Sunday, October 17, 2010

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.


Noot said...

Very Interesting. Trying to get air to go around a corner is a challenge.
Of coarse the bigger the hole - the less velocity - so, we have to take that into consideration. Can we increase compression a little now?

pat said...

Very cool Lee. Thanks for sharing for sure! I spent a bunch of time two winters ago doing flathead flowbench testing. I made a plate that bolts to the bench with a 90* tube welded on with a flange that the cylinder bolts to on the other end. This way I could bolt the jug to the fixture and flow the port through the whole inlet tract with the head bolted on the cylinder. The results were all in line with what you found.

St.Lee said...

Thanks for the comments guys.

Noot, I'm all for raising the compression, but methods for doing that seem to be limited on the 45. Any ideas? Here is a link to an old post I wrote that gets into velocity a little bit, and goes a bit against popular theory:

You'll have to cut and paste cause I am not sure how to put a link in a comment.

Pat, I was pretty sure I wasn't the only one to invest flowbench time on a Flathead, and am glad to hear our results were similar. For the life of me though, I can't quite visualize the bench set up you describe, unless it is to make opening the valves easier. I did my tests with the cylinder directly on the flowbench flange down(with appropriate flange adapter) and the head installed. The only reason I used clay as an inlet guide on the intake spigots was because I didn't have the manifold finished for it yet. Normally I like to flow through a manifold with a velocity stack in place of a carb. The valve opening mechanism I fabricated for Flatheads is ugly & clumsy, but functional and does clear everything with cylinder mounted in conventional manner on the bench.

pat said...

i just mounted the cylinder assembly sideways instead of vertical to flow it. to up the compression, run K model pistons and machine the heads for pop-up. then you can build some squish into the engine. the k model has a taller comp height, smaller ring pack and isnt rounded on the top. they share the same wristpin size and bore sizes.

pat said...

i dont have any good pics, but the pistons pop up about .050" on my engine.

pat said...

copy and paste this link

pat said...

St.Lee said...

Thanks Pat. That is some nice looking work on the individual runner mod.

I was aware of the K piston modification, though it was not applicable or needed on this particular motor since it is a stroker. I did machine the tops of the cylinders to get .035" squish though. And obviously the C.R. was raised by stroking the engine

Joe H said...

Hey Lee,
Thanks for sharing your findings. I've been riding my little WLA for 4 years now and it's a little tank. It's ported and polished with stock valves and raised comp. I've been researching my next stage of modification and this goes a long way to helping me figure out what that's gonna be. There's alot of voodoo out there concerning these engines and it's nice to see some clear documented results.

Anonymous said...

Len Andres proved that higher compression hinders flow in the Ricardo style motor. 5.99 to 1 was his ideal C/R for WR and KR racers...

St. Lee said...

Anonymous, thanks for adding a comment, and I apologize for not responding to it sooner.

The name Len Andres sounded familiar, but I had to do a search to confirm that he was a Motorcycle Hall of Fame racer and tuner. Certainly he would have been a valuable person to learn from. However, I do believe that your statement that he "proved that higher compression hinders flow in the Ricardo style motor" is at best only opinion. Since he retired from engine building about the same time the first flow benches became readily available, he "proved" nothing of the sort, and my guess is that if he were alive today with access to flow testing equipment, he would still be experimenting to improve the total package. In fact, I would suggest that Len himself would disagree with that he had "proved" anything.

It is commonly said that in a flathead motor flow is more important than compression ratio (and I think that may be what you were getting at). That may very well be true to some extent, but it is a fairly open ended statement with extremes at each end that show it not to be a hard and fast statement.

I think what Andres did do was to come up with a package of flow and compression ratio that was close to the ideal for the technology and parts available in his day. For instance, what if he could have achieved more compression ratio without sacrificing flow, and modern magnetos with their much hotter spark allowed that compression ratio to be usable without detonation? And even the 5.99:1 "ideal" C/R was entirely dependent on the valve timing, because the cylinder pressure provided by that C/R would be different with a different set of cams.

Not trying to be overly critical, just want to keep people thinking. Nothing stifles improvement more that taking things that have been "proven" in the past as true when in reality all of the factors involved have not been taken into account.