent increase over 65,883 last year's period. Volkswagen of America, still struggling to re-establish itself in the U.S. market, registered the only loss, selling 1,221 cars, a 28 percent drop below 1,696 in the year-ago period.