Wondering why your X-C is goin slow? The stock air filter is crap. Too restrictive. Get a motor saver 1/10 scale filter and the truck will scream. It will hit 50 when tuned, I know that for a fact. This is why not the X-Cellerator isn't raced though, its geared very high. Although it's fine with the XTM .18 (which is a pretty nice engine IMO), you probably won't get the low end punch you are looking for while running a .12 engine. Those of us on the XTM forum have found that the Losi 51 tooth kevlar spur gear fits the X-Cellerator with no mods. The stock slipper pads fit, and the gear goes right over the shaft. Only problem is that the stock CB doesn't mesh with it correctly. But, if you run the mesh really tight, the gear will wear in after about 3-6 tanks. That way you have race worthy gearing. Racernine (racernine.com) ran his X-Cellerator in the O'Donnel race at Rev Raceway in California last weekend and placed 2nd in the A-Main against factory sponsored drivers. He was running a non-modified SH .12 engine as well, which has no problem keeping up with the Sirios.