Porsche AG says it will move assembly of its Boxster and Cayman models out of Valmet Automotive Oy operations in Uusikaupunki, Finland, and into Magna Steyr Fahrzeugtechnik AG & Co. KG’s plant in Graz, Austria.

The switch will come in 2012, after Porsche’s contract with Valmet expires. That pact was extended last year to run through 2011.

Porsche says it is making the move mainly because Magna made a better offer and is in a better position to handle development work for the German sports-car maker.

“Our decision is in no way a vote against Valmet,” Porsche Chief Financial Officer Holger Harter says in a statement. “Rather, it was the high development capacity and competence of our future partner that tipped the balance in favor” of Magna.

Magna will build all variants of the Boxster, which also will remain in production at Porsche’s plant in Zuffenhausen, Germany.

“This underlines our expertise and competencies in complete vehicle engineering and manufacturing,” Magna International Inc. Co-CEO Siegfried Wolf says of the new contract.

Magna already manufactures components for Porsche, mainly convertible top systems and bodywork parts.

The Porsche deal is the fourth new contract for Magna Steyr within the last few months.

The Austrian plant also is slated to build a new cross/utility vehicle for BMW AG’s Mini brand, the Aston Martin Rapide and Peugeot 308 RC Z.

Valmet Automotive has been manufacturing Porsche Boxsters since 1997 and Porsche Caymans since 2005.

The Finnish company celebrated production of its 200,000th Porsche earlier this month, a tally that includes nearly 160,000 Boxsters and 40,000 Caymans.

Porsche production at Valmet’s plant in Uusikaupunki reached its peak in 2006 with 32,393 units. Last year, Valmet built 24,006 Boxsters and Caymans.

Porsche is Valmet’s only partner. Valmet President Ilpo Korhonen says the company has two to three years to sign a new manufacturing contract to prevent a production interruption at the Uusikaupunki plant.

“We can offer world-class production quality,” Korhonen says. “Unfortunately, this time our offer did not meet Porsche’s future demands.”