The cars are different classes. 17.5 1s is nimbler and faster in the turns. 25.5 is like running a big block but so heavy it's point and shoot down the straights. Some places let other 25.5's run, but the national rule is Novak 25.5 ONLY. So if you put a different motor in you may or may not be able to run it. LSRCC is Novak only for their big race coming up for example. And you'll be adding a lot of lead to make weight for usvta.