I'm getting back pressure while accelerating when it is floored in second, third, etc. What could cause this?

 

Is this happening in your lower torso area? Bowels, or spinal/muscular, upper or lower back? Back pressure could indicate a more serious medical condition relating to internal organs, so I'd suggest a visit to your physician, pronto! Does this only happen when driving the MINI, or all cars?
Good luck,

 

are you sure its backpressure? thats odd...just a shot in the dark but possibly a piston or chamber issue??