[KFreddy]

Getting back to the original question on why the TH400 seems to shift early. The Th400 is factory calibrated to shift between approx 3000 and 4800 RPMs dependent on the original factory application/ rear end gear.

Three things determine shift points, the transmission vacuum modulator, the transmission governor and load/speed.

The vacuum modulator is located on the passenger side just next to the tranny. It has a set screw on it, this is good for minor shift point/ speed adjustments. High vacuum: soft and early shifts, lower vacuum firm and later shifts. I say minor adjustments because this is all in relation to available vacuum. Low throttle = lots of vacuum, high throttle = little or no vacuum, hence the tranny shifts at higher RPM when vacuum is low. You really can't adjust the vacuum when there is none. Ultimately the vacuum modulator is only good for adjusting part throttle upshifts, 200 to 400 RPMs at best, it will not affect WOT upshifts.

More importantly is the transmission governor, it overrides the vacuum modulator at about 3/4 to WOT (again because at high RPM there is little or no vacuum.) The governor is located at the rear of the main Trans case on passenger side on a Th400. The transmission governor is controlled by the detent switch that is located on the gas peddle of a Th400 equipped car or truck, unfortunately factory detent switches are non-adjustable. Lokar makes adjustable detent switches, also know as kick down kits; however this kit needs to be relocated from being gas pedal controlled to carburetor controlled. A better and more direct solution is that the transmission governor is adjustable. These adjustments can be made because the transmission governor is composed of weights and springs that are geared to the Trans output shaft. There are two outer (primary) and two inner (secondary) weights, and two springs. The inner, secondary, weights mainly affect full throttle 1-2 and 2-3 shifts. The outer, primary, weights mainly affect the part-throttle 1-2-3 shifts. If your shifts occur too soon you need lighter springs and/or smaller weights. If the shifts are too late, you need stiffer springs. B&M and TCI make governor recalibration kits. Transmission governor adjustments however will take a bit of trial and error to dial in.

(As a side note there are usually ink stamp numbers on transmission governors ranging from 11,13,15,17 etc., up to 23, 25 and 27. I do not know the correlation of these #s i.e. line pressure, spring tension, weights, rear gear ratio; but I do know that the higher the # the later automatic up-shifts will occur. A 25 or 27 stamp most likely had a 2.73 or 3.08 factory gear and an 11 or 13 stamp most likely came with a factory 4.56 or 4.11 gear.)

The third factor is load and speed and power, taking the above into account the valve body is internally calibrated to up-shift in a predetermined range based on a combination of engine load and speed. They are also designed to downshift when conditions require additional RPM to maintain vehicle speed or more commonly when we nail the fun pedal.

To gain total control of a Th400, installation of both a governor recalibration kit, a shift kit and some very light tinkering with the transmission vacuum modulator should do the trick.