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procharger

Compression test

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I know how to do it the other way the motor is more like a car or motorcycle engine than

an airplane motor. I think either way tells you what you want to know. Just wanted to

know what others might be getting for numbers.

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Compression Test on a Rotax 912UL

Hi all,
What is the TBO for a 912ul and what compression readings would you expect in a static test (PSI).
I have been looking at a 912 with 1750 hours and the cylinder readings are.
1 = 130psi
2 = 126psi
3 = 130psi
4 = 125psi
I cannot find any figures for a static test only the one using compressed air.
Any Ideas ?

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I have been maintaining aircraft for over 45 yrs, I think at this point I know how to do a compression check. From your comment I see why most aviation professionals tell me they do not take these forums seriously, including some senior people at Flight Design and other aircraft companies. As an IA and Aeronautical engineer I look at this forum for new information and try to engage in constructive conversation. You however are a good example of why I feel that this forum has become a joke and not considered worth my time anymore.

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58 minutes ago, Madhatter said:

. . . I feel that this forum has become a joke and not considered worth my time anymore.

If that is truly the case, perhaps it's time to move on. An attack on the forum itself . . is uncalled for.

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I suggest you talk to some of the principal players in this LSA industry. I come to this forum for accurate information and offer information based on my background and experience. I deal with professional aviation engineers, CEO's, etc all the time and they do not act like the guy above. This is not the kind of forum that I wish to be associated with and will no longer participate. 

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20 hours ago, Madhatter said:

I suggest you talk to some of the principal players in this LSA industry. I come to this forum for accurate information and offer information based on my background and experience. I deal with professional aviation engineers, CEO's, etc all the time and they do not act like the guy above. This is not the kind of forum that I wish to be associated with and will no longer participate. 

Well...this is a forum designed around CT pilots and owners, not CEOs and aeronautical engineers.  I think there is a lot of good information here, but it's the internet and sometimes you get people that are misinformed and/or you won't get along with.  That doesn't make it a bad forum, it just means there are actual human beings involved.

I'd love it if there was an official FD forum with news, technical information, and opinion areas on how to fly there fun airplanes.  But there's not, and this has been the next best thing for many years.  Given the lack of a support forum for CTs, I find it puzzling that FD doesn't have more presence here providing news, dispelling misinformation, etc.  Oh well.

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Back to the subject at hand...it would be educational to hear from a 45 year veteran about the differences between the direct vs differential compression tests and why differential is preferred. I have some ideas, i.e. accuracy and diagnostic ability, but I’m no expert. 

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One thing I have noticed, the advertisers at the bottom of the forum page have not updated anything in many years. As a buisiness owner for many years that tells me they do not consider this forum an asset for them. If you look at some other forums such as the RV forum there is a lot of direct input from companies such as Dynon, Garmin and others. Just my observation. 

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The differential compression test, is more commonly referred to as a leak down test,  Typically the leak down test numbers are presented as a percentage of the test pressure. In this case, rotax wants you to state the actual number recorded

Since the compression tester relies on the engine cranking, several uncontrolled variables are introduced which can make the results less useful. A weak battery, a corroded starter cable, or a hot starter can all cause the engine to crank slower, which can affect the test results. Even air density and valve lash can affect the readings. For this reason, it's next to impossible to find suggested compression test results for most engines. On top of that, racing engines are usually modified to have higher compression anyway, so you can't rely on a factory manual for the answer.

The use of a controlled, regulated compressed air source makes a leak-down test much more consistent and repeatable. This means that a leak-down tester(read: differential compression tester) can be used to show when an engine is in need of a rebuild due to wear. If the percentage of leakage in an engine goes up from one test to the next (especially if all cylinders increase a similar amount), you can be reasonably confident that it is due to a loss of cylinder sealing. Depending on your budget and how much power you are willing to give up, you will probably want to rebuild the engine when leak-down reaches what ever Rotax recommends. Fresh engines should not leak more than about 3%. Many engine builders consider 5% to still be quite healthy.

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Well said. This is the kind of info that should be on the forum, straight to the point and no long manifestos. Give this man the cigar. 😎

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I’ve not witnessed this, but heard that you can sometimes figure out where the leak is occurring (exhaust valves vs intake valves vs rings) with the differential test as well. 

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 Rotax maint. manual it said the numbers on direct compression should be between 130 and 174

and that it can be done either way from a 5 year old, some may need to look it up for reference.Doing

the check on old antique airplane engines would be the preferred way I would think, for those who tell me

I've been doing it that way all my life I say to you maybe you been doing it wrong. I only asked for some

numbers, if you didn't know the answer perhaps you should stay out of the conversation.  nuff said

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This forum is great I have learned a lot since owning my CT, thanks to all have added their experiences with these

NEW modern engines and planes, I to have 50 years experience working on boat motors, motorcycles, high speed

packaging machinery, mowers, RC plane engines and every other kind of engine there is, no degrees just real world

experience . I learned a long time ago that other people have good ideas to and for that I am grateful.

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1 hour ago, procharger said:

 

 Rotax maint. manual it said the numbers on direct compression should be between 130 and 174

and that it can be done either way from a 5 year old, some may need to look it up for reference.Doing

the check on old antique airplane engines would be the preferred way I would think, for those who tell me

I've been doing it that way all my life I say to you maybe you been doing it wrong. I only asked for some

numbers, if you didn't know the answer perhaps you should stay out of the conversation.  nuff said

You can do it which ever way suits your fancy, but according to the rotax Line maint manual, Section 12-20-00 page 13

 

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image.thumb.png.90a145dc79b9d207fd43a94c7b2aeb93.png

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The simple answer to Procharger is given by the compression ratio which on the 100 HP is 10.8. So if you take atmospheric pressure of 14.6 PSI and multiply by 10.8 the answer is 145 psi.

I believe that it will be slightly higher if you are turning over the engine at a higher speed with the starter but if you are just pulling it through slowly by hand then 145 is the max possible. (standard altimeter setting of 29.92).

Larry

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2 hours ago, procharger said:

 

for those who tell me I've been doing it that way all my life I say to you maybe you been doing it wrong. I only asked for some

numbers, if you didn't know the answer perhaps you should stay out of the conversation.  nuff said

Let's be fair here.  This is a *discussion* forum.  It's not a "tell me only the info I ask for and otherwise shut the hell up" forum.  If somebody honestly thinks you're doing something incorrectly do you NOT want them to tell you?  I sure would.  I'd feel free to ignore their advice, but if somebody has a legitimate concern about my methods, I want to hear it.  That's the only way we get better and correct mistakes.

Doing something one way your whole life is no guarantee that you're doing it the right way.  We see this in aviation all the time and it sometimes gets people hurt or killed.  Look at how many old timers have hand-propping accidents, even though they have "always done it that way"...

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