Hello all,
My US spec XJ6C, is running with a UK specs engine (S code and HIF 7 carbs), on California fuel.
Engine rebuild is imminent and I wonder what route to go in the likely case that pistons need replacing: Low compression or Standard.
The car is destined to be a pleasant reliable cruiser only.
I wonder what the wisdom is on this matter.
Thank you.
High every time. Higher CR means greater efficiency, slightly more power and less waste heat to coolant. L= 7:1, S=8:1 and H=8.75:1 or 9:1 depending on late or early pistons. H was standard for pretty much all advanced markets except USA but then so were 2” HS8 SUs and the US settled for two 1.75” carbs, and eventually taller gearing to finally knock the ‘sports’ out of sports sedan.
So H then?
I also build vintage racing dirt bikes and the high compression pistons often need 110 octane racing fuel to run without pinging.
I want to make sure the XJ can run properly with just gas from the pump… 
Oh, and it is fitted with HIF7 carbs.
Bikes run higher CRs generally, and dirt riding labors the engine harder than street or circuit. I tuned an XL250 in my second year of uni, using a Yoshi cam and big bore kit around 10:1 and Amal Carb.
Had a CCM 500 4-valve and mostly raced a 500 Rickman Metisse MkIIIA
My 9:1 engine requires 98RON, 8:1 needs 95 , 7:1 can run on 91
(to avoid pre-ignition at normal advance)
these are Australian ratings which are different to USA, there is a formula to convert,
I think 98RON is about 93 (US)
In normal, not hot weather my XJ (euro… so 9:1 right?) won’t ping on 95. I did set the timing to 95 octane fuel as the car came with way too much advance, cooler plugs and all kinds of weird effects.
Can the CR be “fine tuned” with thicker/thinner head gasket?
Yes, some on this list have posted about using “Cometic head gaskets” of varying thicknesses to adjust the compression ratio of their Jaguar XK engines. Seems to me that the usual reason was to continue to use the original cylinder head after it has been skimmed too much and has too high of a compression ratio. The archives should have a lot of info about this if you search for “Cometic”. BTW, there have also been posts about how to measure the compression ratio of your current XK engine setup by pouring a liquid into the cylinders with the pistons at Top Dead Center and carefully measuring the amount of fluid used. I have not had to do this on any of my 4 Jaguars with the XK engine but I did read those posts with interest just in case I encountered problems in the future.
Paul
Update:
new flanged liners, pistons (9:1), rings etc have arrived.
Where did you find high compression pistons? I am interested in your power gain.
-Tony
SNG Barratt. I am not expecting much power gain though , if any at all.