compression ratio
 

Ok guys, im coming down to final assembly of the 6.0 I am hoping to put in my XJS. It is a 94 6.0 from a XJ12. I am installing a 3.3L Whipple SC in the valley. Crank fired ignition, yada yada. I milled the heads out to lower compression ratio. I have CC'd the heads. I also checked the volume of the cylinder at top dead center. I have purchased copper head gaskets. I have calculated my compression ratio, but would like to throw it out to the forum to see if I am in the ballpark.

My combustion chambers are now at 36cc
The volume of the cylinder ATDC is 18CC
The bore is 3.54
The stroke is 3.1
The compressed head gasket (presumably) is .080
The head gasket bore size is 3.64

I have what I think is the answer, but it seems lower than I anticipated.

Can anybody confirm?

Based on a spreadsheet I have (admittedly for the XK engine), and neglecting the slightly larger bore size of the headgasket, I come up with 8.47. Sounds about right for a supercharged engine, the XJR 6 cylinder is 8.5:1, compared to 10:1 for the normally aspirated engine.

Out of curiosity, why copper headgaskets? Why not the Cometic MLS gaskets?

The copper head gaskets were chosen upon recommendation by a fellow that has built a very high compression NA motor. The Cometic gaskets most likely would be fine, yet if all goes well, I would like to push my luck a little with boost.
At 8.5:1, I will need 4lbs of boost just for the engine to respond as stock. At 10-12lbs of boost, my net final compression ratio would be 14-16:1. In a hope to not blow a head gasket, I purchased the copper ones at a much higher price than the Cometic. But I haven't bolted the heads on yet. I just have CC'd them, and this was my last thing before doing so.

Where did you get them from SS6? What is the advantage of copper gaskets, please?
Greg

Here is a link that explains it much better than I can. I have been trying to lower my compression. The copper head gaskets I purchased are .090 thick. The stock gaskets were about .040 thick. I have also milled out my combustion chambers. Composite gaskets are great, yet it was Chadbourn Bolles that suggested the copper due to sealing properties under pressure. He built a 6.0 with almost 15:1 for the flying mile. That was a few years ago, and that engine still lives on. From everything I have read, the 6.0 is the best of the best. I am doing all I can to see just how much streetable power I can get.

Given the Choices.. Why would one use a copper head gasket? ? SCE Gaskets

You mentioned the compressed gasket at 0.080 from a 0.090 copper gasket. copper gaskets do not compress. You should use 0.090 for your compression calculation.

I did at first, as you are correct. They displace, not compress. There is very little difference at all in .010 in a 3.64 area, so all it can be is a little better. I really hadn't thought I would get any lower than 10:1 based on opinions of other members prior to milling out my combustion chambers. My fears were a bit elevated on another thread when I was told that by removing material from the combustion chamber rather than having custom pistons made, that I has removed all of the squish. I went back to reading all I could, and it seems that you can find opinions across the spectrum about squish and quinch, but supercharging is a whole different ballgame than NA, so I've let my fear go, and am moving forward. I want to do the final assembly so that I can take the engine to a friends shop that does my custom stuff for me. The blower mounting is done. Now I have to figure out what to do with the AC compressor and blower snout. Of course I'm optimistic about this project, and hope it turns out well. It really is just a hope though, because I don't have much to go on as far as known results, and certainly not the unlimited funds to research/test a project like this.

i have used copper head gaskets on BBC and had problems with weeping around the edges, small coolant drops setting overnight!

actually not a big deal ,just top it up when noticed.

but maybe you will have better results!