A friend of mine that ran a Mack dealership told me how to turn the RPM up on a 88 R Model with the 1800 RPM Maxidyne with the 6 speed air shift 5.55 Mack rears. I don't know if it was the right way or not but it worked. now turns 2100 and downshifts at 1200-1300. there is a plug on the backside of the pump ( kinda hard to get to ) that you take out maybe 22 mm , have someone turn the engine over while you look in the hole and you will see a slot with a pin in the center, use a long flat screwdriver push it down and turn clockwise about 5-6 turns to start with then you have to turn the engine over and do the other side of the governor( 1/2 way around make sure you turn both sides the same amount ). I don't know if it is good on the 4 valve heads to be turning 2100 - 2300 or not, this was a farm semi and worked good. someone on here should be able to tell you the long term effects on a 4 valve head.
