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Ron's Web Site
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Date: 4-1-2012
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Number of Hours: 2.50
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Manual Reference:
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Brief Description: High Cylinder Head Temperatures
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Various builders have had problems with high cylinder head temps. Prior to switching to Mogas, my temps ranged in the upper 300's, and only rarely went above 400 deg, and that was only on very hot days (100+ degrees). I have an adjustable cowl flap, too. Pure Mogas bumped the CHT's about 25 deg, though. I spoke with Bob about it, and he said O-540's tend to run hot anyway, as much as 475 deg in the climb, but he wasn't happy about using only 91 octane Mogas with electric ignition, although the Petersen STC's have several 260 hp O-540's using that fuel. So, I experimented with 30 - 50% Avgas, which lowered the temp 10- 15 deg while bringing the octane to 93 or better. Bob also didn't like the electronic ignition, which advances the timing, potentially raising the specter of detonation. On the ElectroAir setup I use, the ignition doesn't advance timing until manifold pressure falls below 24 inches, and then it follows a progressive schedule thereafter. Turning off the ignition advance cools the CHT's about 15 deg alone. With the advance turned on, reducing the RPM to 2300 has the same effect, probably largely because it raises the MP to nearly 24 inches at my most common cruising altitude. Finally, I tried VG's on the cowl and cowl flaps (two of the configurations are shown in the fotos). It turns out that VG's on the flap increases the CHT. Three VG's decreases the temp only 5 deg, but a single VG on the air box decreased the temp 10 deg, which was fairly impressive for only a few ounces of aluminum. I'll finalize the VG there with a couple pop rivets after I prime and paint the VG. Net effect: highest CHT indicating 380's with half cowl flaps, 30 - 50% Avgas, 2300 RPM with electronic ignition on.
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Three VG's
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Only one VG - works best
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