Increasing compression in 2.7?
 

A local 911 engine builder claims to have good experience from removing material from the cylinder heads to increase the compression from 8,5:1 to 9,5:1. He uses shims to compensate for the removed material in the valve train. This way I could still use the low milage cylinder/piston set in the car.

Does anyone here have experience with this procedure? It will be done on a set of spare cylinder heads that I have lying around so that the originals will remain untouched.

The plan is to combine this with 964-profiled cams to hopefully get around 200hp from my otherwise stock 2.7 1977 911S (SSI/Dansk headers and exhaust).

sounds like smoke and mirrors
the only way you can remove material to raise comp ratio is to fly cut the head
That would be a lot of cutting, shims reduce comp ratio the thicker the shims get.
the 964 cam is a poor choice for performance , it is an emission cam .
just some thoughts
Ian

He giveth and he taketh away. My example may not be yours exactly, but I put a 2.7 kit in my 2.4 motor, 8.5 to 1. Drove it that way for a number of years. Went back in, cut the heads .040'', had to cut the pockets a bit, (MFI engine with 4.5 S cams) and raised it to 9.5 to 1. I had to massage the left side cam idler arm a bit and made stainless steel lash caps for the tensioner pistons so that the piston didn;t ride too high out of the tensioner body. That extra bit of compression is worth it. Bob

CIS can't tolerate high lift/duration cams, so 964 may be as aggressive as it can take. I've heard SC cams actually work well in 2.7's, wakes them up on the top end.

Yes, I plan to keep the CIS, so that limits the choice of cam profiles. Dougherty Racing also has some which are supposed to work with the CIS.

The procedure described by @r lane is similar to what my engine builder suggests, I guess.

I wonder how a set of non CIS style 9.5 pistons would do in a CIS motor? I'll be in that same boat soon with my stock 2.7, and have been thinking about SC cams and a compression bump..

Here is some info on CIS performance .
https://forums.pelicanparts.com/porsche-911-technical-forum/1073886-3-0-rebuild-performance-advice.html

Ian

Page 1 post # 19. specs on engine

Page 2 post # 22 Dyno sheet

Works just fine. I am currently running a 2.7 with 964 cams, JE 9.5 pop ups and 1974 CIS, pretty snappy set up.