Look for one of the Ofna boxes with a single Hi Torque motor as opposed to one with the two smaller 550 motors. I just picked up an Ofna chrome top box (part # 10250) last weekend and that thing cranked over a brand new Hyper .21 8 port race engine without even trying. The Chrome top uses a 12V gell cell battery (15.00 at Tower) instead of the stick packs however it is way more powerful than the dual motor boxes.

If cash is an issue, the Chrome Top comes with wiring so you can attach it directly to your cars battery and run it off that. I also have one of the Ofna True Start boxes and it struggles with the 8 port.