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henry.voris
Joined: 02 Mar 2007
Posts: 116
Location: Pueo Field, Kula, Maui
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 Posted: Fri Jan 13, 2012 11:41 pm Post subject: He assumed I knew what I was doing… |
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13jan12
Gentlemen,
My pal Lance asked, “So… Do you wanna put the big piston in the front or the rear?”
I assumed he was kidding… He assumed I knew what I was doing… A train-wreck of assumptions. Deliberating shortly, we figured the larger piston would run hotter and should live closest to the fan.
After a couple of hours of swearing, doing-it-again, and fumbling around we got the thundering 447 back together… I’m sure we got most of those dang needle bearings back in the wristpins…
Back on the plane, the engine fired right up and ran strong… But the cylinder closest to the PTO (#2) was running a bit hotter than the cylinder closest to the alternator (#1). At 5,000 RPM and above, the temperature difference (both CHT and EGT) exceeded the difference allowed by the factory (36º CHT and 45º EGT). I have attached a spreadsheet that reflects the temperatures at different RPMs.
I got back to Lance and he said that he had not been kidding about two sizes of pistons… I was sick.
Purchased a dandy little bore-scope from Aircraft Spruce and with more swearing and fumbling about I could see what I have (done)…
Cylinder #1, Closest to the alternator, 67.46, green dot
Cylinder #2, Closest to the PTO, 67.45, red dot
Cylinder #2 is hotter… that shoots my theory about the bigger piston running hotter…
Also the top of piston #2 has accumulated a coating of carbon in only 5 hours of operation, while piston #1 is much cleaner. (Photos attached) I have run only AV-2 two-stroke oil in an effort to minimize carbon build up… I’m somewhat disappointed.
In reviewing the spreadsheet… It appears that each piston is operating within the temperature limits set by the factory for operation. The problem is the temperature difference between cylinder #1 and #2.
Before installing the digital engine monitor, I only knew the temperatures on one cylinder… I would have never have seen this problem and would probably be having a great time out flying, right now.
When Rotax sets limits for the difference of temperature allowable between cylinders, I’m pretty sure they are figuring on both pistons being the same size… (Krauts are like that…) A larger than normal temperature difference in an engine with matching pistons could indicate a problem… However, in an engine with different sized pistons, perhaps this temperature difference should simply be expected.
Comments?
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_________________
Henry
Firefly Five-Charlie-Bravo
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Amy57
Joined: 17 Jan 2012
Posts: 2
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| Posted: Tue Jan 24, 2012 3:43 pm Post subject: Re: He assumed I knew what I was doing… |
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That's crazy, I had no idea Rotax made engines with different sized pistons! ha
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