To install the larger valves, the seats must be cut.  Neway valve cutters work well.  You end up with blisters.  It’s all hand work and it takes a lot of effort.  I do the port work before I do the seat work.  That way, I don’t run the risk of rat bites in a pristine seat.  I made some special laps that use abrasive paper.  They polish off any little burrs left by the carbide cutter.

Now that the intake port was finished and the larger valves installed, it was time to see how it flows.  Prior to starting work on the head, I did baseline flow tests.  It matched the data I had previously taken on my other stock head.  I knew the flow bench was providing repeatable data.  
I also had the flow data from the Stage II head.  

So now, I could test the Stage III and see how things stack up.

The trusty flow bench has two full-size shop vacs (5.5 HP & 6.5 HP).  Even with that much power, the bench can barely keep up with this Stage III head.  It will pull 15” H2O (my standard test pressure) but there ain’t much reserve.

Time to move on to the exhaust port.

The exhaust port has two grotesque impediments to flow.  An item I refer to as the “dipsy-doodle”, and a reduced outlet hole (only 1.3” diameter).  I can’t do anything about the dipsy-doodle.  It’s in the roof of the port (the critical flow area) and I have no way to fill it in.

The super-small outlet is another story.  I can fix that.  Here you see the stock 1.3” outlet with the dipsy-doodle lurking in the back.  The circular scribe line shows where this port is headed.  When complete, the outlet will be in the shape of a capital “D” laying on it’s side.  The Stage II head also uses a “D” shaped port, but it is only 1.5”.  I want to see what will happen when I open the port up all the way to 1.79”, which is the inside diameter of my Mac header pipe.

Another problem is the left-hand exhaust runner.  The bridge in the center of the exhaust port forms a straight vertical wall.  I felt it would flow better if the runner were actually round.  It did flow better.  Here’s that flat vertical wall on the left-hand runner.

Now the 1.79” flow bench adapter mates nicely with the port.  The header pipe will too.

Rounding off the guide bosses was a straightforward operation.  I didn’t get carried away.  I want to leave ample material to maintain the press fit between the cast iron guide and the aluminum boss (don’t want to relax the hoop stress by thinning out the wall of the boss).  Just round off the edge to open the cross section a bit; help the hot gas get past the boss.

Then it was time to address the left-hand runner.  I worked on the bridge a bit to establish a round cross section.  Rounding the left-hand runner improved flow measurably on my test mule.

As you can see, this is a vast improvement over the stock head, and a significant improvement over the Stage II head.  Question is, will the 97mm engine like this free-flowing exhaust port?  Will the port be lazy and kill low end torque?  Will it work well with my modified muffler or will the muffler simply negate any flow improvements I derive from the port?  The flow bench can’t answer those questions for me.  Just have to try it and see what I learn.

The average flow on the stage III exhaust port is 31% better than stock (125.7 vs 96.1) and 12% better than the Stage II (125.7 vs 112.0).