heres something funny on my mrx4 at my local and chaplin ct track i never made runtime once very very fustrating tq pace 90%of the time and run out of fuel everyone tells me my clutch too aggressive i am a very aggressive driver on and off throttle too much im too fast bla bla bla, i tried every combo under the sun pipes, ventrui's, soft clutch, out the box big tires, 7 port 9 port everthing!!! but no luck... late september we got the mrx5 so we had over a month of racing left, since i got the 5 every quil every main every race!!! i was making over 5 min. only thing can figure the 5's drive train is so much more efficent its helping me make run time plus my lap times are faster by 3/10th's, i run flash special tuned, jp eagle 9, kangaroo, throuout the summer all had runtime issues same motors with the 5 no problems now....