Continuing my search for improved engine cooling and lower cylinder head temps for my middle cylinders, I've been experimenting with baffle modifications. I went thru a series of air dams and deflector tabs (different heights and locations, some vertical, some canted back at an angle), but none of these really encouraged the air to go where I wanted it. It seemed to be mostly an ineffective approach.
The real solution is to put the air in close proximity to where you want it, without introducing a bunch of turbulence that merely robs energy from the cooling air like air dams and deflector tabs will do. In fact, the turbulence behind the tab creates a significant eddy downstream, and you can see the effect by the temps going up for the cylinder behind the tab.
Instead of dams and tabs, I fabricated a controlled constriction directly over the middle cylinder. This has the effect of accelerating the flow and moving it closer to the head, thereby improving the rate of heat transfer. The constriction acts sort of like a venturi, with a smooth transition downstream of the narrow point to smoothly transition the airflow back to the rest of the baffle. I call this shape the "coke bottle".
The coke bottle is attached to the inside-roof of the existing baffle. If you have hinged baffle sides this is quite simple to accomplish, and allows you to fine-tune the size and location. The coke bottle works extremely well! After installing it my middle cylinder CHT's dropped by 30 degrees, and there was no apparent increase in temps of the back cylinders. The coke bottle simply made better use of the cooling air I was taking in. That's a win-win!
With the cowling mods, enlarged exhaust exits, and now the coke bottle installed, my CHT's are fantastic. Typical cruise CHT is in the 250 deg F range, varying from 180-200F for the front cylinders (coolest), to about 230-250F for the other 4 cylinders. Even during a hard climb in the Mississippi heat the CHT's rarely go over 300F, and the engine just runs great.
NOTE: photos link to full size image