Here’s a few question that I can probably get answers to by spending a day or two analysing parts-lists and Googling but am taking the easy approach and asking other enthusiasts. And experts.
Try not to get too angry with your responses
My understand is that you could get any V12 with A or B grade liners and H (high) or S (standard) compression.
This is on top of the differences in compression ratios between say the carb engine & the HE.
So my questions, in no particular order, are:
can you change your pistons & liners from A to B or vice versa? Let’s say one piston is lost or damaged and your engine has A liners/pistons but you find a set of 10 x B pistons and liners and you can’t source any As. Can you put one B in amongst the As?
you have two engines with very similar engine numbers (ie they are same era). One is H, the other S. What differs on them to make the compression ratios different? Pistons? Head, valves?
was there always an option of H & S throughout V12 production? A glance at JHM1092 (JDHT amalgamated parts list) seems to show early engines only came as S
Kirbert
(Author of the Book, former owner of an '83 XJ-S H.E.)
2
You can swap A and B pistons and liners in and out as long as the piston and the liner grade match.
I personally wouldn’t replace just one or two with the different grade. It would make a very slight difference in displacement, and it would just bug me. However, I would be willing to run with one entire bank grade A and the other entire bank grade B. That’s because each bank of a V12 is a balanced inline 6 anyway.
In the early pre-H.E. engines, the US got low compression (7.5:1 I think) while the ROW got high compression (9:1 or so). In 1981, with the introduction of the Digital P EFI, the engine went to a 10:1 compression, I think both US and ROW. Bywater says it’s the best performance V12. A year or two later the H.E. was introduced, completely different pistons and heads, but once again the US got 11.5:1 compression while the ROW got 12.5:1 compression. After Jaguar got bought out by Ford, the engine was upgraded from 5.3 liters to 6.0, and with that change everybody got 11:1 compression. It was still an H.E. engine, but they stopped calling it that.
You can swap A and B pistons and liners in and out as long as the piston and the liner grade match.
I personally wouldn’t replace just one or two with the different grade. It would make a very slight difference in displacement, and it would just bug me. However, I would be willing to run with one entire bank grade A and the other entire bank grade B. That’s because each bank of a V12 is a balanced inline 6 anyway.
Very helpful and interesting Kirbert1.
I ask the question more out of interest than necessity but it’s good to be aware of options.
I will hopefully soon be revealing my 12 pistons on a stuck engine. I doubt all 12 will be rusted but have been pondering the options available for any that are not re-useable
In the early pre-H.E. engines, the US got low compression (7.5:1 I think) while the ROW got high compression (9:1 or so). In 1981, with the introduction of the Digital P EFI, the engine went to a 10:1 compression, I think both US and ROW. Bywater says it’s the best performance V12. A year or two later the H.E. was introduced, completely different pistons and heads, but once again the US got 11.5:1 compression while the ROW got 12.5:1 compression. After Jaguar got bought out by Ford, the engine was upgraded from 5.3 liters to 6.0, and with that change everybody got 11:1 compression. It was still an H.E. engine, but they stopped calling it that.
Again. Very informative. You saved me a lot of research
So basically over the 3 different eras there was a compression-ratio variance of 5:1 but within each era the difference between S & H was about 1.5:1
