Cly head volume question 911 S

[Lyndon302]

67 S 911 2.0L 81mm bore.
Trying to work out the compression ratio for a 38cc positive some piston.
I'm after the chamber volume .
Deck height
Gasket thickness.
Any experts out there.
Thanks Lyndon

[r lane]

I am not an expert, but have gone through this procedure on numerous occasions. You need a clear plex plate to seal the cyl head, and a burette. I use a heavy chassis grease to make the seal. You check the chambers volume. My testing bench is right next to the safety clean, so I use that fluid, but any number of fluids will work. I then put the top ring in the piston and grease the ring area up and insert the piston into the cyl. I use the same plex plate to seal the top of the cyl, push the piston up to touch the plate, and check its volume. You will need to measure the dome height. You use the dome height to calculate the cyl volume above the ring, subtract the fluid you injected into that area and you can confirm your dome volume from that. You now have cyl head chamber volume and the dome volume. You now need to check the piston to head (deck height) distance and calculate that volume. The rest is math/geometry. Not complicated, just tedious.

[stownsen914]

In Bruce Anderson's book he mentions that combustion chamber volume on 2L heads is usually in the 70-75cc range, depending on which valve sizes the head has. That's a wide range - not useful for a compression ratio calculation. Remind us the valve sizes for a 67S, maybe someone will know more specifically for that head combo.

You have to measure and set the deck height. Most say to aim for .040-.050", though I think the factory spec may say up to .060". Measuring deck height is trickier on the 2L pistons, which if memory serves correctly don't have the flat ring around the perimeter where you would normally measure deck height.

[BURN-BROS]

Using approx. values 38cc dome will generate about 9.1/1 for 67-68S and 69 T and E's