I'm gonna walk through the compression ratio math, and explain why along the way.
First some accurate statistics:
351C nominal deck height = 9.215"
351C nominal connecting rod length = 5.78"
351C nominal piston compression height = 1.65"
351C nominal deck clearance = 0.035"
Nominal combustion chamber volume:
1970 D0AE heads = 63cc
1971 D1AE heads = 66cc
1971 D1ZE heads = 75cc
1972 D2ZE heads = 75cc
1973 D3ZE heads = 78cc
1974 D3ZE heads = 78cc
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The formula for static compression ratio is: (Vs + Vc ) ÷ Vc = static compression ratio
Everything is converted to cubic centimeters to perform the calculations (0.061 cubic inches = 1cc)
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Vs = volume of the swept area, i.e. the bore x stroke of one cylinder
351 cubic inches = 43.875 cubic inches per cylinder = 719cc per cylinder
357 cubic inches (0.030" oversize) = 44.625 cubic inches per cylinder = 732cc per cylinder
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Vc = volume of the clearances above the top piston ring at top dead center
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+ combustion chamber volume
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Clearance Volumes (use both):
+ volume of head gasket (1cc per 0.005" of compressed thickness)
+ deck clearance (1cc per 0.005")
(Note: with a flat top piston I like to keep the sum of these clearances,
which I call the "total clearance", in the range of 0.045" to 0.080";
i.e. 9cc to 16cc. This insures good resistance to detonation)
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Piston Top Volume (use one):
- volume of domed piston top
+ valve notch volume of flat top piston (normally 3cc-4cc)
+ volume of dished piston top
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+ clearance between the piston and the cylinder wall (0.2cc or less)
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To calculate the Vc of your Pantera's motor
+ 66cc combustion chamber volume (D1AE/1971 cylinder head = 66cc nominal)
+ 8cc volume of head gasket (0.040" compressed thickness normally)
+ 15cc deck clearance (9.215 - 1.75 - 5.78 - 1.61 = 0.075" nominal)
+ 8.5cc volume of dished piston top
+ 0.2cc cylinder wall clearance
Vc = 98cc total (rounded to the nearest 1 cc)
the compression ratio is therefore
(732 + 98) ÷ 98 = 8.47:1 compression ratio
The variables would include whether or not the block has been decked, whether or not the heads have been milled and the actual volume of the head gasket used. For what its worth, the Cleveland is compatible with 91 octane US/Canadian pump gas (equivalent to 95 octane anywhere else in the world) set at 10.0:1 compression equipped with all but the mildest or hottest of camshafts. Quench heads or open chamber heads, makes no difference. That is in fact a conservative recommendation, it is in no way pushing the limits of detonation if the "total clearance" dimension is less than 0.080". If the alloy heads you are purchasing have high swirl combustion chambers, they should be able to tolerate a half point more than that (10.5:1). Of course, too much compression is even less of a concern at high altitudes.
However, notice that the sum of the head gasket thickness and deck clearance is 0.115". That's too big of a gap between the head and the piston top, which takes the Cleveland combustion chamber out of the range of having excellent resistance to detonation and is headed into the range where it is less resistant to detonation. This much clearance is OK if the comprssion ratio is 8.5:1, BUT it would not be a good choice to leave this clearance this large AND raise the compression ratio to 10.0:1.
If it were me, since you're removing the heads anyway, I would pull the motor to make replacing the heads easier, and replace the pistons with a set of Speed Pro L2379F forged flat tops while the heads are off.
That change alone makes the compression ratio 9.5:1
New Vc of your Pantera's motor
+ 66cc combustion chamber volume (D1AE/1971 cylinder head = 66cc nominal)
+ 8cc volume of head gasket (0.040" compressed thickness normally)
+ 7.6cc deck clearance (9.215 - 1.75 - 5.78 - 1.647 = 0.038" nominal)
+ 4cc valve notch volume of flat top piston
+ 0.2cc cylinder wall clearance
VC = 86cc (rounded to the nearest 1 cc)
(732 + 86) ÷ 86 = 9.5:1
IF you replace the dished pistons with flat tops then ordering the new heads with 66cc chambers will give the motor 9.5:1 compression if that's what you want. Or 61cc chambers would give the motor 10.0:1 compression. Or 57cc chambers would give the motor 10.5:1 compression. The "total clearance" (sum of the head gasket thickness and deck clearance) would be 0.078" which insures good detonation resistance; and all the dimensions are back to being factory/normal, nothing has to be machined or milled to an extreme or wonky amount to achieve a healthy normal compression ratio in the range of 9.5:1 to 10.5:1.
Also FYI, Ford's recommended maximum amount to mill from Cleveland heads is 0.060". Every 0.006" milled from a quench chamber head reduces chamber volume by 1cc. Milling a 1971 D1AE head the maximum amount would reduce the chamber volume by 10cc, i.e. nominal 56cc combustion chambers. Its not uncommon to mill open chamber heads quite a bit, but its usually not necessary for street motors with quench chamber heads (if they have the proper pistons installed
).-G