CR bump at small end of rod
 

well reading through Waynes book...

I was wondering upw much of a CR bump can you get at the small end of the rod...

Assuming one has a 7R case with Mahle RS cylinders and pistons...Stock is 8.5:1

can a full point be achieved using the rebushing method? or what is the typical increase...just curious

MJ

I am also interested in an answer to this question. Also is there a machine shop that has done a number of these that anyone would recommend, or is it such a standard task that any machine shop can handle it?

I remember my boss Dave telling me that he once had a rod lengthened 3mm by offset boring the small-end of the connecting rod. I can only assume that it entailed a lot more than offset boring the bushing, but rather had to have involved offset boring the small-end bore to an oversize as well.

So if you can figure out how much 3mm would affect the CR, then you'll know. It would be the same as changing the deck height 3mm, and it requires knowing the CC volume.

Assuming RS pistons at 8.5 CR, I get a combustion chamber
volume around 60ml. Each milimeter change on the deck height
is worth "about" 1 point in compression. Here's a more exact
look up table, it's not a linear relationship:

0mm 8.5CR
1mm 9.4CR
2mm 10.5CR
3mm 12.0CR

Watch your valve clearance!

F.

The deck height on the 911 motors I've seen is about 1mm so you really can't raise the compression much using this method before you have problems.
-Chris

What Chris means by problems, would be the piston potentially hitting the edge of the cylinder head.

-Wayne

thanks

I was just musings if I do build a RS-ish motor...I know Wayne has been a big advocate of rasing the mild 8.6:1 CR...

I guess I am wondering if it can be done with Mahle RS piston kits instead of using JE's....

MJ