I measured the combustion chamber volume after reassembling the head.  It is now 55cc.  Those new intake valves take up a little extra room.  I will need this value to confirm my static compression ratio.

I’m not an advocate for polishing.  I prefer to test it with the flow bench and look for significant, measurable improvements in flow.  The finished ports are relatively free of imperfections.  There are no big chunks of stuff sticking out into the air stream.  Good enough.  Let’s face it, after a few miles of operation the crud is gonna start accumulating and that pretty mirror finish will be buried.
 
The completed cylinder head flows much better than the Stage II.  It can handle a cam with up to .420” lift.  It’s ready to install on the big bore engine.  I’m chompin at the bit to give it a try.

I hope you find this report informative and I welcome your questions and comments.

Knowledge is power.

Mike

Amazing!
Thanks for sharing your hard work.

Finn, it is my understanding that the "D" port is intended to mitigate exhaust reversion.  The hot gas flows easily over the bottom edge as it exits the port, but any reverse wave (acoustic or pressure) will run into the flat face on the bottom.  In effect, it should reduce the reverse flow (it serves as a dam).

I have seen articles and books that discuss problems associated with exhaust reversion, and it can get so bad that it actually forces the fresh intake charge back out of the carburetor.  Reversion that bad is usually associated with open headers (straight pipes).  It can be a real torque killer since a carb doesn't care which way the air is flowing, so each time the charge reverses flow it picks up more fuel.  The mixture goes grossly rich.  In severe instances, it can change direction several times before the exhaust valve closes.  Long duration cams with high TDC lift and lots of overlap aggravate the condition.

Another reason to leave the bottom of the port alone is the exhaust gas tends to run along the top of the port, so opening up the bottom only increases port volume, which decreases gas velocity.  I suspect that in the case of an exhaust port, the differential across the port is so high that the gasses are going to fill that port regardless of the inertia developed in the column of hot gas.  Even though the mass will want to run along the top of the port, the pressure will force the mass to fill the port top to bottom.  But as the pressure bleeds off, the smaller port volume will start to yield dividends by maintaining gas velocity and dragging the burnt charge out of the cylinder.

I don't have any hard evidence that this "D" port theory works any better than a round port.  I just know that a number of reputable head porters use "D" ports, and their heads are generally recognized as good performers.  Jerry Branch, HeadQuarters, and Edelbrock all use "D" ports.

The "D" port isn't going to result in better numbers on the flow bench, but it shouldn't hurt flow either.  If it mitigates reversion, then I think it's a good idea.