Since the last test run had the engine running quite rich, I adjusted the carb a bit leaner and tried again. It was definitely leaner, but the differences are getting small enough that I think I have to let the EGTs settle for a longer time to get a stable reading. This means I’ll basically have to do a run at full throttle with a fixed mixture setting, go down to idle and let it cool, do another full-throttle run with a slightly different mixture setting, and repeat until I get enough data points to get something reliable.

I did also decide to try a run with the cowling on, to see what the effect on temperature was. The cylinder head temperatures were overall a little bit higher but showed less spread than without the cowling. In both cases #4 was the first to overheat. Without the cowling, the CHTs when #4 hit redline (set at 400F, or 204C) were 189C, 167C, 182C, 204C. With the cowling on, they were 203C, 185C, 195C, 204C. So the spread without the cowling was 37C but only 19C with the cowling on. I guess this indicates that the cowling actually does help force the air through the cylinder cooling fins, making the air flow more equal.

However, the thing that stood out the most was that temperature sensor I had mounted on the cylinder #2 intake runner where it passes near the exhaust header. With the cowling off, the temperature at that point rose moderately, hitting 50C as I pulled back from full throttle. With the cowling on, it just kept rising: 60C, 70C, 80C… As it went into the 80s, I pulled back the throttle to idle, since that’s far higher than what the nylon they’re made of can take. With the cowling on, however, the temperature just kept rising, even after I shut the engine down. When it hit the 90s I jumped out and quickly took the cowling off to vent the hot air. The runners were noticeably soft when I squeezed them.

So, there’s no doubt that the runners need to get replaced with stainless tubing. I’m in the process of cutting and welding those right now. I also fabricated two exhaust shields from 24-gauge stainless that I bent into a cylindrical section and welded tabs for hose clamps on. These will prevent the headers from radiating directly onto the runners that exit the plenum. I’m really hoping this will be enough to keep the temperature down enough for the plenum to survive.

The last ditch solution would be to use exhaust wrap to keep the headers from radiating all their heat inside the engine compartment. I’d prefer not to have to do that since this raises the temperature of the exhaust pipes and can lead to premature failure. People often use this with cars, but cars don’t spend a lot of time at full throttle, whereas airplanes do. We’ll see what kind of temperatures we see with the heat shields.

I’m hopeful I can weld up the stainless intake runners and get another test run in this weekend. Stay tuned.