Head/piston dome cc/ compression ratio

[edc153]

Hi gents!
I'm working on my first 912 engine after years of fixin 911s, and 944 turbos. It is a 65, VIN 124 Porsche factory coup.
The 912 has low compression, about 110psi on all cylinders. The leakage is only 5% with new p's&c's. The compression should be around 160 if it is actually 9 to 1 ratio. I calculated my assumed compression ratio using 54cc clearance volume, but it could be as low as 8 to 1. I was told to use an extra 1mm base ring to keep the compression with in the use limits of today's gas. But even at 8 to one, I should be getting about 150psi on the compression test. It is a big bore 86mm kit.
This is all quite puzzling, because if the compression is so low, does it mean I have smaller than normal piston domes? Does anyone know what the clearance volume is supposed to be? Or the Dome cc, and head cc?
This thing is very hard to start. The compression is so low. The carbs are rebuilt, rebuilt heads, new bearings, everything.... restored.
Fresh gas, clean tank, new fuel lines and filters, all new tune parts. Oil pressure is fine when I get it to run.
What am I missing?
Thanks for your help!

[Christopher E]

Sounds strange? Did you actually measure the volumes yourself or just calculate it? Ist the head volume correct - 54cc's seems a bit small - as far as I've learned normal head volumes are around 58-60ccs? And you have to make individual calculations of clearance and dome protrusion as there are a lot of different stuff out there? What does the factory stamp say on your heads? (Or has it been skimmed off?)

There is extensive info on calculations of CR, 356, 912 and super90's in Harry Pellows book "Secrets of The Inner Cirle" - I recommend it! Might be helpful in going over it in your head once more and tracing it down.

[Christopher E]

I read up on it - 912 heads have 60cc's as the "median" but ranges up to 62cc's.

Original factory 912 pistons have a dome volume of 18.75cc's.

I assumed you calculated a clearance of 1 mm on 86mm bore? Have the heads been flycut at any time? Were the addititional shims to lower the compression or to balance the volumes?

There must be a gremlin in there somewhere - and the smallest error in volume makes a BIG difference in CR? Good luck!

[edc153]

Thanks for your reply.

Taking the heads down means lowering the head volume, thereby increasing the CR.
I calculated clearance volume not knowing either head cc, or dome cc, assuming a 9 to 1 ratio. I am guessing the pistons are actually for 8.5 to 1, leaving me with about 7.5 to 1 with the 1mm cylinder spacer. Nothing seemed different about the heads or pistons. They all look the same. I will be measuring the clearance volume today with the heads on using a viscous fluid..
I've got to get the compression pressure up to at least 130psi.

Glad to see there are a few old Porsche left in Sweden. Mine will be up for sale in a week or two.
It is one of several I own.

[Christopher E]

Yep - there are still a couple of nicely kept old cars on the roads here - not that many 912's though... I found my car in the south of Germany a few years ago and she's spent most of her life in the US. Bellingham, WA - actually... Perhaps I should do some research on that some day? :-)

Sent you a PM.

all the best // Chris

[edc153]

The heads have a stamping on them reading "61" probably referring to the actual volume. I cc'd the chamber with piston at tdc, and came up with 60cc +- 1cc. The piston is 2mm negative deck. All of this confirmed my calculations so now my known minimum combustion volume is 53cc +- 1cc.
I believe I fixed my problem when I cut out the 1mm spacer. The low compression was probably due to excessive gas in the cylinder which washed the walls of oil. Now my CR should be about 9 to 1. Will be testing pressure again tomorrow.

Everything just needs to seat in I guess.

Thanks Chris!