Porting the 45 Inch Flathead, Conclusions, Conjecture, and Caveats

In part one of this series, I did a flow test on a stock Harley 45" Flathead cylinder and published the results here. In part 2 I added porting work utilizing stock size guides, again publishing the results. Part 3 saw the addition of larger intake valves. In this concluding piece, I would like to show the before and after results, along with my interpretation of those results.

First the overall results:

Exh stock .100" lift - 41.9cfm / finished 58.3cfm (+16.4)
Exh stock .200" lift - 82.4cfm / finished 99.8cfm (+17.4)
Exh stock .300" lift - 104.4cfm /finished 120.3cfm (+15.9)
Exh stock .350" lift - 110.6cfm /finished 128.3cfm (+17.7)
Exh stock .375" lift - 113.3cfm /finished 132.0cfm (+18.7)
Exh stock .400" lift - 115.7cfm /finished 133.8cfm (+18.1)
Exh stock .450" lift - 119.3cfm /finished 135.2cfm (+15.9)

****
Int stock .100" lift - 48.2cfm /finished 53.3cfm (+5.1)
Int stock .200" lift - 83.8cfm /finished 90.5cfm (+6.7)
Int stock .300" lift - 98.7 cfm /finished 112.4cfm (+13.7)
Int stock .350" lift - 102.6cfm /finished 119.0cfm (+16.4)
Int stock .375" lift - 103.8cfm /finished 122.1cfm (+18.3)
Int stock .400" lift - 104.3cfm /finished 124.0cfm (+19.7)
Int stock .450"lift - 104.4cfm /finished 128.0cfm (+23.6)

That gives an overall increase at .350 lift of 16% on both the intake and the exhaust. Not too bad, but is that all there is to be gained in porting work on the Flathead 45? Certainly not; in fact I already have some ideas in mind for the next set.

So, what have we learned? First off, the exhaust side of the equation requires a minimal amount of work to get it up to snuff. A good valve job blended into the port, with the ports "cleaned up," and a 30 degree backcut on the valve are about all that are required. I personally would not sacrifice any compression ratio by relieving the exhaust side.

On the intake side, the results were just as good, though perhaps not so clear cut. A larger intake valve is obviously worth while. It provides several advantages. The first is the most obvious. The flow potential is greater on a larger valve. Second, the larger diameter puts more circumference on the bore side where the flow is easier to achieve. The third advantage of the larger intake valve, is that it moves that circumference closer to the cylinder bore. That means you will actually be removing less material when relieving, which in turn leaves more compression ratio.

The possible drawback to using a larger intake valve is the increased shrouding by the wall of the head on the back side. Any airflow from the "back" side of the valve is going to have to travel around and over the top of the valve to get to the cylinder. The question becomes, what is the ideal valve size, keeping in mind that the increase in flow on the bore side from the largest valve may outweigh the small gain a smaller one would provide on the back side. In the picture below, the pencil is pointing out the area in the head that shrouds the intake valve. Incidentally, I opened up the small portion marked in black which corresponds to the edge of the head gasket in the area around the intake valve.

And that leads us to the question of seat angles. I spent a little time (very little, obviously) making a drawing to help explain what I believe to be the difference in flow characteristics between the 30 degree and the 45 degree valve seats. Note that the drawing is not to scale whatsoever.

Notice the difference in the shape of the green line which indicates my interpretation of airflow tendencies between the 30 and 45 degree seats. The 30 degree seat and valve tend to direct the air in a "flatter" trajectory toward the cylinder bore on the bore side, at the expense of a sharper turn to get around and over the top of the valve on the back side. In contrast, the 45 degree has a slightly better flow path on the back side, but is directing the flow higher in the chamber on the bore side.

With these paths in mind, here are some thoughts;

• A 30 degree seat is probably much more effective in a Flathead than it would be in an OHV, since you would want the airflow directed deeper into the cylinder on the OHV(cylinder directly below the valves; flip the drawing upside down).
• A smaller valve, with less shrouding would flow better on the back side with a 45 degree seat. Whether or not this might make up for the 30 degree's better path on the bore side would require more testing.
• A 45 degree seat would benefit less from relieving than a 30 degree. That is not to say that a 45 degree will not benefit.
• Depending on the amount of clearance between the top of the valve and the head at full lift, flow from the back side of the valve may actually decrease as the valve nears full lift. This could be alleviated by taking material out of the head at this point, at the cost of lowering the compression ratio.
• A 30 degree valve seat theoretically would flow best with a smaller choke diameter than a 45 degree in order to allow room for more angles, resulting in a gradual redirection of the flow (the choke is the smaller diameter in the port just below the valve seat)

A word (or three) on the transfer port:

I have heard it theorized that the reason relieving works is because it increases the area of the transfer port (the transfer port being the "window" between the top of the cylinder bore and the roof of the head). I tend to think this is incorrect. I do believe that it would be relatively simple to test though. A well ported and relieved intake should show a significant increase in flow by relieving the exhaust valve IF the transfer port is the restriction. Similarly, the intake should show an increase by removing material from the roof of the head in the transfer area. Easy enough to test, however either method risks needlessly lowering the compression ratio.

Here is a tip if you are having trouble visualizing the transfer port. Lay a head gasket on a piece of light cardboard and trace the outside and headbolt holes. Open up the headbolt holes in the cardboard and place it on a cylinder using bolts to align. From the bottom of the cylinder trace the bore onto the cardboard, remove from cylinder, and cut out the round circle you just traced. Now by laying the cardboard cutout onto the head and aligning it with bolts, you will be able to see the actual transfer port size and shape. Just eyeballing it would lead one to believe that the transfer port (at least on a normal 45) is probably large enough. A few measurements should either confirm or dispute this. Unfortunately time restraints precluded me from checking it this time around.

This finishes up my series on porting the 45 inch Flathead, at least this particular one. There is more porting work on Flatheads on the horizon though, so if I get some positive feedback on this series, I may do another. And by positive feedback, I don't mean that you necessarily agree with my conclusions, just that you found it worth reading.