AC 301 or Cummins??

 

Turn on the garden hose sometime, put your hand over the stream of water out away from the hose, as you move your hand closer to the end of the hose, the water flow will tend to "back up" in the hose, the closer you move your hand to the end of the hose, the higher the water pressure will get at the discharge.  Air is no different. 

 

That chamber can help keep the air speed up, further away from the valve and help the flow.  If you tested the flow, then milled the chamber off to the point of haveing a flat head, it would flow less air.

 

Every orfice (meaning port or valve curtain area) have a coefficent of flow, every inlet and outlet have a coeffieicent of flow.  Take a flat plate, make a hole with 1 square inch of area in it, the hole can be round, square, or any shape so long as the area is equill to 1 square inch.  Put it on the airflow bench and pull a negative pressure under it, say the standard 28" H2O and see how much air that plate will flow.  It depends on the envrorment that day and it depends on the edges of the plate, how sharp of an edge is on that orfice.  Anyway just say it flows 85 CFM of air.  Put a rolled edge on the inlet and it will flow more air, put a rolled edge on the outlet and it will flow more air yet.  Put a tube on the inlet and outlet with a large radiused end on the tube like a tuba or horn shape and it will flow more yet. 

 

Typically you want to see a certain amount of flow or air density for a given area in a port, just say 100 CFM per square inch, or 120 CFM per square inch when the valve is at maximun lift.  To large of a port and it's to slow of air speed, or has dead areas, to small and the air speed gets to high, and even though the flow bench shows XXX flow, the small area will hinder the performance of the engine.

 

They say about 142 to 146 CFM per saueare inch of area is optimum for a valve curtain area.  Meaning that a valve seat area X it's lift is the curtain area.  Thers some verry well designed heads that can flow more than this yet, but look at our old tractor heads, they will not come anywhere near this, those ports are just to kinked around, sized wrong, sharp corners, etc.

 

These Moline heads are pretty good, but one can't start with a clean slate and make a perfect part, there's still a lot of compromises to make to get the head bolts in, the manifolds to fit, keep stock appearance and dimensions for the bolt on accesories.  I'd like to make one for an Allis sometime, but the market just isn't there.  The cost of development and pattern costs is verry high.  There's aftermarket Farmall 450 heads available, but they really aren't any improvement over the originals other than the fact that they are available and reasonably priced.  I can sell these Moline heads fairley reasonable, but it takes a lot of time and money in the R&D and pattern work that I need to add a fee to every one made.