The XJ's underfloor structure is derived from the XJ8, but the new car's mechanicals come from the XF. The front and rear suspension are similar to the XF in design but use active air suspension instead of conventional coil springs. The similarities between Jaguar's two sedans don't end there: if you're familiar with the XF, you'll feel right at home the moment you sit in the XJ. That's because the driving position is exactly the same - the proportions of the imaginary triangle formed between the driver's heels, hips, and hands were lifted straight from the XF, which isn't a bad thing.
Thanks to the aluminum, the XJ weighs about the same as the 6.4-inch-shorter XF, so as you can begin to imagine, the cars feel quite similar on the road, too. The ZF steering rack (taken straight from the hot-rod XFR) is unusually quick, and the brakes are so immediately responsive that they verge on grabby - but the combination makes the XJ feel surprisingly agile and light on its feet.
Powertrains, too, are carried over from the XF, so the XJ offers a choice between normally aspirated (385-hp) and supercharged (470-hp) V-8s. You won't find one sitting on the dealer lots, but if you special order your XJ, you can check the Supersport option, which instructs the engine management software to allow 510 hp of thrust. On paper, the difference between the base engine and the Supersport doesn't seem nearly as gargantuan as it feels. Jaguar says the 385-hp car will accelerate to 60 mph in 5.4 seconds; the Supersport in 4.7. That's the biggest 0.7 second you can imagine - the base car is lively, but the Supersport is fast enough to illicit a string of four-letter words from a nun.
Both engines sound magnificent from inside the car. In the base XJ, an induction tube pipes a satisfyingly snorty intake growl into the cabin when the engine is under load. That type of diaphragm-based resonance tube doesn't work with boosted engines, so instead, the supercharged engine uses what basically amounts to a microphone in the intake tract. The signal is then piped to a speaker in the dash, which faithfully recreates the guttural noise you'd hear if you stuck your head into the intake manifold. Minus the permanent hearing loss.