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Old 05-29-2014, 12:11 PM
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DanH DanH is offline
Join Date: Oct 2005
Location: 08A
Posts: 9,871

Originally Posted by Titan-Xpert View Post
I am suggesting that this design has worked very well for over 50 years and it was purposely designed to have a small regulated airflow in this relatively cool part of the head (Front of # 2, rear of # 3 head area).
Quick sketch, just to illustrate the issue...

Baffling in this area of the head should not be so tight fitting that it totally blocks the flow. That’s why most use a washer between the head and baffle where it attaches to the head. The only way it could have zero airflow, would be if this area was silicone shut.
Apparently you do feel some step must be taken to ensure "regulated airflow" to the fins below the restriction (red circle). A washer or two on the baffle mount bolt (the bolt hole is seen here below the black rectangle) does indeed space out the baffle, allowing lots of airflow through an area with no fins. That's a useless leak, fine for GA slugs with huge inlets and exit areas, but not so spiffy for efficient cooling.

Most Lycoming type engines and their clones go to TBO. They would not if they were not being cooled properly. This cooling design is well proven, something must be working right.
Have to agree. There's a pretty good argument for each cylinder being an individual engine. Temperature deltas between cylinders probably don't compromise TBO, assuming the hot one isn't too hot.


Bobby, could you please identify the source document for the pressure measurement drawing in the previous post? The baffle buttons are straightforward, but I can't quite tell what is intended with the Section A-A probes, or how they are mounted.

For those interested in such things, CR3405 made an excellent comparison of probe types and arrangements. See pages 14 through 20, and Figs 11, 12, and 13. The investigators preferred baffle buttons for upper plenum pressure and piccolo tubes for the lower plenum, as they found long piccolos in the upper plenum read slightly low compared to buttons. That's not a surprise, given that the buttons are not shielded against local flow velocity.

I chose foot-long piccolos for both upper and lower plenums, for several reasons. They are easy to make, mount, and plumb, in an identical fashion, important if global, easily comparable results are expected from a group of homebuilders. Two, they work pretty much the same regardless of plenum volume as they shrug off dynamic pressure. Three, they are self-averaging over the majority of the plenum volume.
Dan Horton
Barrett IO-390

Last edited by DanH : 05-29-2014 at 12:14 PM.
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