The XJS and XJ12 are magnificent touring cars, covering vast distances in comfort at great ease, the glorious V12 engines effortlessly lopping along, albeit at higher revs than necessary.

The magnificence of these cars is only hampered by the very old and out of date 3 speed Turbo 400 gearbox. For relatively little expense and effort, these gearboxes can be changed for the later model 4 speed T 700R4 gearbox, (or the later model again 4L80E which requires additional electronic control box to operate) which also has a torque converter lockup to further enhance fuel savings as well as the improvement in performance.

Why choose a T700?

The 4 speed T 700R4 is better matched to the power range of the V12, and the 4th gear overdrive provides economic lower highway cruising revs, also providing the opportunity to change the diff final drive ratio for more spirited performance without sacrificing fuel economy.

GM Transmission Ratios 1ST 2ND 3RD 4TH

GM Turbo 400 2.48 1.48 1.00 n/a

GM T 700R4 3.06 1.62 1.0 0.70

Parts required

T 700 gearbox with manual kick down (from VN Commodore V8) (approx $700, overhaul $1,250)

Commodore rear gearbox mount $50 (from wreckers)

Low-vacuum switch - GM #14014519 $210 (from US)

Adapter plate gearbox to engine block $250

Shorten tailshaft $250

For the adapter plate and GM 700R4 gearbox to fit pre HE cars, the original Pre HE large starter motor must be replaced with either a later model HE smaller one or a small geared type.

Starter motor holes are tapped, so holes in the starter motor were cleaned out so that bolts were inserted from front edge into adapter plate, allowing for easier removal if necessary.

Alternative gearbox

An alternative to the T 700R4 is the later model 4L80E gearbox, (approx $1,500, overhaul $1,500)

In addition to the other parts, this gearbox also requires a Compushift electronic controller ($1,700) to control the gear shifts and torque converter lockup.

The standard Commodore rear gearbox bracket and rubber mount can be used, however I designed a different rear gearbox mount and had it fabricated to enable 21/2" exhaust pipes and crossover pipe.

 Showing adapter plate bolted up to block ready for gearbox

The standard existing gearshift and cable is used, only having to change the plastic plate to show 4 gears instead of 3. I have put a lock out plate to prevent selection of 1st gear so cannot select it accidentally when changing down manually.

The standard Speedo drive, same as the Turbo 400, is used, just changing the little plastic gear for correct Speedo reading.

The tailshaft is shortened to suit the longer length of the T 700R4 gearbox and spline connection.

Installing the gearbox in the car is fairly straightforward.

The T 700R4 has a lock up which will work on 3rd & 4th gear which is terrific, it's where the fuel consumption savings are made. It switches on and off via a manual switch (to engage over 80kph) via a vacuum switch, which senses when the accelerator is pressed, to disengage the lockup, and allow to re-engage when cruising, and disengage when braking.

700R4 Torque Converter Lockup—Non Computer Control