If you don't know by now, the XK8 and XKR power band weaknesses are the exhaust over axle piping, almost cringe flat and the highly restrictive air intake.
This is how it should be fixed


The 4.2 and 4.0 are high revving,high compression Dual Overhead Cam Engines, that like to breath air in and out.

This is just a XK8 non supercharged after those 2 issues are resolved.
According to the last dyno found in the UK,with traction control off, The power jumps from 290 HP @ 6,100 rpm; 290 ft lb , @ 4,250 rpm, to a staggering 325 HP, and 330ft lb torque non supercharged 1998 xk 4.0.

That's about the max your going to get out of just 4.0 liters before losing torque with oversize headers/exhaust and ported heads.

The supercharged version will have higher number of course and the stock 4.0 xk8 loves nitrous.

Cool boys testing the Jaguar XK8.flv - YouTube

 

Is that a custom fit, or does someone make that setup off the shelf?

 

Off the shelf, however,an exhaust shop will charge you way less to ,make a system.Id never pay the prices I see for a off the shelf system for our cars, total rip off.


Jaguar XK8 and XKR Exhaust Systems - The Driven Man - Specialty Auto Performance Parts & Accessories

 

Not unlike the design of the OE system in the XK :



(click on the image to enlarge it)

Graham