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In March 2003, the ACO announced changes to the Le Mans Prototype (LMP) classes that first took effect from the 2004 race. The former Le Mans Grand Touring Prototype and Le Mans Prototype 900 (LMP900) categories were merged and renamed Le Mans Prototype 1 (LMP1) and was limited solely to manufacturers. Since the Le Mans Prototype 675 (LMP675) category had no car capable of challenging for the overall victory, the ACO designated it a lower class and renamed it Le Mans Prototype 2 (LMP2). LMP900 and LMP675 cars built in compliance with the ACO technical regulations for the LMP and LMGTP categories could enter until 31 December 2005. Skid blocks were made thicker and the air restrictor size was reduced by five per cent. Teams in LMP1 and LMP2 could choose between an open or a closed cockpit. The maximum weight of LMP2 vehicles was established at and for LMP1 cars. Engine displacement for normally aspirated engines set at , turbocharged engines were limited to and engine displacement for diesel power units was restricted to .