How to distinguish 4.0 AJ6 or AJ16 - is there such a difference ?

Hi, I've come across a 4.0 XJS - registered in 93. VIN decodes for the engine as being
    3.6 4VD or 4.0 4Y
But its a 4.0 ... so what does the 4Y mean ? Perhaps I have misunderstood something, but is this a 4.0 AJ6 , or is it a 4.0 AJ16 engine ? Were there any 4.0 AJ6 engines ? or if its a 4.0, its an AJ16 by default ?
Any information welcome,
Regards, Stan
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Hello Stan,
The AJ6 came in both 3.6L and 4.0L versions, and although the 4.0L had more torque it really did not make more power than the 3.6L.
Over the years as emission standards became stricker, it became obvious that this engine needed more computing power and gadgetry to reverse what was happening to this engine; it was losing power whilst competition was making more and more power.
Along comes the AJ16
The block casting was lightened yet made more rigid and allowed the designers to place mounting ears for knock sensors right where they would work best. It boasted a new fuel injection system and a throttle valve with the idle control built right in. The head was revamped and received new valves and castings to mount individual ignition coils over the spark plugs. Newer piston design raised the compression to over 10:1 and a higher lift camshaft raised both torque and power. The EGR system was removed and the belt driven air injection pump was replaced with an electric driven pump. The new engine control was upgraded to a microprocessor type that was re-flashable.
Sadly all of these improvements was not enough; Power went from the last year AJ6 rated at 219HP to only 237HP with the AJ16. The writing was on the wall. A new V8 was ready for production and it was cheaper to manufacture, 12" shorter, lighter, and made 290HP. The death of this engine was due to the fact that the AJ6 was designed to use the head off of the V12 (which was a really bad combination), and the spacing of the head bolts prevented using modern head designs. So in short, in order for Jaguar to compete a completely new "6" would have to be designed from the ground upwards. FORDs response: "Here's the V8 we just made, use it."
So no, the 4.0L does not make an AJ16. It was a short-lived modified version of the AJ6 that made a AJ16. Very few parts will interchange between the two.
I hope this answers your question. DieInterim
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