This doesn't sound right, who would manufacture a compressor that ran like this? what happens when you turn it on? it goes full throttle? When you shut it down are they shut down while in full throttle? If they are newer models I would think they are adjustable by the outlet valve. It just sounds like the rental guys want to protect themselves which is fine but a good way to have you go look someplace else, if they are renting out lots of these to larger companies they might not want to bother with renting you one for one day.