Dazed and Confused

[Ray Hamilton]

Hello,
I'm at the end stage of a 2.8 with a 3.1 crank and pistons rebuild. Here's my original thread for background: http://60degreev6.com/showthread.php?t=35749. So I'm about to put the heads on when I ran into this:

" GM 3.1L Crankshaft (67mm journals - 1985 blocks and above only) - GM P/N 10078981

Note: This crank is intended for aluminum heads and dished pistons. However, it can be used with 8.5:1 3.1L flat top pistons. IT CAN NOT BE USED WITH 2.8L FLAT TOP OR 3.1L DISHED PISTONS WITH IRON HEADS . The correct combination is 3.1L 8.5:1 flat top pistons and iron heads. I have not found 3.1L 8.9:1 flat top pistons yet. Second, it is internally balanced so it requires the associated neutral balanced flywheel or flexplate."

http://www.geocities.com/citation_x11/history.html

This flies in the face of all the research I've done on this. I was under the conclusion I would end up with a CR around 8.9 to 1. I used the HP562CP iron head pistons with the .105x2.913 recess, with iron heads. Also, you can see the dished pistons with this upgrade being used at http://www.engine-parts.com/GMV6/gm28stroker.html, that sell 3.1 stroker kits ready to eat. So, can someone flesh this out for me? I can live with 8.5 CR although 8.9 would be better. But if the above is correct, the dish is going to drop it below 8.5. (I hope I'm wrong on that).
The sad part about this is I've got enough money in this to almost buy a GM 3.4 Crate Engine.

[Guest]

If you have Desktop Dyno 2000, plug the numbers into it and it will give you the SCR.

Also there is an SCR calculator on this site, though I have not used it.

From what I remember of these combinations you should get 8.9:1 SCR with what you are using if you have a .040" head gasket.

I don't know whey it would say that this crank can't be used withaluminium heads, unless I missread that nad it's talking about the piston and head combo.

[Ray Hamilton]

Thanks for the reply. No, I copied that just as it was written. I couldn't figure out what he was saying. It certainly didn't agree with all the other information out there.

[Guest]

I also wanted to note that there won't be any "8.9:1 flat top pistons", since the 3.1 with flat tops will have a higher SCR than 8.9:1. IIRC somewhere closer to 10:1.

[Ray Hamilton]

If a 3.1 has the same bore as a 2.8, and the cast heads have the same combustion chamber volume, then according to the calculator here, if I figure the compression for a 3.1 with iron heads .040 over that comes out to 8.6. Wouldn't that be the same?