I think it's got to be as it is, just by changeing plugs and doing valve clearance it revs noticeable better above 3000 rpm and the air filter should help it out even more, it's always had this air filter on since built and on the Dyno plot in 2012 the hp started dropping off at 4400 rpm and torque peaked at 3700 rpm which seems to low to me even for an old V8