FT-300 Compression
10-10-2005, 07:25 AM
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FT-300 Compression
Bax,
I just finished breaking in my new OS FT-300 Twin. With the 20-10 prop recommended for break-in, I can get about 6400 to 6500 rpm out of it. Does this seem reasonable for this engine with a 20-10? I'm going to run it next with an 18-10 which is what I will be using for my Christen Eagle, so expect the rpms to go up, but thought I would get a few more rpms out of it with the 20-10 than I am?
Along this line of thinking, I have ordered a compression gauge, so don't have any specific numbers to pass along yet. However when slowly turning the prop over, there is a noticeable difference in the compression between the two cylinders. The forward most cylinder has good compression and feels like it holds it fairly well, but the other cylinder feels like it has less compression, and does not seem to hold it very long. I ran the engine in according to the book, and did not have any heat or performance issues through the break-in period. I have checked the valve clearance to make sure they are closing properly, and have not noticed any oil around the head, so don't think I have a head gasket issue. I'm beginning to wonder if I have a bad ring since I noticed the compression difference between the two cylinders right out of the box when the engine was new. Guess I was thinking that after everything worked itself in during the break-in period, that things would seat better and the compression difference would work itself out. However I am still noticing the difference, so now wonder if there is something more serious wrong. I'm guessing there's not much you can tell me until I run compression checks on the engine and can give you some hard numbers, but in prep, can you tell me what I should expect for compression readings from each cylinder and how long it should hold that reading? Also, do you have any suggestions on things to check? Thanks................
I just finished breaking in my new OS FT-300 Twin. With the 20-10 prop recommended for break-in, I can get about 6400 to 6500 rpm out of it. Does this seem reasonable for this engine with a 20-10? I'm going to run it next with an 18-10 which is what I will be using for my Christen Eagle, so expect the rpms to go up, but thought I would get a few more rpms out of it with the 20-10 than I am?
Along this line of thinking, I have ordered a compression gauge, so don't have any specific numbers to pass along yet. However when slowly turning the prop over, there is a noticeable difference in the compression between the two cylinders. The forward most cylinder has good compression and feels like it holds it fairly well, but the other cylinder feels like it has less compression, and does not seem to hold it very long. I ran the engine in according to the book, and did not have any heat or performance issues through the break-in period. I have checked the valve clearance to make sure they are closing properly, and have not noticed any oil around the head, so don't think I have a head gasket issue. I'm beginning to wonder if I have a bad ring since I noticed the compression difference between the two cylinders right out of the box when the engine was new. Guess I was thinking that after everything worked itself in during the break-in period, that things would seat better and the compression difference would work itself out. However I am still noticing the difference, so now wonder if there is something more serious wrong. I'm guessing there's not much you can tell me until I run compression checks on the engine and can give you some hard numbers, but in prep, can you tell me what I should expect for compression readings from each cylinder and how long it should hold that reading? Also, do you have any suggestions on things to check? Thanks................
10-10-2005, 11:02 AM
#2
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RE: FT-300 Compression
Your RPM numbers are about right. With an APC 18 x 10 prop, we usually see about 7,300 RPM or so. RPM in the mid-to high 7,000's would be the upper limit you'd want to run that engine. Any prop turning in the mid-7,000's or so would be about right. Adjust the prop diameter/pitch size to get your airplane flying the way you want.
As far as compression is concerned, we really don't have any particular numbers. O.S. doesn't have a specification. If the compression between the two cylinders is very dramatic, then it's possible that one of the cylinders either didn't have the ring seat properly, or there was a bad piston/ring/cylinder fit to begin with. Usually, we run that engine at full throttle and as rich as possible and still keep it running for about the first 20-30 minutes. After that, we'll start to work the engine towards peak RPM. We'd consider the engine ready to fly after about 1-1.5 hours' worth of running on the bench. If the engine has a good top-end, a good idle, and good transition, then we don't worry about it. Many times, a compression "feel" is eliminated because combustion pressures "blow" the ring out to make the seal...this is true for ALL ringed engines.
Only a good comparison of exhaust gas temperatures would tell you if there's a lot of difference in the actual running.
As far as compression is concerned, we really don't have any particular numbers. O.S. doesn't have a specification. If the compression between the two cylinders is very dramatic, then it's possible that one of the cylinders either didn't have the ring seat properly, or there was a bad piston/ring/cylinder fit to begin with. Usually, we run that engine at full throttle and as rich as possible and still keep it running for about the first 20-30 minutes. After that, we'll start to work the engine towards peak RPM. We'd consider the engine ready to fly after about 1-1.5 hours' worth of running on the bench. If the engine has a good top-end, a good idle, and good transition, then we don't worry about it. Many times, a compression "feel" is eliminated because combustion pressures "blow" the ring out to make the seal...this is true for ALL ringed engines.
Only a good comparison of exhaust gas temperatures would tell you if there's a lot of difference in the actual running.
10-10-2005, 12:22 PM
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RE: FT-300 Compression
Bax, In your response you stated that the combustion may actually cause a better ring seal than I have with just hand turning the engine over, so is it worth checking the compression with a gauge? You also mentioned exhaust temp as a check. Can I check that by just checking header temps against each other, and what would this tell me if one cylider is running cooler or hotter than the other? Also, what is a normal running temp and what would be considered non-normal? Thanks
10-11-2005, 09:48 AM
#4
My Feedback: (11)
RE: FT-300 Compression
Because there aren't any specs, a gauge would only give you a comparison. Same for exhaust gas temperatures. You'd measure the exhaust stream as it exits the cylinder and adjust the mixture to peak it. You'd compare both cylinders. Adjust to the hotter one. Then see what you get.
Checking cold compression is a guide to helping you find a problem. If one cylinder's 'way down, then there's a problem that needs to be investigated. The two should be fairly close.
Checking cold compression is a guide to helping you find a problem. If one cylinder's 'way down, then there's a problem that needs to be investigated. The two should be fairly close.
