Thorp Air Command - T18.net

I have an 0-290 on my airplane with about 10 hours SMOH. I have installed a new oil temp gaug, a new 4 probe CHT Temp gauge and a new tach gauge. I have sealed up my baffles pretty good. There are a few minor places air could leak out but overall the baffles are sufficient. The cowl air inlet area is sealed off as well. I have flown with all the fairings installed and all the fairings removed. While on the ground, I lean my mixture until I see about a 50 RPM rise (I have gotten to where I taxi with full rich mixture to try and keep CHTs down). My CHT temps are the same in all configurations and this is what they are:

From startup to taxi to the hold short line it takes me probably 5 minutes. My CHT #1 290 #2 290 #3 325 #4 300 at the hold short line. Oil temp is about 110-120 degrees. On takeoff, I go full RPM to 2700 and climb out at about 800ft/min. Since my #3 CHT is the hottest, I monitor my #3 CHT during takeoff. Before I can make the turn to crosswind, my #3 CHT will be hitting 425. Once I see that temp, I lower the nose and climb at about 200-300fpm and reduce RPMs to 2300. I have to stay at 200-300fpm otherwise it makes my CHT temps rise. Oil temp is still good at this point and #3 CHT will maintain 400 degrees with a 200-300fpm climb. After I make it to cruise altitude (level flight), I set the RPM to 2500 and lean the mixture for best power. I have also cruised at both 2500 and 2700RPM and the CHT do not rise over 400 (both full rich). During all of this, my oil temp will stay about 185 degrees. CHT are as follows in cruise #1 325 #2 350 #3 400 #4 390. If I go full rich mixture and reduce RPMs to 2200-2300, I can get #3 CHT down to 380. The engine runs smooth and strong throughout all phases of flight.

I think my CHTs should not get so high during taxi. I would think I could climb at at least 1000fpm to get to 2000 feet AGL without overheating the engine. I would have also expected my CHTs to be a lower during cruise.

Anyone have any ideas on ways I can get the CHTs down? I'm almost out of ideas at this point.

If you are using straight mineral oil, you will see these types of temps. You engine is not broken in yet. You need to get to altitude w/out going much past 400, then level off and run as much throttle as you can to maintain 400 or below for at least an hour. If you haven't changed your oil since overhaul, it's time to do it. Mineral oil breaks down fast in the first few hours.

_________________
Bob Highley
N711SH
SN 835
KLAL

I was prepared for high break in temps, but I thought I would be able to climb better than 200 fpm during break in. Because the cruise temps were ok and the climb CHT temps increased so quickly, I thought I had an issue of not properly moving the air over the cylinder heads (baffle seal issues). I have changed the oil twice in the 10 hours and I am using AD oil.

Do you have the thermocouples under the spark plug? The Lycoming published temps are for the bayonet probe. I changed probes on my plane and all CHT temps went down 75 degrees. Also, you have to add degrees the amount the outside temp is above 70 degrees, and subtract degrees the same number the outside temp is below 70 degrees. Look at your guage instructions to see if this is your case too.

Jeff Blanton

I did realize Lycoming recommended the bayonet type as opposed to the thermocouples (which I have) but I wouldn't have thought a 75 degree change was possible. I might look into changing it out. Thanks for the temp adjustment info as well. I wasn't aware of that adjustment.

I used this checklist. see step 11. http://www.sacskyranch.com/eng23.htm

I called Lycoming, and they said they recognize the bayonet style thermocouple. I made a flight with the ring style under sparkplug three and got 425 degrees on climb out. I then changed the probe with the bayonet style compatible to my guage. I then flew again and got 350 degrees. This was this just my experience. I spent all last summer trying to make it run cooler with cowling and baffle mods, when all along I think my temps were fine.

Thats good info Jeff, thanks. You're results have just about persuaded me to make the switch to bayonets. I was just a little surprised at the rate of CHT increase during taxi and climb so I thought there may be some specific issue with the cowl/inlet area/exit area that I wasn't aware of.

Dick, I got your message. I agree with you. Local advice was that AD oil would be fine. I wasn't comfortable with that, but I followed that advice anyhow. I think I'll switch to mineral oil now.