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.
magnificence of these cars is only hampered by the very old and out of
date 3 speed Turbo 400 gearbox.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
choose a T700?
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.
Transmission Ratios 1ST 2ND 3RD 4TH
Turbo 400 2.48 1.48 1.00 n/a
T 700R4 3.06 1.62 1.0 0.70
700 gearbox with manual kick down (from VN Commodore V8) (approx $700,
rear gearbox mount $50 (from wreckers)
switch - GM #14014519 $210 (from US)
plate gearbox to engine block $250
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.
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 to the T 700R4 is the later model 4L80E gearbox, (approx
$1,500, overhaul $1,500)
addition to the other parts, this gearbox also requires a Compushift
electronic controller ($1,700) to control the gear shifts and torque
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.
adapter plate bolted up to block ready for gearbox
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.
standard Speedo drive, same as the Turbo 400, is used, just changing the
little plastic gear for correct Speedo reading.
tailshaft is shortened to suit the longer length of the T 700R4 gearbox
and spline connection.
the gearbox in the car is fairly straightforward.
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
700R4 Torque Converter
LockupóNon Computer Control