Transmission line pressure
 

I've read from place to place that it isn't a good thing to boost line pressure in the transmissions in our trucks, or at least the 4r100's. How and what does the programmers do to quicken and tighten the shifts in our trucks? Any help on this subject would be greatly appreciated. I've not purchased a programmer yet but will be in the very near future and I'd like to make sure I don't do anything to harm my truck. It's served well for 200K+ miles and I'd like to keep it that way!

This is a very good question. Let's see if I can answer it properly without creating more questions. :D

As of late, most manufacturers (Ford included) are under the misguided impression that trucks should shift like a Cadillac. After all, nobody wants to spill their tasty, hot cup of Starbucks right in their lap, causing all sorts of unpleasantness, not to mention that we can't have the Mrs. smearing her makeup in the mirror. There are a few things that manufacturers do to soften the shifts:

1. Reduce power output during the shifts (Known as Torque Reduction).
2. Lower line pressure during shifts to soften them.
3. Increase the delay between unlocking of one clutch set and locking of the next (Known as Slip Time).

When modifying calibrations to improve the firmness and holding power of the transmission, we sort of have to undo what the manufacturers do.

The first thing (for a performance program) is reducing the torque reduction. This gets rid of the "dive" that occurs between each shift. Now if we eliminate torque reduction with no other considerations, you are going to end up with worn out clutches. This brings us to step 2...

Next we reduce the amount of slip time in between each shift. This reduces the amount of wear on the clutch plates by reducing the time it takes for a shift to complete.

Finally, there are two ways to modify line pressure: Line pressure modification during shift (for each gear), and overall line pressure modification (also for each gear).

For most performance situations, increasing pressure on each shift improves the shifts by causing a quicker, more positive lock of the clutches. This reduces wear. Once the clutches are locked or engaged, the additional pressure backs off and you run the desired main pressure. There usually isn't much need for increasing the overall pressure in these situations as the clutches don't have any problem holding the moderate power gains you'll normally see with a programmer. Of course if you significantly change power output with a supercharger, turbo, nitrous, etc. then you may run into an issue and those needs are addressed in a different fashion.

For towing, not only do you want improved shift pressures, but you'll usually want a slight increase in overall pressure to ensure the clutches don't slip with heavier loads. Since we design towing programs with much of the torque reduction still in place, we don't need to get too aggressive on shift pressures. This helps prevents stressing the overall drivetrain, including the transmission.

It's quite true that increasing transmission pressures can cause problems with the transmission, including excessive tranny temps. However, most of the files that cause this damage are often running anywhere from 15 to 50 PSI higher than stock on both shift and main pressure. Our tuning is much more conservative, and usually up to about a 10 PSI increase will do the job nicely.

In any case, it's a matter of balance and being able to weigh the modifications with the needs of the customer and vehicle.

Hope this helps.