I will tell u guys y, and this is true also for your regular street cars also. I used to have a cousin that worked at ford when I was younger. ok so they start a production run of lets says 500 motors. now to cut out a motor the machines that cut the blocks and pistons and so on, like everything else those machines that do the cutting have pieces that ware out as the production goes on and after every x amount of cutting the pieces need to be changed weather its blades or water jet nozzles or so on they dull, so lets say the fist 50 motors cut from the new blades or nozzles and so on will have the sharpest edges or cuts or lines so naturally the motors that are cut from the new blades or nozzles or the first motors that are cut right after they change the pieces will be faster. believe it or not. one time my family bought 2 of the same cars and so happens my car was a beast compared to my sisters car so after finding out the production numbers mine had a lower number. so something to think about..