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.

 

We'll blame it on the five-o-clock shadow that old head is sporting.  Yeah, that's the ticket!

 

How about a picture from the other side of the head?

 

While the Allis head might seem perfect in terms of shrouding from an intake point of view, its not.  The trick is to contour the chamber (even just a little) to accomdate the natural "plume" emerging from the valve's edge.

 

From an exhaust perspective, you want to reverse the situation and find the right chamber shape to "funnel" the exhaust gases out more efficiently without restriction. 

 

Airflow around the Allis' valves into and out-of the chamber is kinda "confused". Definately not laminar - or even close.  But!, the piston head shape can help that situation - hence the heart shaped M&W Allis pistons.  Jaguar did the same thing back in the day, shaped the chamber in the piston rather than the head (it was flat like an Allis).

 

The MM head is gorgeous - a nice "soft" chamber design that impedes detonation and I'm sure meets the airflow demands at design.

 

Wi50 - did you wet flow or smoke that head to come up with your chamber radius?

 

The Cummins head sucks, but the Allis head is much worse.  I'd run the Cummins hands down compared to the Allis, and if you want to run on methanol then deffinatly the Cummins.  The Allis is not going to carry fuel in the port near as well as the Cummins will, and in sheer airflow capacity the Cummins will win. 

 

The Cummins is going to be less costly in the end and a tougher product.  The cylinder head parts fo the Cummins are going to be off the shelf parts used from another application, the Allis are going to be special order.  The Cummins block is stronger, the crank has some counterweight and it's easy to get one counterweighted. It will also have more value in the end if and when you chose to sell parts.