I just bought a nr 5kt. I am currently running p5x b/c I have not decided if I am modding the 5kt yet. I don't have time to break it in yet. The p5x is running great so it really does not bother me. R/C car in the july and august issues have reviewed the c6bb turbo and the new jx ninja. the RB review the reported the rb as an on/off mill, that lacks low end torque. The did not see that the new c6 was a bad motor, but would be better suited to more open tracks seen in Europe. The new Ninja seems to have many of the same improvements as the speed, at a lower price. It got a good review. I race on short technical tracks mostly so I really like the low-end torque mills. If I need more high end speed I would rather change the gearing and be faster with a faster time from corner to corner than be the fastest in the straight. Straight speed to me is like the old golfers term drive for show, putt for dough. Time the sections on the track. Count passes on each section. When you fall five feet back in the straight how much real time is that. Is it made up when all the car pile up on the first turn? To me the turbo seems to have a faster power band. The engine has the same general characteristics, but reacts a little faster to commands. The none turbo has a smoother power band. This can be useful in low traction tracts, but you can tune out the quicker power band of the turbo with gearing and clutch engagement. I believe the Turbo just offers a little more flexibility when setting up for a track. So when it comes to engines for racing, pick an engine for your track and driving style. Most all these engines will work well and be competitive.