It is worth noting that
when the XJ-S was raced competitively (a 4-speed manual was offered
as a limited edition for this purpose), the rear brake rotors were
always replaced with ventilated discs -- whenever it wasn't against
the rules, of course. There are kits available to do this today (see
XK's Unlimited,
GT Jaguar, or
Bob Greene Developments).
Most of us don't use the brakes that hard repeatedly (the stock
brakes work fine once) but if you drive fast down mountain roads,
take note.
Ventilated Rear Brakes
According to Chad Bolles, the rear brake rotors from a Maserati
Quattroporte are ventilated disks and will fit the XJ-S -- and are
even reasonably priced. It is then a relatively simple matter to
fabricate a spacer to go between the halves of the stock calipers,
and to revise the emergency brake setup a bit.
Jeffrey Gram installed ventilated rotors in his car. The
interesting thing was his reason for doing so: to reduce the tendency
for the brakes to cook the final drive seals. He expects that the use
of ventilated disks, even if braking performance is insignificantly
improved, will extend the life of his final drive unit.
Conversion to Ventilated Discs
A couple of items to note regarding this conversion. First, the
lower swingarms may contact the wider discs at the bottom of their
travel. This must be corrected, usually by some minor grinding on the
arms themselves. This is described in the instructions with the kit,
and doesn't seem to cause any problems -- this swingarm has plenty of
strength to spare!
Second, the widening of the caliper to accept the wider disks
results in the bleed screw being moved farther outward. It already
tends to foul the exhaust pipe, and this mod may require considerable
fiddling with the hangers to prevent banging. And, when done, it may
make bleeding the brakes even more difficult than before. All of this
is reportedly NOT mentioned in the instructions with the kit.
A note regarding the ventilated rear disk kit from
GT Jaguar: At least one person who
purchased this kit was far less than impressed. He had the following
complaints:
- The rotors were two-piece, consisting of a cast iron
ventilated disk bolted to an aluminum hub. As a result of the
multiple mating surfaces involved, he ended up with a runout far
beyond tolerances -- on a brand new installation. Much fiddling
was required to get the disks true.
- The bolts holding the disk to the aluminum hub were too long
and fouled the differential case, had to be ground off.
- The hub didn't fit the output shafts on the differential.
Apparently the manufacturer knew this, and in the instructions
suggested cutting on the output shafts to make the hubs fit!
- The kit didn't include a yoke for holding the emergency brake
pads in place with the widened calipers, so the original yokes had
to be cut and a section brazed in. Apparently it is possible to
use the sectioned yoke without brazing it back together, but this
owner didn't care for that idea.
- The general workmanship was unimpressive.
Judging from a picture in their catalog, the
XK's Unlimited kit also
appears to have a two-piece disk.
If you wish to install such rotors, perhaps the following
procedure should be followed:
- Modify the aluminum hubs to fit the differential output shafts
properly.
- Assemble the disks to the hubs using proper length bolts (or
the original bolts properly shortened). Mark the disk and the hub
so that it can always be disassembled and reassembled in the same
position if necessary.
- Assemble the disk assemblies to the differential and measure
the runout. If not within specs, remove the disk assemblies and
have them turned without separating the disk from the hub.
An even better plan might be to perform steps 1 and 2, then
reassemble the whole car. Drive it for a while to get everything
settled in, and then drive it really hard a few times to get the
brakes very hot. Then perform the runout measurements on the rotors,
and if excessive disassemble and have them turned. This way, whatever
distortion occurs within the first few times getting hot will also be
corrected by the turning. It's a lot more work, though.
According to Jeffrey Gram, the ventilated disks offered by
Bob Greene are one-piece
castings.
Cooling Air Flow
The race cars also added an oil cooler for the differential. Taken
together, this all means there is a lot of heat in that area. Perhaps
the performance enthusiast would be well advised to improve the
cooling air flow to this area. A scoop on the bottom of the car
directing air up into this space might help, or perhaps some ducting
is called for.
Brake Lines
Chad Bolles suggests that the brakes on the XJ-S or any other car
can be made to feel firmer by replacing the flexible lines with
steel-braided lines. The stock lines expand somewhat when pressure is
applied, causing a spongy feel. The steel braid won't stretch, so the
pedal feels more rigid.
On to Weight
Reduction/Relocation