I read somewhere (possibly here from XJR Engineer?) that the optimum compression ratio for the XJR was about 9.3, but for reasons I don't recall it was settled on 8.5 for the production version.

Knowing the difference is in the pistons, would it make any sense to take the 10:1 pistons from the NA engine and mill a recess in the top to drop the compression to 9.3 and/or use a thicker headgasket to arrive at 9.3 compression in the SC engine? I'm thinking that would improve thermal efficiency and boost mid range torque, which is my goal. I'm not concerned with top end horsepower as much as low to mid range torque. I typically operate the car at 3500-4000' elevation so I'm thinking detonation isn't as much of a concern as at sea level. I already have the timing bracket and also have a larger SC pulley ready to fit.

Thoughts? Or is the improvement not worth the effort?

 

Interesting question. Sounds like a lot of work for what is probably minimal gain, but will be interested in opinions from a technical perspective.

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FWIW the supercharged 5.0 in the F-type is 9.5 to 1 compression. The compression ratio in the 3.0 V6 based on the same block minus two cylinders is 10.5 to 1. Most people here think the V6 is pretty highly tuned to get 380 hp out of 3 liters.

The issue may be getting it to produce power with the extra compression rather than having preignition issues, resultant retarded timing etc. as long as you have no carbon build up or other issues I would think it would make a little more power with the extra compression.

 

Perhaps, but it is always a cost & reliability to benefit ratio one would consider, and one's appetite for either

I suspect for most owners, cracking open the block to replace pistons would be a pretty high cost and risk with an engine that has never been taken down since assembled from the factory and is known for its "bulletproof" reliability.

So depending on how much power such a modification would produce, that would probably weigh heavily on most unless their XJR were a pure toy and they have a lot of funds and patience.

But of course, some likely do!

So it would be interesting to see if it can be done and what the benefit would be from a vicarious perspective for most of us

I suspect someone like Andy would be a good resource to offer an opinion on the feasibility and benefit. Hopefully he'll chime in as it certainly is an interesting idea/question.

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