700R4: Preparation & Install

The 700R4 fitted with the B&M Tork Master 2000.

After pulling the old TH350, the first hurdle of the 700R4 swap was the physical length. The 700R4 case is longer, meaning the original crossmember position needed adjustment. I had to slide the crossmember back several inches on the frame rails to align with the new mount location at the tail of the 700R4.

The original driveshaft, which is now roughly 3 inches too long.

Because of that extra transmission length, the driveshaft had to be sent out to a local shop to be shortened and re-balanced. You cannot “eyeball” this; if the shaft is too long, it will bottom out in the transmission and destroy the thrust bearing in your engine.

Shortened and balanced. Final fitment looks solid with proper slip-yoke travel.

Simplified 12v switched wiring for the internal lockup solenoid.

The 12v wiring you see here manages the Torque Converter Lockup. Because this is a 1992 “late model” case, it features an internal normally open 4th gear pressure switch. By providing 12V to the pigtail and jumping the correct internal wires, the transmission will automatically lock the converter once hydraulic pressure confirms it is in 4th gear, and automatically unlock it the moment it downshifts out of 4th.

This setup creates a hands-free highway cruise. I haven’t added a manual toggle switch to the dash yet, but I may if I find the engine lugs too much when the lockup hits during lower speed 4th gear cruising.

Pro-Tip: The 700R4 does not use a vacuum modulator like the TH350. Instead, it uses a TV (Throttle Valve) Cable. This cable controls line pressure based on throttle position. If you don’t adjust this perfectly, you will burn up the clutches in the transmission in less than five miles. It is the most critical adjustment in the entire swap.

This swap brings a huge performance benefit to the Turbo 250 setup. The 700R4 has a much lower first gear (3.06:1) compared to the TH350 (2.52:1). This gives the Inline 6 a significant mechanical advantage off the line, helping the car get moving and building boost much faster. By the time you hit second gear, you’re already in the meat of the power band.

Everything is buttoned up and ready for me to figure out how much fluid the 700r4 came with.