[Walt Fricke]

What's your deck height? That's the distance the circumference of the piston top is below the cylinder top at TDC. You need to know that (and you can just measure it doing a trial assembly of one cylinder) to calculate CR if you don't have a burette handy (they are more expensive than one might think, at least glass ones are). You should do this measurement with the cylinder base copper gaskets in place, and the cylinder reasonably snugged down, using some cylindrical spacers to nuts on the studs so they can press the cylinder down.

You know your cylinder diameter (84mm nominal).

I believe J&E has given you the volume of its dome, as cut with those pockets: 28.5cc.

You should be able to find the nominal value of the volume of your heads and use that as a reasonable approximation. Our moderator John Cramer once posted this as 68ccs if they are stock '73s.

Then you just do the math. Which you can easily look up - it is in most of the books on engine rebuilding, and not even specific to 911s.

I might try to do it for you (it is really simple), but it needs deck height, which you haven't specified. If you don't know how to measure that, search this forum for it - there are plenty of descriptions and tips out there.

When I have done this with a burette, I used a shortcut. Assemble a piston and cylinder and put at TDC. Put an assembled head on and torque the nuts some. On the engine stand, make the spark plug hole point as straight up as you can make it. To make things easier, I busted the center out of a spark plug, and epoxied a piece of fairly transparent plastic tubing into the plug, and screwed it into the engine. Fill with Marvel Mystery Oil (or some other fluid of your choice) until it starts to rise in the tube. Rotate the engine some to see if it burps some trapped air. Mark the fluid level on the tube. Drain the oil (or just rotate the crank some so you can drain the oil when you take the head off). Block the base of your spark plug, and measure how much fluid it takes to fill to your mark. This you subtract from the total fill volume. That is your combustion volume directly measured.

Then the formula is simple: displacement in ccs plus combustion volume divided by combustion volume = compression ratio.

The textbook method is a bit different. With piston and cylinder installed, lower the piston until the top of the piston is right at the top of the cylinder (or below it) and measure that distance down. Measure the fluid volume needed to fill the cylinder to the top. Calculate the volume down to that point if the cylinder wasn't there, and subtract the measured volume from it. That gives you the volume of the piston dome (the 28.5ccs).

Then put a head upside down with a spark plug in it (and valves, of course) and fill it with fluid up to the mating surface. This is usually done with a clear plastic circle plopped down on the mating surface, and sealed with a little grease before it is pressed down. It has a small hole in its middle where you drip the fluid in, and you can watch for and deal with bubbles if they appear. That gives you your actual cylinder head volume, which should be close to the factory figure, but more accurate.

Of course, if you are anal you will do this for all heads, as they can vary a bit. Not likely to be useful for a street motor, as you aren't going to be adjusting for this by taking metal off of individual heads or pistons to make the volumes exactly equal to the least common denominator, and then having special base gaskets to adjust the CR up to what you want (if needed) by changing the deck heights.

At this point you can use one of the standard formulae for CR which use bore, stroke, deck height, piston dome volume, and cylinder head volume to calculate how much volume there is at BDC, and divide that by how much volume there is left at TDC to give CR.