Cylinder Head Temps - Cylinders 5, 6

[brian.olson]

My A&P recently installed a new Aerospace Logic 6-cylinder CHT gauge in 62C so that I could keep an eye on all six cylinders (I had a top overhaul done at the beginning of the year and want to protect the investment). The CHT probes are spark-plug type (the cylinders do not accommodate bayonet probes).

After a few hours' worth of test flights I've noticed that cylinders 1-4 seem to read appropriately for the outside air temp and flight conditions, but cylinders 5 and 6 read almost alarmingly low.

Some data points for you:

1. CHT probes fit under the spark plugs; the cylinders do not accommodate bayonet probes. Manufacturing documentation indicates this type of probe may read up to 25 degrees F low;
2. Outside air temp was approximately 70 degrees F on the ground, 65 degrees in the air. The sample photo was taken at approximately 2,000' AGL.
3. Photo was taken during straight/level flight after about 30 minutes of flight time.
4. The engine baffles were replaced two years ago with a complete Airforms kit and are a thing of beauty - they seal well and work great;
5. Engine operation at all times seems correct. RPM correct, power is correct, engine runs smoothly, mag drop on the ground and in-flight is "normal".

In-flight photo follows. To interpret, it is set to show a graph of all six cylinders with a callout of the lowest temp and the highest temp of the six. In this case, the lowest temp is cylinder 5 at 151 degrees F, with cylinder 6 slightly higher:

Photo immediately after shutdown:

You will note that in the photo immediately after shutdown all six cylinders have equalized and cylinders 5, 6 are now actually reading higher than in-flight - in a range that I would think is "normal" for ground operations.


What I have tried:

1. All probes have been installed per manufacture directions, all cable routings double-checked. No cables are touching any exhaust components nor are they touching any of the ignition components (ignition harness, p-leads, etc.)
2. Anecdotal comments by other smart folks at the airport seem to point to the fact that cylinders 5, 6 are right up front, face-first into the prop blast and the expectation is they will be running cooler (though to me it seems they are almost "too" cool);
3. Immediately after shutdown I hit various points on cylinders 5, 6 (barrel, fins, spark plug) with an infrared thermometer and the temps indicated on the thermometer are directionally inline with the temps that the gauge is showing.


My gut tells me the gauge is working fine, the probes are working fine, it's just that cylinders 5 and 6 are running very cool, maybe too cool. Not sure what to do about that.

Thoughts? Suggestions? Guidance? Especially interested in observations from other folks running 6-cylinder CHT gauges.

BTW, I'll give a full review of the gauge at a later point, but thus far I like it a lot.

Thanks -

Brian

[counsellj]

Brian, That is completely normal for 5&6. They are right up front and the C-170 baffling didn't include the flap piece that sits along the front/bottom of the cylinders like on the C-172/O-300 installation. I typically see mid 200 degree CHTs on 5&6, while everything else is low to mid 300's. I have seen people put aluminum tape along the bottom from fins on 5&6 or modify their baffling to match C-172 design.

[jlwild]

Brian, many years ago my IA, during replacement of all engine baffles, installed front cylinder baffles on #5 and #6 as used on early Cessna 172 that have the same nose cowl as Cessna 170. They work great.

[brian.olson]

Jughead and Jim - thanks for the quick responses! Jim, I'll speak with my A&P to see if he has thoughts around the front baffle mod. Appreciate the suggestion.

Brian

[4583C]

Brian I installed an EI 6 probe EGT/CHT during annual this year and my experiences parallel yours. I have been contemplating the extra baffles.

[GAHorn]

Brian, can you post a pic from the front of your cowling? (And remind us which SERIAL NUMBER/model 170 you have?)

Items 1A and 7A were added to the early 172’s which resolved this issue.

[brian.olson]

George:

Thanks for providing the snapshot from the IPC. My 170A has serial 19716 and I can confirm the baffles do "not" include the two additional pieces (1A and 7A) that you referenced.

[cessna170bdriver]

Same issue, same numbers (summertime) with my later B-model (sn 26541). In the winter here in Michigan, my #5 & #6 CHTs struggle to reach 200F, even with winterization plates installed. We need to check Airforms website and see if they stock those 172 baffle parts.