Re: Launch Limit 4,000 rpm ??
One wheel burnout.
With no mechanical limit slip in the open rear differential the power will take the easiest path (whichever wheel spins easier) and leave the car near motionless while burning off one of your tires.
It's usually not a problem if you can keep the weight even across both tires (like a launch on a flat level surface), but you're not always so lucky pulling out on street surfaces.