What makes the difference between 8.5 and 9.3 CR on US-SC engines?

[Gunter]

All piston domes on OEM pistons are for CIS.
'78-'79 = 8.5 CR, '80-'83 = 9.3 CR
Stroke is 70.4 mm on all
If the rods, cylinder height, wrist pin location is the same for 3.0 liters, what makes the difference in CR?

[alphie]

I'd like to know this too...Is it the location of the rings from the top of pistons???

[emcon5]

The 2 obvious possibilities are piston dome volume and combustion chamber volume. It doesn't take much of a change to either to change the compression half a point.

Punching numbers in the spreadsheet I did for my engine, 2 CCs less head volume, and 2 CCs more dome volume bump the compression up by about .6

Just because the domes look the same, doesn't mean they CC the same.

Tom

[javadog]

The pistons changed in 1980. For trivia buffs, there are also different part numbers for 911SC heads for '78, '79, '80 and '81-on.

JR

[Gunter]

emcon: You're right, the CIS dome may look the same but may be slightly different in volume.
I think that, if everything was the same, the location of the wrist pin on the piston would determine CR.
True, JR, the mystery deepens: I can see why '78-'79 heads are different (Larger intakes) but why would the '80 be different from the '81-'83?

Cylinder Head (sold individually), 911 (1978-79) engine type 911.04/06
Brand: Genuine Porsche
930-104-029-08-OEM $1441.00
Cylinder Head (sold individually), 911 (1980) engine type 930.07
Brand: Genuine Porsche
930-104-028-03-OEM No Longer Available
Cylinder Head (sold individually), 911 (1981-83) engine type 930.16
Brand: Genuine Porsche
930-104-028-05-OEM $1340.25

Any more thoughts?

[javadog]

No clue what the differences are. The part numbers for the complete head assemblies for '78 and '79 are different. The castings had to change somewhat, as the rest of the parts show to be the same. In fact, the only changes in minor parts shown for the entire series occurred in 1980, when three of the studs changed lengths. So one would have to assume that the castings for the heads differ slightly between '78, '79, 80 and 81-on. The change from '79 to '80 is obvious but the others aren't.

JR

[carmen78sc]

'78 and '79 heads have the provision for the air injection near the exhaust ports. Don't know why '80 would be different than '81-'83.

[andrew15]

It's piston dome size that makes the difference in compression. They all work with CIS but the 9.3:1 and 9.8:1 pistons are noticeably bigger than the 8.5:1 versions
I know I had pics of all 3 - If I find them I'll post.

AM