I shot this picture at Jaguar’s Castle Bromwich plant last week. On the left is a rear door from the new XJ saloon. On the right is a rear door from an XF saloon.

This little hands-on demonstration was designed to show us just how much weight can be saved through the use of aluminium. The XJ door weighs just 8.9kg, while the XF door weighs 13.9kg. That’s a massive 5kg saving, making the XJ door nearly a third lighter.

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What struck me straight away was the seemingly comparative ease of fitting aluminium doors to the rather weighty XF saloon. I suppose if we have four doors the total weight saving would be 20kgs, and we could deduct another 25kg by using an aluminium bonnet and boot lid.

Of course, the door hinge faces would have to be made of aluminium to prevent rampant galvanic corrosion between the ally door structures and the XF’s steel monocoque, but some bright spark can surely develop a bi-metal design, that bonds an aluminium face onto a steel hinge plate.

Interestingly, if I heard the engineer on the shop floor correctly, the XJ production equipment can produce a full set of ‘closures’ in just 206 seconds. So perhaps this clever robotic machinery has the capacity to make aluminium doors and lids for the XF as well? It might be good idea for the car’s mid-life facelift.

Nobody at Jaguar would give me a clue as to whether the next XF would also be made entirely of (relatively expensive) aluminium, but I was told that large volume production demands that an aluminium car is made from stamped sheet rather than square-section aluminium extrusions, like those used in the Jaguar XK and Lotus Elise. And Jaguar has clearly invested in a serious aluminium stamping operation.

Could Jaguar base the next XF off the XJ’s floor structure? Why not? BMW’s 5- and 7-series cars have long been basically the same vehicle. Such a move would raise the prospect of this Castle Bromwich production line building the same basic saloon car at a rate of, say, 55,000 units per year. That has to be a sound recipe for profitability.

Incidentally, the hunks of green-painted metal in the background are the press tools used to stamp out the panels that make up the inner structure and exterior skin. I asked how often the tools are taken in for inspection and repair.

"Once a year or every 25,000 operations," said the Jaguar engineer.

Does that mean Jaguar wants to build 25,000 XJs per year?  If it does, it would be huge jump over the sales of the old XJ. But they deserve to do so.

The XJ’s body engineering is a work of art.