Regular readers of this blog will know that I have been working at intervals over the last 2 years to try and reduce engine operating temperatures following a rebuild from 150bhp to 160bhp.
I had reached the point a few months back where I could run at around 350 degrees (using bottom bayonet CHT probes) in level flight on all cylinders except #3.
At the recent annual inspection, Jesse discovered that there was a crack in the rear baffling panel that hangs down underneath the rear of the engine casing. This baffle panel has to be able to move forward and aft in order to be able to rest against a flange on the lower cowling. This seals the lower rear of the engine compartment, preventing cooling air from exiting at the rear of the cowling. The air instead has to rise through the engine compartment.
This baffling is riveted to other baffling at the rear of the engine, but part of the aluminum has fractured, so the baffling is now able to float and drop when the lower cowling is in place. As a result the baffle "floats" backwards past the cowling flange once the plane is in the air and positive pressure is generated under the engine. After landing the other day on a local flight, I could see the baffling bent back at an angle.
The result of the displacement of the lower baffle is a 15 degree rise in CHTs across all cylinders, and a 10 degree rise in the oil temperature. This takes the average operating temperature of #3 cylinder up to (or above) 375 degrees) at which point I begin to get oil blow-by from that cylinder. I am still not sure (a) why #3 is always 15-20 degrees hotter than the other cylinders, (b) why the oil blow-by occurs. I suspect some issue with ring seating or sticking, but it would probably require a boreoscope exam of the cylinder to confirm or deny that suspicion. I have a medium-term plan to replace the current stock cylinders with Millenium cylinders in any case.
This weekend, the fractured baffling will be replaced with a new cut-to-measure piece of aluminum. This will ensure that the plane is truly ready for longer trips in September and October. I have a trip planned to New Mexico and then there is Rough River approaching soon.
After Rough River, I will fit and test Ed Spracker's modified carbureter venturi (see previous posting) and test it back-to-back against the standard one-piece venturi.