In case anyone is interested, the well known Australian Jaguar expert Mr Norman Lutz made the point that the rear inboard calipers could be bored out and resleeved to take larger pistons of the same diameter as the individual front ones and thus give substantially greater rear brake effort, as explained in these two attached spreadsheets showing the braking effort available with the OEM and with the rear pistons modified.

 

Bigger pistons will remove rear brake bias.
We racers change bias all the time depending on track conditions. But that’s not something you just play with.
If the rears lock up before the fronts the car will tend to spin. If the fronts do too much of the braking the car will tend to go straight ahead ( that’s called understeering in road race parlance or push in stock car parlance)

Very smart engineers set the brake balance for the best braking under normal conditions.

 

Just my lousy two cents on brake upgrades....

Personally I wouldn't recommend putting a lot of effort or expense into brake upgrades unless your particular driving habits require them. How hard will you really be driving the car? Are the stock brakes really being over-taxed?

I've always been happy with stock brakes on my Jags, even when driving fast thru my favorite sections of mountains and curves. I've never felt any heat-related brake fade. Of course there are provisos. My idea of "fast" might be the guy's idea of slow-poke. I get that Plus I try to be mindful of the brakes all along by relying more on engine braking.

On the open road, even at illegal speeds, I've never felt the brakes were sub-par at hauling me down to a stop. The proviso here is that I've never been in a high speed driving situation where I've had to make repeated stops from high speed in rapid succession. I make it a point to NOT drive fast in a road/traffic environment where that is likely to be required. I like fast driving but there's a right time and a right place for everything. If I have to make several hard, rapid succession stops from high speed, well, that's a situation where I shouldn't be driving so fast.

As I write this I'm also thinking of my TR6 experiences driving fast through the curves and mountains. It has a 5-speed. Again I rely a lot on engine braking. I really don't spend too much time on the brakes; usually just a quick stab does the trick for setting up the next corner. If I had a manual transmission XJS I'm sure I'd do the same. After all, isn't that part of the beauty (and fun!) of of having 5 (or more) forward gears?

Cheers
DD

 

Well said
We use stock brakes on our vintage race cars. Tracks like ELKHART LAKE where we get up to our top speed (150+) at 3 points in the race track. followed by tight 90 degree turns ( without banking)
Yes we put fresh brake fluid in before every track session and we duct fresh air to the brakes
On a 4 mile long track with 14 corners it’s 3-4 laps before the brakes start to fade.
When we upgrade the brakes to racing caliper stuff they don’t stop any faster. They just fade less.
We can use modern calipers and rotors but that puts us in a class of much much newer cars.
Cars that start out with hundreds more horsepower than we have. Much wider and taller rims with very low profile tires.