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Stephen,
Did you ever come to a conclusion on the CHTs changing with the firewall exit change at the bottom? Is there any positives or negatives to the PA-22 vs. the PA-20 firewall? Considering replacing one and trying to decide which route to go. Thank you.
I am just back from an 8 day flying and camping trip in Idaho. We did a lot of hard flying including Moose Creek, Johnson Creek and Red's horse ranch. The outside air during our flying ranged from 65 to 85 and we once climbed to 10,000 feet. My oil temps never got above 185 and were mostly ran in the 170's. I will block a little more of the oil cooler. The CHT's on my two rear cylinders at cruise were usually around 395 and 405 or a bit lower. On climb out I usually hit the 430's on number 4 and once hit 445 before I adjusted my cruise to cool it off. Taking off above 3000 feet elevation I have to lean to keep my engine running decently but, I keep it as rich as possible for cooling purposes until I am at cruise altitude. I adjust my mixture for both performance and CHT. After installing my baffling this past winter I spent time to seal air leaks. I also enlarged the lower cowling exit, increased the size of the lip and sealed around the carburetor inlet. All of which I saw little change in CHT's. I have been told that the exit should be 30 to 50% larger than the entrance. I have no further adjustments planned.
My real question is that if I am replacing the firewall on a PA-22/20 with a new one from the Cub Doctor, should I replace it with a PA-22/20 firewall or a PA-20 firewall? Wondering if anyone has done this, I am sure that someone has. Is it worth the trouble of switching styles? Is it advantageous to keep the tunnel exit at the bottom of the firewall?
I replaced my PA 22/20 firewall with Cub doctor's stainless PA 20 firewall two years ago. I had removed the old tunnel and had cut down my old firewall a few years back. This modification required rebuilding my lower cowling. Last spring I made all new cowling. I liked the looks of the profile of the PA 20 engine cowling better than the PA 22. I was curious about possible changes in cruise speed but, I did not see a change. I did open up the lower airflow exit to compensate for the removal of the tunnel. I have also replaced my engine baffling recently. My oil temperatures rarely get to 190.