In 2002 in an effort to have an Australian car manufacturer competing in the top category and to have them competitive rather than just making up the numbers, PROCAR allowed Holden to use the 7.0 litre, 427 cui GM LS6 V8 engine (as used successfully at Le Mans in the Chevrolet Corvette C5-R and C6-R's) instead of the 5.7 litre Gen III V8 as used in the Monaro CV8 road cars.

This caused some controversy as it was felt that allowing Holden to use a larger engine than available in the road cars gave them an advantage over their rivals, with some fans feeling that this decision was a big factor in the eventual demise of the series.

During practice for the PROCAR run 2002 Bathurst 24 Hour race, triple Nations Cup champion and seven time Bathurst 1000 winner Jim Richards labelled the Monaro as a "Better V8 Supercar" (even though its fastest time around the Mount Panorama Circuit would be some 7 seconds slower than a V8 Supercar at the time), while NC regular and veteran driver John Bowe said that while he had no problems racing against the car, he believed the 7.0 L Monaro was against the spirit of the rules.

Other cars that were eligible to race in Nations Cup, but rarely (if ever) did included the Mosler MT900R, BMW M3 GTR, Nissan Skyline R34 GT-R and Lotus Esprit.

The regulations differed and not all Nations Cup cars were eligible to race in the GT series, the controversial Holden Monaro 427C the most notable such example.