Originally Posted by
ModMech
I'm certainly no authority either, BUT it was explained to me in person, the basic fluid dynamics of the situation.
There are two ways to increase Tq capacity, apply pressure, and friction elements. Since we cannot alter the elements from outside, all that's left is line pressure.
The single most critical thing to an ATs life is cooling/lube. Here's where it gets messy....
Increasing line pressure as some "shift kit" sellers and tooners would have you do, actually does three things, NONE are good.
1) Raises the line pressure ALL the time. Unnecessary, hurts mileage, and is hard on the internals.
2) REDUCES cooler flow. If you double the pressure, you cut the flow to 1/4, NOT good.
3) Reduces lube flow. As above, PLUS the cooler return IS your mainshaft lube circuit.
If the line pressure is high enough, long enough you WILL distort the apply pistons, causing leaks and/or blown seals. This is made exponentially worse because your cooler flow is reduced, so it will operate hotter.
The ONLY correct way is to only raise line pressure with 1's & 0's just before, and during the upshift, then to return it to "normal".
As I understand it, the line pressure is "adjusted" in the EEC to match the Tq capacity of the vehicle being modified. It is also tied to "load", to that at high loads it will shift quicker and with more pressure to prevent slipping and the resultant heat. The theory goes that if you are putting around, the shifts should be more firm (quicker) due to slightly closer solenoid timing, and increased line pressure, BUT at high power levels, the line pressure is raised to match the Tq production.
Basically, you get more overall "during shift" line pressure than stock all the time, and better solenoid timing as well. And as the power comes on, you also get enough additional pressure to handle it.