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I've been tweaking engine cooling from a slightly different angle and wanted to post it here in case it might help others. I haven't ruled out installing cowl flaps, but I wanted to fine tune everything else first. I enjoy learning about this sort of stuff and we don't have much else to do here in NZ on the weekends, so I got to thinking about how I could get the best from the cooling setup.
For my IO540 I used an exit area of 130 ² in (numbers that I found on the forum that seem to work very well on my engine), and I fabricated a small fiberglass lip to further lower the exit air pressure. That all worked well for the engine break-in and test flying, but I did need to refine it. Initially I have been controlling CHT's during the climb with airspeed and generally climbing with full rich mixture.
Recently I did a GAMI spread test which is fancy way of saying that I climbed to 7000ft, set full throttle, and very slowly leaned the mixture back until the engine ran rough. I repeated his 3 times, then landed and downloaded the data. I was then able to view the graph on my computer using Savvy Aviation's free software. You're then looking to see what the fuel flow (mixture knob position) is as each cylinder hits its peak EGT. Ideally I wanted all cylinders to hit peak EGT at the same fuel flow. Actually they have a tool that checks this automatically in the software.
What it showed me was that the #1 cylinder was running richer than the rest. This meant that when I leaned the mixture in the cruise, the other 5 cylinder EGT's were all Lean of Peak with correspondingly cooler CHT's, while the # 1 was running much hotter near peak EGT (or even on the rich side). This made it difficult to run LOP at lower altitudes because the #1 CHT was always hot while the others would run rough from being too lean.
Airflow Performance sell individual restrictors for standard Lycoming fuel injectors, at a reasonable price, so I installed a slightly smaller restrictor (a 5 min surprisingly easy job) and repeated the test. I can now run LOP even at low altitudes with all CHT's grouped roughly even. I'll fine tune even further and will further lean both #1 and #3 cylinders now that I know how to do it. My #6 CHT still runs hotter than the rest - but for another reason (that's where the oil cooler is ducted from), but not by much and they're all fairly closely grouped now.
Next, I realized that the original cooling lip was doing well on the cooler days, but not so much in the summer heat. No surprise there, because when I made it I simply estimated the size based on the TLAR method. I've now made a second larger cooling lip. That dropped my CHT's another 10°c and will be used during summer. It's a 5 min job to switch over (7 screws) and the original one should work well during winter.
That leaves one last cooling aspect which is my oil temperature. With the original cowl exit lip my oil temperature is usually around 75°c - slightly lower than ideal. The larger cowl exit lip drops the oil temperature even further, but this should be negated by using it during the summer months. I do need to double check my Vernatherm setup, but assuming it is correct then I'll add a small baffle to restrict airflow into the oil cooler scat tube. This should also leave more air to cool the #6 cylinder.
Here's the "GAMI SPREAD" results - you can see the #1 cylinder in red doesn't hit peak EGT until a lower fuel flow than the rest indicating that it was running richer than the others - it's EGT was still climbing as the others peaked.
image_11636.jpg
For my IO540 I used an exit area of 130 ² in (numbers that I found on the forum that seem to work very well on my engine), and I fabricated a small fiberglass lip to further lower the exit air pressure. That all worked well for the engine break-in and test flying, but I did need to refine it. Initially I have been controlling CHT's during the climb with airspeed and generally climbing with full rich mixture.
Recently I did a GAMI spread test which is fancy way of saying that I climbed to 7000ft, set full throttle, and very slowly leaned the mixture back until the engine ran rough. I repeated his 3 times, then landed and downloaded the data. I was then able to view the graph on my computer using Savvy Aviation's free software. You're then looking to see what the fuel flow (mixture knob position) is as each cylinder hits its peak EGT. Ideally I wanted all cylinders to hit peak EGT at the same fuel flow. Actually they have a tool that checks this automatically in the software.
What it showed me was that the #1 cylinder was running richer than the rest. This meant that when I leaned the mixture in the cruise, the other 5 cylinder EGT's were all Lean of Peak with correspondingly cooler CHT's, while the # 1 was running much hotter near peak EGT (or even on the rich side). This made it difficult to run LOP at lower altitudes because the #1 CHT was always hot while the others would run rough from being too lean.
Airflow Performance sell individual restrictors for standard Lycoming fuel injectors, at a reasonable price, so I installed a slightly smaller restrictor (a 5 min surprisingly easy job) and repeated the test. I can now run LOP even at low altitudes with all CHT's grouped roughly even. I'll fine tune even further and will further lean both #1 and #3 cylinders now that I know how to do it. My #6 CHT still runs hotter than the rest - but for another reason (that's where the oil cooler is ducted from), but not by much and they're all fairly closely grouped now.
Next, I realized that the original cooling lip was doing well on the cooler days, but not so much in the summer heat. No surprise there, because when I made it I simply estimated the size based on the TLAR method. I've now made a second larger cooling lip. That dropped my CHT's another 10°c and will be used during summer. It's a 5 min job to switch over (7 screws) and the original one should work well during winter.
That leaves one last cooling aspect which is my oil temperature. With the original cowl exit lip my oil temperature is usually around 75°c - slightly lower than ideal. The larger cowl exit lip drops the oil temperature even further, but this should be negated by using it during the summer months. I do need to double check my Vernatherm setup, but assuming it is correct then I'll add a small baffle to restrict airflow into the oil cooler scat tube. This should also leave more air to cool the #6 cylinder.
Here's the "GAMI SPREAD" results - you can see the #1 cylinder in red doesn't hit peak EGT until a lower fuel flow than the rest indicating that it was running richer than the others - it's EGT was still climbing as the others peaked.
image_11636.jpg
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