How did Jaguar offer two different compression rates for the same pre H.E. V12 engine? I assume the block, crankshaft and flat heads are identical since those parts are expensive to modify. So what is different, the rods or the pistons?

How hard would it be to change a 1976 pre H.E. V12 engine with U.S. emissions components and U.S. compression rate to the European compression rate? (BTW: I just realized that I should have bought the 1976 RHD XJ-S that I resisted.)

 

The pistons were different between the 8:1 and 10:1 engines. To change compression, all you would have to do is change the pistons.

That being said, the pre HE and early HE blocks are the same, so you probably could update to the HE pistons and heads. I've never done that, so check with V12 experts. You can probably buy a running HE and drop it in cheaper than buying pistons and rebuilding the engine.

 

So to get from pre H.E. U.S. compression to pre H.E. UK compression I only have to change the pistons? Sounds doable.

 

The compression ratio on the US cars was 7.8:1 & the European cars were 9:1. It is a matter of changing the pistons. You will have to use premium fuel or the car will ping. Another old upgrade was to install the 1974 XJ12 down pipes to eliminate the catalysts. Once this is done advance the timing to as close to 20 degrees before tdc or until the car starts to surge & back it off until it stops. To set the mixture you remove the cap from the pressure sensor & adjust the allen screw. If you want more acceleration find a 3.54 LSD diff.

 

Don't forget the rather rare 10.0:1 engines made from mid 1980 to early 1981! If changing pistons, might as well go for the gusto!

Cheers
DD

 

From 7.8 to 9 would also need the ECU remapped I think????????

The NO Cats front pipes would be sweet.

I have a very DIM light in the grey shiit, that the 10:1 PreHE was a P Digital EFI system, BUT, I stand corrected.

 

A remap is not needed, just adjust the co via the allen screw. It will make a big difference.

 

Wasn't there a 10.5:1 compression ratio engine aswell? I have that number in my mind... Mostly for UK models and then only catfree and IIRC Marelli ignition...

 

Not ringing any bells.

AFAIK all Marelli cars were 11.5:1

Cheers
DD

 

Well, then it was 11.5:1 But that was IIRC only thr catfree Marelli!?

 

According to Bywater the 11.5 was standardized for all cars/markets/emissions when Marelli was introduced....including the V12 Series III sedans which kept the Lucas ignition

cheers
DD

 

Darn Jaguar! All these different rates and figures... Main thing is: it works.

 

As Doug said earlier, there were a very few late PRE HE models (from the bodywork point of view)which had a 10:1 CR and a non-HE but somehow different or revised pre-HE fuel injection and ignition system. These were reputed to be the fastest of all factory V12s. No v12 production cars with the May head were made with a lower CR than the 11:1 AFAIK. Certainly not marketed.
Greg

 

Cheers Greg

 

Man is still learning
Maybe I have problem. I have XJS 1988, Hess&Eisenhardt, Lucas, V12, US spec car. The original engine was completely cooked and blue. The previous owner drove without water until the engine stopped. So I bought European XJS, I took the bare engine (block) and put on it the US spec accessories from the old engine.
Now I have: European (UK) engine but with US accessories (sensors, emission system, catalysts, ECU).
When I read this discussion, I feel I have a little problem. Or do not I have?

 

Should be the best of both worlds - higher UK compression and the O2 sensor feedback injection. Does the car run well?

 

Thanks,
My feeling is that yes.
Only two shortcomings. But maybe it's not.
First. When idling (cca 750 RPM) the right (A) bank have on the exhaust other running than B bank. Make "plop, plop,.." and it seems, that occasionally some injectors will not open. And I also feel this stumbling on my "***". (Sorry, I donīt know correct word in English, maybe "backround"?) But they are cleaned individually (not on the rail) and if I connect the tester, then it is heard that they are opening properly. At over 1200 RPM, everything is fine.
Second. My real fuel consumption js higher, than computer consumption. But I donīt know, how is computer precise. And I make short trip (cca 150 - 200 km) and 20% is in village or in town. My computer say me 16 l/100 km and true consumption is about 18 l /100 km.

 

The computer is everything but accurate, so don't worry about that. Have you checked the spark plugs and HT leads? Some times a worn plug or wire can cause idle issues.

 

Had no idea there were so many different compression ratios over the years. I test drive cars via my butt dyno. Any healthy XJS V12 should:

- light the tires from a standing start
- happily get sideways
- chirp the tires full throttle from 1st to 2nd shift at extra-legal velocities

 

.

hey Greg, does anyone (by a miracle) have a picture of the 1980 10-1 comp. piston, also maybe a pic of the 9-1 Euro pistons ,for comparison!

seems when bought new set of V12 pistons&rings from Welsh 25yrs ago, they said they were Euro hi-compression Pre-HE , thats all they said!

when i put them side by side with my old 7.8-1 ,the bowl was shallower, so i figured thats good,ilike compression!

but makes me think which euro pistons did i get? 9's or 10's. neither ones had any valve reliefs.