The Detroit Pistons made up a 13-point deficit in the last quarter and defeated the Washington Bullets, 109-105, last night in an exhibition game at Madison Square Garden.

The Bullets playes without Elvin Hayes, who was sidelined with a groin pull. They closed out their exhibition season with two victories and six defeats. Detroit was led by Kevin Porter, who scored 20 points and handed off for seven assists, in winning its fourth game in six starts.

Mitch Kupchak started in Hayes' forward spot, but Coach Dick Motta also readied him for the Friday National Basketball Association season opener by playing him at center an using Wes Unseld at forward. Kupchak responded with 25 points, hitting 10 of his 18 shots from the field.

"We did pretty well with Mitch at center and Wes at forward," said Motta. "Mitch is going to be our first front-line guy off the bench.

"This was a good workout for us. We gave our young kids a chance to play. It was also the first time I used out timeouts. The only thing wrong was that we haven't put in our out-of-bounds plays yet."

The Washington offense was at its best in the third period when it produced 35 points, six fewer than in the entire first half. With Greg Ballard scoring eight of his 14 points, the Bullets hit on 15 of 23 shots.

Trailing, 64-62, after a Porter jumper with 1/2 minutes remaining, the Bullets outscored the Pistons, 17-6, and closed out the period ahead, 79-70. Two free throws by Charlie Johnson and a short jumper by Unseld increased the lead to 13 points before Detroit started a comeback of its own that tied the score at 86 with 6 1/2 minutes remaining.