The little 125C transaxle ( 3 speed with locking torque converter) worked well, but it used a TV cable to control its line pressure, based solely on the amount the throttle was open. This worked great for an engine where the amount of torque being produced is based entirely on throttle position. But with a turbocharged engine, it would shift soft at part throttle, because the engine develops nearly full torque at part throttle!

I installed a Transgo shift kit, which improved it greatly, giving crisp, european-feeling shifts, but at about 25% throttle, it still was overwhelmed. Finally after 42,000 miles the torque converter lockup clutch failed, and second gear started to slip... Time for an upgrade...

I aquired a THM 440-T4 transmission, (heavy duty 4 speed with locking torque converter) from a Cutlass Ciera with a 3.8 SFI V6. The 440 uses a TV cable to determine shifting speed, and a vacuum modulator to determine shifting firmness. This is perfect for a turbocharged engine, because even though the throttle is only open a little, the engine vacuum is gone because of the turbo starting to spool up and produce boost. The modulator senses this, and increases the transmission's line pressure so nothing slips. But the TV cable still tells it to shift at a slow, part-throttle shift speed. The result is very smooth light-throttle cruising shifts, but very crisp, and properly timed part throttle acceleration shifts.

Just as a good measure I installed a Transgo kit in this one as I rebuilt it. These "system correction" kits are engineered to address common failures in the different parts of the transmission as it ages, beyond what a basic overhaul will fix.

The new trans. is quite a bit larger and heavier than the old one, and it took some bracket modifications to get all my custom turbo parts to fit around it. Also the CV driveshafts had to be replaced due to larger output shafts on the bigger transmission.

This upgrade really improved the car's drivability. It's geared lower (higher numerically) for more acceleration, but also has the overdrive 4th gear, so the overall ratio gives a lower cruising RPM.

Old 125C
Final drive ratio (including chain ratio) 3.18:1
Converter stall speed 2795 RPM
Transmission Charge Pump: VD Vane pump
Line pressure control: Throttle-valve cable
Shifting speed control: Throttle-valve Cable

New 440T4
Final drive ratio (including chain ratio) 3.33:1
Overdrive ratio (incl. FD & chain ratios) 2.33:1
Converter stall speed 2100 RPM
First gear in the 440 is lower than in the 125C. I don't know the exact ratios.
Transmission Charge pump: Variable-Displacement vane pump
Line pressure control: Vacuum modulator
Shifting speed control: Throttle-valve cable
Converter Clutch Regulating valve eliminated - locks with line pressure.
Oil cooler drainback check valve eliminated - improved cooler flow.

I've already noticed about a 25% increase in fuel efficiency, going from a malfunctioning 125C (2795 RPM converter with broken lockup!) to the new transmission. That is a Good Thing!

Sorry there will be no pictures of this modification as my camera was stolen last year. However I do plan on adding another sound file (see bottom of the linked page) so you can hear the shifting difference between the old and new transmissions.