c-4 in 89 mustang

If you put a stock C4 behind the 2.3, several things may be playing into this.

Most likely the governor in the trans came from a car with 2.40 to 3.08 gears or similar, so it sees the same rpms at a much slower driving speed. I have this problem with my 82 and the original trans out of the Capri that came with a 2.47 gear, but it does not bother anything. I have not researched governors, but perhaps a governor or the governor weights from a C4 pickup that had 355 to 410 gears would help.

There are a couple mph of adjustment (+/- 4 mph, I think) in shift point available by using an adjustable vacuum modulator, and adjusting it for higher shift points. I can't remember if screwing it in or out raises it. You can do a similar adjustment by changing the length of the rod that the modulator uses to operate the valve, and, or, installing different thickness gaskets between the modulator and case. This works best with push in modulators vice the screw in one. Call TCI or PA and ask.

You may be able to get some improvement by swapping valve bodies, but I would consult a trans vendor again.

I also run a kickdown linkage, primarily because I street drive it a good bit and I wanted it to have all the functions.

Oh, forgot about boost. I would call TCI or PA and ask what they think about setting up a C4 for boost.

You may be fine, because low vacuum, ie, full throttle, is what contributes to full throttle shifts, but I would check.

You may be best with a fully manual valvebody.

The kickdown could most likely be fabbed using just a cable from the throttle body to the kickdown lever. Note that Ford used a rod to push it vice a cable, that would need to pull.

Might be out of ideas here, I have not done anything like a boosted C4.

Try this on the 5.0 forum, since there are probably some guys that have a blown 5.0 with a C4.