All you have to do is get the sirio .18 to keep up with .21's? mabey for half a tank or so.. after the .18 look to replace the tranny with a good forward only conversion, replace all the plastic sliders with CVD's, get Al chassis braces and alloy skids (stops chassis flex and striping spur gears). A friend of mine decided to go sirio .18 and he ended up buying all the above which made his maxx reliable again. Yeah it was decent quick but still in a drag race he was no faster than my 247 powred maxx or my friends 247 powered savage. then he submerged his sirio maxx into a creek after a failed jump and soon after his sirio snaped a con rod after the bronze bushing let loose. (fyi a p/s and con rod for a sirio is about $140!) Needless to say he gave up and parted out the maxx and just got a hyper 7 pbs. when it was a stock 2.5 maxx he never had problems breaking things left and right like he did with a .18. A maxx with a 2.5 is at its limits of what the stock driveline can handle. anyhitng more and it begins to have driveline failures that aren't due to wear.