The Pentagon made "an unfair presentation" as it blamed Congress this week for failing to allow a second contractor to build turbine engines for the Army's new M1 tank, the ranking Republican on the House Armed Services Committee said yesterday.

Rep. William L. Dickinson (R-Ala.) said the Army had failed to make the case for contracting with another company besides the Avco Corp. of Stratford, Conn., to make the engines.

Dickinson said Undersecretary of the Army James R. Ambrose presented a confusing picture of the Army's need for a second contractor to lawmakers two days before the House-Senate conference on the fiscal 1984 defense authorization bill.

"I have no parochial interest at all in the engines," he said. "I was just trying to find out what was the right thing to do in the conference about the engines. Ambrose's figures were soft."

The Army's initial projection called for purchasing 10,000 engines for 7,000 tanks. The House conferees concluded that a second source was not needed to produce that number of engines. The Army then increased the figure to 13,000.

Dickenson, who requested the meeting with Ambrose, said the Army official "made a very unconvincing presentation."

"That was the first time he had asked for almost two engines for each tank, and he couldn't justify the new number," Dickinson said.

The pending fiscal 1984 defense authorization bill would forbid the Army from awarding an M1 engine contract to a second company.