Fiat will double the power train warranty on its 1978 cars as part of an effort to improve its marketing in the United States, the president of Fiat Motors of North American said yesterday.

Dr. Claudio Ferrari, in Washington for one of a series of national dealer meetings, said the Italian auto maker would announce details of the new 2 year, 24,000 mile guarantee on the engine, transmission and drive train and continue the standard 12-month, 12,000 warrantly of the rest of the car.

The extended warranty will be made retroactive to cover all 1978 models, said Ferrari, who is completing a series of meetings with dealers after recently becoming chief executive for Fiat in the U.S.

He said Fiat, which has about 0.5 per cent of the U.S. auto market, has modest goals for boosting its sales next year. The target is at least 66,000 Fiats, up from a projected 63,000 cars this year.

Roughly half the Fiats sold in their country are sports cars, for which the total market is only 200,000 units a year. Boosting the company's already substantial share of that market will be difficult but there are opportunities to get a bigger piece of the much larger small sedan business, added Ferrari.

Fiat will introduce a replacement for its 128 model last next year, a front-wheel-drive car to compete with the Volkswagen rabbit. In the past introduction of new Fiat models has been delayed as much as two years behind their production in Italy. To get the new car - which has not yet been named - onto the American market quicker, the lag will be cut to about six months, he said.

Fiat plans to introduce a restyled version of its 131 series last next year, increasing the size of its four cylinder engine from 1.8 liter to 2 liters.

Fiat will also begin in 1978 to expand its higher priced Lancia line, which after being imported for about two years is "still in an introduction and consolidation phase," Ferrari said.

About 5,000 Lancias will be sold here this year. The cars once starts at $5,000 and the line will be extended upward with introduction in 1979 of a coupe and sedan to complete with Mercedes Benz in the $20,000 price range.

Fiat, a diversified industrial giant that gets only about half its total sales from cars, is increasing its investment in other businesses in the United States, Ferrari said.

This year Fiat boosted its share of the Fiat-Allis heavy equipment manufacturing business in the U.S. from 64 to 77 per cent: Fiat-Allis is a joint venture with Allis Chalmers. The company also recently acquired controlling interest in Hesston Corp., a Kansas manufacturer of farm equipment.

The latest American venture for the company is its Iveco truck division, importer of medium-weight deisel trucks, in the 17,000 to 31,000 gross weight class.

Next May Iveco will begin importing Magirus trucks, which are made in a Dutch plant jointly owned by Fiat and a German firm.