The cut-rate financing war among domestic auto makers pushed car sales in late August to 71 percent above the rate for a year ago, shattering all records for the period, according to company reports yesterday.

The Big Three posted spectacular gains over last year's Aug. 21-31 performance, with Ford Motor Co.'s sales up 79.6 percent, General Motors Corp.'s up 73.6 percent and Chrysler Corp.'s up 76.1 percent.

"They're a boomer. I don't think anybody was predicting this. It's amazing," said Gary Glaser, an automotive industry analyst with First Boston Corp. in New York.

The performance put sales for the month of August at 23.4 percent ahead of a year ago.

Showroom traffic had been falling this summer and a Teamsters Union car haulers strike cut deliveries to dealers for three weeks. But on Aug. 15, GM offered a fixed 7.7 percent interest rate on leftover 1985 models, sparking a cut-rate financing war.

Ford matched that interest rate and threw in rebates. Chrysler fought back with rebates and a 7.5 percent interest rate.

The domestic car makers sold 405,080 cars in the Aug. 21-31 selling period, 71 percent better than the 237,078 of a year ago.

There were 10 official selling days in the period, putting sales per day at 40,080, shattering the old Aug. 21-31 record of 31,811 sales a day in 1978.

The all-time auto sales record was 53,959 set in the third reporting period of September 1972.