I am experiencing my 3-4 shift earlier than I would prefer at certain throttle angles. Unless I have the throttle matted (>95% tps) I shift up to fourth gear when over 80mph. I run a 4L60E (with TransGo shiftkit) so I have control over line pressure, shift points, tc lockup, etc. and I have my normal kickdowns set where I want them so I don't believe it is an issue of shift points. I feel it may be related to line pressure.

Consequently, my question is: how does line pressure affect xmission function? Will increased line pressure allow the gearbox to hold a gear longer at a given load/throttle angle or does it affect how hard the shifts are?

As your RPM's increase it also increases the line pressure in the transmission. If you allow you line pressure to increase your RPM's will also increase Hence so will your speed. If you allow your transmission to achieve greater line pressure before it shifts you will have firmer shifts and they will also occur at a later time. I'm not familiar with the shift control function of the 4L60E but the line pressure aspect if it is the same through all transmissions. So if you let your line pressure increase you will hold 3rd gear longer. and have a firmer shift into 4th. Hope this helps you out.

Later,
Lars

Is this an aftermarket controller, or a PCM. What are you running?.
3rd gen with a L60?.
Do tell...

lars92RS: thanks, that matches my intuitive grasp of whats going on.

Grumpy: check my comments more closely on the last thread you participated in that I started. (I'm an interloper)