The ACR has a lot more
The ACR has a lot more downforce and less weight. Downforce and weight are far more important when it comes to laptimes than mere power. That said, the backing the viper team had was much better than the backing that Jim Mero had. Chevy went to the nurburgring ONLY to test the ZR1, and Jim had to literally beg Chevy to let him do more 2 flat out laps. Interestingly tho, the segmented times of those laps was 6:57. Further, the Nurburgring organizers let the viper team have even more time than they had arraged in order to set that lap. Also, part of the track that the ZR1 set the 7:04 time was still wet as it was raining on the track.