Jag xke stroked to 6.8L

Ok I am new here and didn’t won’t to bother you guys with stupid questions but after a lot of looking, I can’t seem to find the answer I need. I have recently acquired a 1973 Jaguar XKE and the previous owner put a 6.0L v-12 stroked out to 6.8L in the car. It has a 10:1 stroke is all I really know about the setup. I have been able to establish that the engine has oversized sleeves from rob beere racing. I am possibly trying to figure out what bore I have and stroke? I was trying to start with a standard 6.0L and get that to work out and then toy with how they got to 6.8L but I can’t figure out the compression chamber volume or. Plume above the piston or even a concrete CR on the original 6.0L to start with. I have seen where head volume is 30cc, gasket .04mm, stroke is 78.5mm and bore is 90mm and the piston is .07mm down from top of deck on the original 6.0L but still can’t recreate the what I think is the CR for an original 6.0L nevertheless get to the 10:1 Never-mind get to the 10:1 ratio for the 6.8L perhaps I have something off here? Any help would be appreciated.

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They may have reworked the combustion chamber to get 10:1. Stock CR on 6.0 facelift XJS engines for US market was 11:1. Simply doing a bore and stroke increase would raise the CR above 11:1.

The combustion chamber volume had to be increased. Thicker head gasket, piston top profile change, and taking material out of the combustion chamber can be used to accomplish this. These are the only methods I can think of off the top of my head.

The answers to your questions can only come from the engine builder, or documentation from same. Anything else will just be conjecture/guessing.

Am curious to know why you “need” this information.

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This engine was built around 2011 and the builder couldn’t remember all the details. He did say he got to the 10:1 mostly by increasing the stroke and bore. I don’t actually need to know the info, was just curious what the engine setup is. I have no intention of doing anything to the engine further just thought it would be fun to know what the stroke etc of the engine is when talking to others.

Could you measure the stroke by measuring piston travel, a close fitting wooden dowel in a plug hole ?

0.04 mm is a pretty thin head gasket.

There are no stupid questions here so ask away. Its been decades since I have had anything to do with engine building but don’t think bore will impact CR, stroke and pistons yes. That said I believe Steve is correct, to ascertain what you’ve got needs info from the builder, short of taking the engine apart to find out.
pauls

Bore will have a definite impact on CR. It’s math. Any increase in cylinder capacity, due to bore or stroke (or both) without an increase in combustion chamber volume will increase CR. As will an increase in piston displacement into the chamber (piston dome volume) which gives an effective reduction in chamber volume.

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There are tools that can get a very close measurement of cylinder volume without disassembly (sometimes used by tech stewards in the amateur racing world) that can help confirm the engine size. Pretty expensive as I recall, but perhaps can be borrowed from a racer or done by a race prep shop. And you can use various methods to measure stroke through the plug hole. Some basic calculation can then give you a close approximation of bore size. But if the engine is healthy and running well, do you really care?

Steve, I have no idea how much its been bored but for the XK engine (not the V12 AFAIK) max was .030" but .040 was done. Without chamber increase it would seem like CR could have some impact but not be anything like the difference in pistons between 8/1 and 9/1 CR pistons. I have not done nor I even know how to do the calculations but would like to be enlightened.
pauls

Well the way I understand the CR equation it is the total volume above the lowest piston position divided by the total volume above the piston at is highest point. So it seems the displacement / # cylinders + volume in head + volume inside gasket + volume from piston dish would be the total volume above the lowest piston position and then the volume above total piston position would be this same volume less the piston travel? So if that is true then if the bore can be determined and the stroke can be determined then perhaps we can get close. Bob Beere piston, sleeve and camshaft kits appear to have 96 mm as a typical sleeve for the V-12 6.0L that would increase the displacement from the 90mm bore which was the standard bore of the 6.0L or so I have read? The piston volume ie dish or mound on the piston as well as the stroke and perhaps the head volume (30cc) someone said would still be the unknowns? But maybe I am oversimplifying this or missing something.

The only increase the stroke would be with a different crankshaft, surely the previous owner know who that was sourced from, otherwise it’s just a scam.

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This link says something about offset grinding the stock crank for a longer stroke and using Chevy pistons and rods. So a machine shop could increase the stroke on a stock crank. You would not need to buy a new crank.

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It’s possible the builder could have used the low-compression pre-HE heads and pistons and through boring and stroking achieved the quoted compression ratio. I have a 6.0 sitting in the garage for a long time now that I was planning to rebuild with Heron style chambers for my S2 XJ12.

There is a fair amount of information on the V12’s and their various configuration and sizes here: Jaguar V12 engine - Wikipedia

This gives you bore and stroke of the different sizes. To determine displacement, calculate the area of the bore multiply by the stroke, then times 12 (cylinders). I’m not sure why you are discussing compression ratios?

I believe the largest size of engine - 7 litre was used in racing and some were put into Lister XJS’s. It’s possible your car has one of those engines, or the p.o. who did the conversion got a crank pistons etc from one from somewhere else… Wiki describes the bore and stroke dimensions of these Lister engines - I haven’t confirmand them by calculation.

Back in the old hot rodding days folks would have the rod journals welded to to increase diameter than have them ground to a different center, that center increasing the distance from the center of the main journals - hence a longer stroke. (By 2X the increase in the distance.) Jaguar would have used new crankshafts specifically made for purpose. That would necessitate a shorter rod or a higher pin in the piston, to prevent the piston from exceeding deck height. It also increased compression as there was now more air/fuel in the cylinder going into the stock combustion chamber. That could be controlled by a different piston crown or changes to the chamber shape.(for example putting the combustion chamber partially in the crown). The only way for you to determine each accurately is to take the heads off and measure each.

If you have a Lister engine, or Jaguar/Lister made parts to increase a stock engine to 7 litres - well lucky you.

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If it runs good, go for a drive and don’t worry about it. Don’t poke the bear and start opening things up just to see. I’ve never asked a single person what their bore and stroke was. :upside_down_face:

Tferrer, You may be right, it is the Engineer in me that wants to crunch the numbers! I think there is just too much info that is unknown to get the right answer. Don’t know the volume of the head, piston, bore or stroke or even the distance down to piston from the top of cylinder. Everyone throws this CR around like it is the number of all numbers and just wanted to understand how they got it for my car.

Thanks

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I don’t know what you are trying to achieve. But i would start from scratch. Pull the motor completely apart and compare the crank to a stock one. Measure the cylinder bore for certainly. Then go in what ever direction you started. Good luck

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Houlo,

I am sure that is the way to figure it all out, but I am quite happy with the motor and it runs great ! just a huge curiosity for me is really all that is driving me to be able to calculate this.I am an engineer by profession and we love to understand the numbers so that is a curse for us at times.

Thanks

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What are the compression numbers? That will get you in the ballpark.