Engine CHT question

Have a 10 hr since SMOH O320E3D that has the CHTs run up very rapidly to 425 plus on take off and have to throttle back to keep the temps down for take off climb. Once the engine warms up in flight the CHT issue goes away and settles down in the 375 range even at WOT. Oil pressure is around 75 and oil temp stay in the low green

Initially though it was a newly overhauled thing but it does not seem to be clearing up at 10 hours. I am going to replace all the baffles with red silicon baffle tape this week to see if I can get a better seal but don't think it is causing this issue.

What CHT range are others getting on take off?

Could this be a vernatherm problem? Just need more time? Other ideas?

My CHT's stay below 400F in the immediate climb to pattern altitude. If I continued a steep climb at (say) 100 knots or less, I would likely see temps like yours. To prevent this, I usually climb at >120 knots.

You might check and verify the timing is correct. Advanced timing will generate more heat rejection. Can't hurt to check.

Pete - if you have fuel flow indicator, what is your full throttle, full rich fuel flow at/near sea level? If you have a fixed pitch prop, also note the rpm at the fuel flow. What are the OAT's? CHT will follow nearly 1 for 1 the OAT, everything else being equal. Kyle mentioned climb IAS, that is a biggie.

The archives are stuffed full of threads on this topic...

PeteP said:

Have a 10 hr since SMOH O320E3D that has the CHTs run up very rapidly to 425 plus on take off and have to throttle back to keep the temps down for take off climb. Once the engine warms up in flight the CHT issue goes away and settles down in the 375 range even at WOT. Oil pressure is around 75 and oil temp stay in the low green

Initially though it was a newly overhauled thing but it does not seem to be clearing up at 10 hours. I am going to replace all the baffles with red silicon baffle tape this week to see if I can get a better seal but don't think it is causing this issue.

What CHT range are others getting on take off?

Could this be a vernatherm problem? Just need more time? Other ideas?

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10 hrs isn't always enough time to fully complete a break-in.

Searching archived threads will be helpful because there is a lot of things that can have in influence (some which have already been mentioned here).... often times it is a number of them put together.

It is rare that a first time builder has a baffle system that is actually performing up to the level that it can. Sometimes there are major issues. A dark hanger and bright light is your best troubleshooting tool for that. Search for anywhere air can flow that it is not moving between cyl fins.

I just went through similar CHT issues with my IO-320, with new cylinders, on my RV-8. My CHTs during the first 30 hrs were getting to 425 right after take off and I had to pull the power back below 75% to keep them from going higher. I am now at 53 hrs and the CHTs now stay right at 400 in a max perf take off and climb. A few items that did help; The baffles will "settle in" a bit after about 25 hrs, go back and look for gaps and fix them. I found the lower "finger" pieces that wrap lower parts of the cylinder heads had grown a 1/4" gap in places. Make sure that they are snug around the fins. Also, the well documented fix of putting a 0.030" washer under the #3 cylinder aft mount screw (#8 screw) seemed to make a big difference on my #1 and #3 CHTs. Now, even the balmy Alabama summer days of 98F, my CHTs stay at or below 400 and the oil temp peeks at 190 and runs at 180 ish in cruise.

Fuel Flow?

My limited experience says that takeoff and climb CHT temps are highly dependent on timing and fuel flow ----- not sure what a 320 fuel flow should be but I know on the 360, 180hp, anything much below 17.5-17.8 gph is going to result in higher CHTs.

R.

What Ron said. Check the jet size and fuel flow.

My wingman just finished troubleshooting a similar issue on an RV-6 he purchased. Climbout was severely limited due to rapidly climbing CHTs. No problem in cruise.

It turned out to be the jet size on the carb. The engine was running too lean on climb. He called one of the major carb overhaul shops and gave them the carb model number. They confirmed that that it was the wrong carb for an RV.

Traded his carb in for an overhauled unit. Can now climb without limitation.

RONSIM said:

My limited experience says that takeoff and climb CHT temps are highly dependent on timing and fuel flow ----- not sure what a 320 fuel flow should be but I know on the 360, 180hp, anything much below 17.5-17.8 gph is going to result in higher CHTs.

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That would drown a fixed pitch 320. 17.8 ~17.9 is right around best power for an IO-390 at sea level, at 2700 RPM... 13:1 air-fuel, or 0.077 FA.

Fixed pitch 320, climb RPM....

((320 x (2500 / 2)) /1728) x 60 = 13,888 cubic feet per hr @100% VE

13,888 x .0765 lbs per ft^3 sea level density = 1,062 lbs per hr.

Best power air-fuel ratio is 13:1, so 1062 / 13 = 81.6 lbs fuel

81.6 / 6 lbs per gallon = 13.6 GPH

12:1 air-fuel is 14.75 GPH...and remember, these are sea level numbers on a 59F day. Hotter or higher requires less fuel.

Throwing more fuel at it is just a power reduction.

PeteP said:

Once the engine warms up in flight the CHT issue goes away and settles down in the 375 range even at WOT.

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Well, it IS summer in VA, you know. Those climb temps with a new engine do not sound abnormal this time of year. After level off, you are accelerating and getting more air through the engine so it cools off.