I am in the process of reassembling the cylinder head on my 1994 XJ6 and I am horrified by the poor restrictive exhaust port design and the terrible finish quality of the AJ6 cylinder head.
Here are some photos of what I am seeing.


Intake ports on the bottom of the photo

See the ridge that is in the transition area from the valve seat down into the port? The smoothness of this transition where it goes from the back edge of the valve seat and leads down into the port is critical to good flow.
I know it is difficult to see in this photo but this ridge really protrudes into the port and restricts the size of the cross section of the end of the intake port as it leads into the back side of the valve.
The exhaust ports look exactly the same in this same transition area from seat to port. However the actual outlet of the exhaust ports out of the cylinder head are even worse.



Every other four valve head I have ever seen has a large oval shaped opening for the intake and exhaust ports. For instance, see this exhaust port photo of similar Ford four valve head.


Ford exhaust ports

The Jaguar exhaust manifold has nice large oval shapes for each cylinder.


AJ6 cast iron exhaust manifold
You can see when I lay the old exhaust gasket on the cylinder head, how much material could be removed to enlarge the size of the exhaust port.


Exhaust port with gasket in place

So why did Jaguar make these little individual round ports for each valve when the manifold has nice large oval shapes for each cylinder?
This just kills the flow of the exhaust ports.

The flow possibilities for these heads would REALLY benefit from a lot of work on the smoothness of the port to valve seat transition area and opening up these exhaust ports into a more standard oval shape for each cylinder.

 

I have no idea, but would be interested to hear from experts.....

I have seen cases where porting or opening up manifolds, etc actually decreased performance as it turned out the engineers had designed in certain flow characteristics for a given performance target. I have no idea if this is the case here though.

 

This is almost certainly the case. Engineers spend countless hours designing, testing, and perfecting an engine. Just read the article about the AJ-26 engine development recently posted. What may look like a design or manufacturing flaw was actually purposefully done; be it for performance, fuel efficiency, or refinement.