Well, things are slow here in the engine tech forum, so it's a good time for a little show & tell. This will be aimed at those who have never had a 351C engine apart, and have wondered what the terms open chamber & closed chamber actually referred to in regards to the cylinder heads.
2 aspects that differentiate 351C cylinder heads are the design of the combustion chambers and the size of the ports. At the bottom of this post is a picture of the two combustion chamber designs.
On the left is an open chamber cylinder head, on the right is a closed chamber cylinder head. The open chamber head has a round, bowl shaped combustion chamber that is approximately the same diameter as the cylinder in the engine block. The closed chamber head has a smaller combustion chamber that has been shrunk to tightly enclose the two valves and the spark plug, creating flat surfaces above & below the combustion chamber. These flat surfaces are known as the "squish" areas.
On a 351 cubic inch motor, replacing an open chamber 4V head with a closed chamber 4V head will raise the compression ratio about 0.8:1, eight tenths of a point. If you have a '73 to '74 cobra jet motor with a dismal 8.0:1 compression ratio, replacing the open chamber heads with closed chamber heads will yield a compression ratio of approximately 8.8:1. Increasing the compression ratio is one of the advantages of the closed chamber heads.
It has been long ago determined that reducing the clearance between piston dome and the cylinder head “squish” surfaces at top dead center to 0.045” +/- 0.010” on a street motor improves the thermal efficiency of the motor (it produces more torque) and increases the motors resistance to pre-ignition (aka knock or pinging). It is common for racing engines to set this clearance even smaller. As delivered by Ford, this clearance is approximately 0.070 which is too large to realize the benefits of “squish”. Squish aids the engine during the compression and exhaust cycles. As the piston stroke reaches top dead center during the compression cycle the squish area squeezes the air and fuel into the more compact combustion chamber so as to be more easily ignited by the spark plug while also increasing the turbulent motion of the air and fuel so that upon ignition the air and fuel burn more completely and energy extracted from the fuel air mixture is maximized. During the exhaust cycle, the squish area and smaller combustion chamber improves exhaust gas scavenging by forcing the exhaust gases nearer the open exhaust valve as the piston stroke reaches top dead center.
With a 0.070” gap between the piston and cylinder head at top dead center, the fuel/air/exhaust gases are not completely squeezed into the combustion chamber, the fuel and air left behind during the compression cycle are actually shielded from the flame front during ignition and remain partially unburned, which reduces the energy extracted from the fuel air mixture. The unburned fuel increases hydrocarbon emissions, which is why Ford switched the 4V Cleveland motor from closed chamber heads to open chamber heads in 1972. In a similar manner, the exhaust gases left behind in the squish area during the exhaust cycle are shielded from the exhaust valve and exhaust scavenging is reduced. Exhaust gases left behind in the cylinder reduce the cylinder area available for fresh fuel & air that are drawn into the cylinder during the intake cycle and dilute the fresh air that is drawn in. So unless a short block is “decked” to reduce the clearance between the cylinder head and piston dome, a closed chamber head’s only value is that of increasing the motors compression ratio. The motor shall not profit from the extra torque and pre-ignition prevention of squish.
An open chamber head can extract equal power from a non decked short block, as long as the motor’s compression ratio is adequate for the fuel and camshaft being used.
Your friend on the DTBB, George
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