Thats probably withnitrox mate

 

definately 900 with nitrous . . and then your engine dies . . . i am not even sure the crank and rear end etc . . .would handle even 600 . . . at least not for long. I would say with 400 the car would scream . . . the stock euro models were all at 300 . . . so you can always retro the air inject, air pump, cats, but you get into fuel mixture fun . . . the o2 sensors etc . . . what year is your car? some quick horses come from better inlets, port inlet tubes, bigger butterflies are available as well, a better breathing exhause would help, larger pipe all the way back with one set of less restrictive cats and or mufflers . . . i think you can get a quick 30 that way. But in reality, these motors are also limited by their ignition systems, direct fire ignition would help, with 12 cylinders to fire from a rotor, jags have very little time to build up a charge, thus the 2 coils on older cars and the solid state amp etc. Direct Fire ignition, larger and less restrictive inlet pipes and butterfly mechanisms, much better breathing exhaust, get rid of the clutch fan and go electric, 8 horses there, lose the air pump, remove smog crap . . . that should take you to 300 for around 800 to 1000 dollars. Look into the racing jags of the late seventies early 80;s they modded those cars pretty nicely. The other thing is the jag weighs a ton . . . a lighter hood, fiber or aluminum, there is so so so so much dampening material on that car. Lose weight equals faster car on what horses you have. You can also very cheaply buy a more agressive adjustable vac actuator on your tranny, has a screw to change the shift points allowing a bit more agressive revs . . .this gains some speed. any car with 6 shocks or 4 shocks and 2 inserts . . . you know ride quality was a little higher up the list than horses. . . .it is well known the inlet tubes on your air boxes are restrictive for a reason, old trick, make the inlet smaller, that way the air expands when it gets in, speeds up just as it is going in and thus deadens the engine noise trying to get out the hole from inside the engine. . . . look into retro fitting an MSD ignition module, or direct fire unit, revs build quicker . . . if you want to drop cash, rebuild bottom of engine, more agressive piston or heads, some years of the XJS had like 11 to 1 or 14 to one compression, but would not pass us emmissions . . .made more power. If you are planning on making more power you WILL need better cooling, aluminum radiator, single pass, better cooling and 20 lbs lighter . . . there is so much you can do . . .look on amazon for books on it . . they exist . . .and get the kirby free book . . .www.jag-lovers.org . . .free download word doc . . .1000 pages . . . see TWR . . .and watch the video below . . build one of these

http://www.youtube.com/watch?v=if2j6yqZg0Q