With a Modiflied motor the XX-4`s triple belt drivetrain is smoother than a single belt layout.
Also the XX`s supension layout works better & is tuffer too !
I personally seen a sponsored driver go the1/2 sec faster with the Xx-4 compared to his XXX.
I also get to see the two cars battle on my track week after week . Watching the two run together makes it easy to see which one goes faster....
If Losi allowed their driver`s to run a XX-4 over the XXX I`m sure we would see a few choose the older car over the new one.