Question for Paul McBride, the General Aviation News engines expert: Is it possible at 3,000-plus feet to lean your engine to the point where it could cause cracking in the exhaust port?
I have a Lycoming O-320-E2C with 9:1 pistons with 65 hours on them (with a total time of 120 hours) and corrosion was found in all four cylinders. The cylinders were pulled and they were sent off to be chrome plated. All four cylinders failed, had cracks in the exhaust port, valve guides were out of tolerance, and valve seats out.
Before I go back to the mechanic and discuss these failures, especially on a low time motor, I wanted to have some idea that if I’m in cruise for 1.5 to 2 hours @ 3000-plus feet with motor lean out, would this cause the temperature to get high to cause this exhaust port cracking?
I believe my indicator that shows exhaust temperatures shows the highest of any cylinders is in the low 1,300° range.
Appreciate your feedback,
Bill
Answer: Bill, when I read the first sentence, my first thought was….no way. Then after I continued to read the information you provided, I had second thoughts.
This may not come as good news for you, but it’s my honest opinion that the results you received from the cylinder shop regarding the condition of your cylinders comes as no surprise. I feel that the condition of these cylinders was a direct result of extremely high operating temperatures during operation.
Going a little further, I think I can point my diagnostic finger at the fact that your engine was originally certified at a maximum of 150 hp @ 2,700 rpm with a compression of 7.00:1.
It would be difficult for me to even begin to calculate the horsepower that you might be pulling out of this engine with the 9:1 compression ratio pistons without knowing other facts like engine operating RPM. My guess would be it’s going to be greater that the certified 2,700 rpm, and that’s without considering the accuracy of your tachometer.
So, my thoughts are that the condition of your cylinders was most likely caused by the higher compression ratio pistons and I suspect higher engine RPMs.
Now for my gut feeling: I suspect the cause of all of this would be that when the higher compression ratio pistons were installed, the carburetor was never compensated to provide a higher fuel ratio to support the higher horsepower being taken out of the engine.
It all goes back to the old saying “there is no such thing as a free lunch.”
You must always remember the Lycoming rule of thumb: You may lean any Lycoming engine at any altitude, providing you are not exceeding a calculated 75% power. Do you think you may have been exceeding this rule?
I’d like to caution you as you move forward with this project. Whatever you decide to do with regard to engine modification while looking for more horsepower, be absolutely certain that the fuel that you’re putting in is capable of supporting the horsepower you are taking out of the engine.
In addition to that, use caution when leaning at any altitude and make certain you don’t exceed 75% power of the engine when leaning.
CHT is the result of EGT offset by ambient airflow volume and temperature.
Exhaust cracking is the result of thermal and/or mechanical stress, or a defectively produced component.
EGTs don’t tell you how hard the cylinder is working or how much stress the engine is under. For that you need to monitor your CHTs.
Insufficient information but that’s a lot of damage, along with corrosion, for 65 hours.
I don’t think that the carb jetting is the problem. the engine is still 320 cubic inches so the carb is seeing the same air volume and velocity. It is just compressing the mixture more to get more power from the fuel.
Also, the O-320-E2B and D use the same carb but have 8.5:1 compression, to allow it to make 160 HP vs 150 HP.
The added compression with the 9:1 pistons will get about 6-8% increase in HP, putting it at about 160 hp., from 150 hp.
There is no mention of the cylinder temps, which is what will cause cracks.
So, my guess is that poor baffling is causing excessively high cylinder temps, which will lead to cracks, usually at the spark plug threads.
I concur with Jim H. My recollection is that the stc regarding the compression increase limits the take-off power to redline rpm. With increasing altitude, as rpm decreases, the throttle can be advanced thus maintaining sea level hp slightly longer in the climb. Of course, cruise power will be limited as normal. Exceeding redline (tach error is a factor) due to power/prop combination can be the cause of heat damage.
Absolutely a great assessment of his problem, I’m very impressed 👏.