WIth my 5.0 MPI I changed from a 21 to 19 and made a big difference. The 21 was at 4700, within the motors range of 4600-5000, and the 19 put me at 5100. All I do is cruise and I still like the performance with the 19 better than the 21. Tbh I think it actually uses less fuel. Top end stayed...