Thanks, Nik - that helps me visualize it better.

Quote Originally Posted by nik12937 View Post
The only input I can offer is this since I haven't personally done tuning myself, just read up on it. A shift kit is modifying how the accumulators work in your transmission.

In a basic sense, an accumulator will take pressure being applied to a clutch or band fluid circuit and help regulate how the pressure is applied. The fluid presses against the accumulator piston, which in turn presses against the springs and "gives" to an extent depending on the tension of those springs. In a shift kit (I'll use the Transgo kit as an example), you install springs with different tension and spring rate to alter how the accumulator regulates or dampens the fluid pressure. There can also be shims which simply limit how far the accumulator can actually travel. I've probably dumbed it down quite a bit, but that's a short description if you will. A shift kit which deals with an accumulator will work the same general way in any automatic transmission

A tune on the other hand (from what I understand) is affecting what is commanded by the PCM and deals with overall line pressures and such. The transmission will compensate for a shift kit to an extent (really depending on internal wear and achieved shift times), but the overall effect will remain. Two transmissions, one with a shift kit and one with a tune, both achieving the same shift times will still feel different and wear differently due to how the pressure is applied in and out of gears.