The F engines have about 8.7:1 compression ratio. What yours has will vary
slightly depending on the height of the valve seats, etc, not to mention
whether you have the original (thick) headgasket or the thinner headgasket
used on both B and E engines. You can gain a little by using the thin
headgasket but calculations are confusing in determining head overall thickness
because both the depth of the combustion chamber and the thickness of the
gasket were used to determine the compression ratios of the various different
engines. Most of the fellows who really know performance engines say that
the thinner the space between the flat head surface and the top of the piston,
the better the combustion and thus the performance. They call this space
the "squish zone". Without any lab tests to back it up, I can tell you that
I have used the thin headgasket and milled 0.040" off the head with good
results. If you really want to do it right, you need to "cc" the head and
the volume within the gasket. The gasket volume can be calculated by
multiplying the diameter of the gasket's opening (3.570" for a stock gasket)
squared (multiplied by itself), multiplied by pi (3.1416)/4 (.7854) to get the
area and then multiplying it by the compressed thickness of the gasket. It will
simplify calculations somewhat to convert all measurements to centimeters
before you start so the result will then be expressed in cubic centimeters.
A centimeter is 0.3937", an inch is 2.54 cm.

(For those who wonder why I used pi/4, it is because I used the diameter in
the calculations rather than the radius. Diameter squared is 4 times the
radius squared so I use pi/4 to make up for that.)

The volume of the combustion chamber itself is not easily calculated because
of its shape but can be measured directly with the head sitting level with
valves and spark plugs installed and filling each chamber with oil from a
graduated cylinder which you have previously filled to a recorded volume.
Then the difference for each chamber before and after pouring the oil into
it will be the cc of that chamber, to which you add the volume of the gasket
space calculated above. This gives the total volume (cc) for that combustion
chamber. To calculate the compression ratio, you divide the swept volume of
one cylinder (496.5 cc for a standard B20 bore) plus the volume of the
chamber and gasket space (full cylinder and combustion chamber with piston
down) by the volume of the combustion chamber and gasket space only (piston up)
and that will give you the compression ratio.

Let's say that you have a headgasket whose compressed thickness is 0.0315"
which is .08cm when installed in the engine. The holes for the bores are
3.570" = 9.068 cm. Square that and you get 82.23, multiply that by pi/4
(.7854) and you get 64.58 square cm. Multiply that by the thickness (0.08cm)
gives you 5.17 cubic centimeters.
Now suppose you found that your combustion chamber holds 52.8 cc of oil.
(You will have a hard time measuring closer than to the nearest cc, by the
way but this is just for the sake of example). So your total of the combustion
chamber and gasket space is 57.97. We'll round it off to 58cc.

Add that to your swept volume for one cylinder: 496.5 + 58 = 554.5 cc.
divide that by 58cc and you get a dimensionless quantity, the compression
ratio of 9.56 (usually expressed as 9.56:1).

You really need to do this for more than one cylinder to confirm that the
combustion chambers are all indeed the same volume, although unless you have
gotten a lot of valve seat recession, they will probably be as close as you
can measure.

Once you figure out what you have you can make a pretty good estimate of
how much you will have to remove to get the compression ratio you want.
Remember that the combustion chamber is biggest at the surface and the
more you mill off the faster it gets small, so GO EASY. The option of
putting on a thicker gasket to get an overmilled head back down to a
usable compression ratio is NOT a good one because it makes the squish
zone too thick. Therefore it is better to mill less and check your volume
again.

The compression ratio of the B20E in stock condition is 10.5:1 which is
pretty close to the upper limit for street driving near sea level with
commercial gasoline, and you may have to use the best grade you can buy
to keep from getting objectionable pinging/pinking under load. If you
live at higher elevations, 5-6000 feet (Denver, for example) where atmospheric
pressure is lower, you can get by with a higher compression ratio to make up
for the lower atmospheric pressure. In fact 50-60 years ago there used to
be "Denver heads" for the old Ford V8 engines because their power was greatly
reduced at those elevations and higher compression heads were furnished to
help that problem. Naturally these were highly sought after by hot rodders.


--
George Downs, Bartlesville, Oklahoma, Central US