Reply to "AFD Alum heads"

Good morning all, I'd like to address some things Ron mentioned back on page 13.

Ron, regarding weber mainfolds, there are 2 Australian manifolds with 2V size ports, the Cain (no longer in production) and the Redline. Outside of those 2 manifolds, to my best knowledge, all the other 351C Weber manifolds have 4V size ports. The Hall Pantera manifold seems to be the most readily available manifold. They change hands a lot.

Regarding the CHI 4V head, John of CHI shared some photos of the head with me earlier this year, the ports of the head in the photos were unmistakably 4V size ports.

Regarding open chamber heads, there is a lot of opinion on the subject of open combustion chambers verses closed combustion chambers. Here's the facts: All Cleveland racing heads and aftermarket heads feature closed combustion chambers. The designers of these heads obviously believe the closed combustion chamber design is superior. An open combustion chamber is a “hemispherical” pocket in the cylinder head containing the valves and spark plug that is the same diameter as the cylinder bore. Closed combustion chambers feature a smaller combustion chamber containing the valves and spark plug that is surrounded by flat cylinder head surfaces creating an area referred to as the “squish area”. Ford’s purpose in equipping some motors with closed chamber heads was to raise the compression ratio of the motor. The smaller volume of the closed chamber heads does just that. However in order for the squish area to aid in producing additional horsepower, the clearance between piston dome and the cylinder head at top dead center must be 0.045” +/- 0.010” on a street motor. It is common for racing engines to set this clearance even smaller. As delivered by Ford, this clearance is approximately 0.070”. This clearance 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. A closed combustion chamber head used with a zero decked block is good for at least 20 ft/lbs of torque on a street engine. 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 and increases hydrocarbon emissions. 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. 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, it could be argued that an open chamber head can extract more power under this situation (i.e. if a block is not zero decked), as long as the motor’s compression ratio is adequate for the fuel and camshaft being used.

your friend on the PIBB, George