Ford Motor Co. will begin making and selling the smallest American automobile engine in its 77-year history sometime this fall for its 1981-model successor to the Pinto.

The car, code-named the Erika until a few days ago, will have a 1.3 litre base engine with an optional 1.6 litre power plant. Both will be four-cylinder, in-line and transversely mounted in the front wheel drive vehicle.

The smaller version, just under 80 cubic inches of displacement, is about one-third the size of the old Model T, which was built from 1908 to 1927. It is also smaller than almost every imported car engine in the nation today.

Engine size traditionally is measured by the amount of air-fuel all pistons replace when moving from their lowest to highest points in the engine cylinders.

John A. Betti, Ford vice president for powertrain and chassis operations, told the Detroit Auto Writers Group recently that the 1.3 litre engine will be sold in the United States.

By comparison, the smallest engine sold here for the imported Ford Fiesta is a 1.6 litre four-cylinder. But Ford has an engine in the Fiesta, sold only in Europe, that is even smaller than the one coming this fall. It displaces slighly less than one litre.

There are several reasons the smallest Fiesta engine -- and some other small engines Ford makes -- are not sold here. They don't have enough power to meet American emissions limits, which almost always reduce output. And they don't have the capacity to power comfort-assists such as automatic transmissions, air conditioning and power brakes.

The Erika has been formally named the Escort internally at Ford. Its Lincoln-Mercury counterpart will be called the Lynx. The car initially will be sold only as a two-door hatchback here, a version that is sometimes called a three-door car.

The Escort/Lynx will have a 94-inch wheel base and an overall length of 167 inches. The Pinto is about 2 inches longer overall, with a 2.3 litre engine or 140 cubic inch power plant.

Curb weight of the Pinto is about 2,450 pounds; that of the Fiesta, is 1,760 pounds. The Escort/Lynx is expected to fall somewhere in the middle and will probably be priced higher than the Fiesta.

Most imported cars, against which the Escort/Lynx will be aimed, now have slightly larger engines.

In 1980 models, the Honda Civic engine has 82 cubic inches of displacement (CID); the Renault Le Car in 85 CID engine; the Mazda GLC an 86 CID; and the Toyota Tercel an 89 CID. But the Datsun 210 engine, at 75 CID, is 5 cubic inches smaller than the Escort/Lynx.

All are four-cylinder engines; the Tercel and Le Car have front wheel drive, as will the 1981 Escort/Lynx.