RV8Squaz
Well Known Member
This text below is from an email I sent to my partners in a F33A Bonanza. The Bonanza has a Continental IO-550B. I also own an RV-8 with a 200hp IO-360A1A. I've been doing my own compression checks since I got the airplane flying more than 10 years ago. Yet, I still learned something new. I'm not an expert at this. I haven't spoken with any engineers at TCM or Eastern Technology. Please let me know if you see any inaccuracies, but the info below I feel is correct and is FYI.
"Hi Guys,
It seems like since we started with the pre-buy inspection, we have been getting very different results each time we have had the differential compression tests done. As you know, when the pre-buy was done we had a couple of cylinders that failed. Later, when we were getting the spar checked in Waco, we had another compression check and it was miraculously better. We even did our own compression check with excellent results. And finally Bob did a a compression check with very poor results. The mystery has now been solved.
Eastern Technology makes basically four different testers. Two do not have the Master Orifice feature and two do. They all have an internal orifice. The Master Orifice is used to establish the minimum acceptable pressure for that day's atmospheric conditions. The models are:
E2A: No master orifice, internal orifice of .040
E2A-1000: No master orifice, internal orifice of .060
E2M: Yes master orifice, internal orifice of .040
E2M-1000: Yes master orifice, internal orifice of .060 This is the one I have.
Lycoming does not specify the use of a master orifice to establish minimum acceptable compression. Continental does. This minimum is typically about 40-45 psi and is determined with free flowing air without hooking up the tester to a cylinder by opening the master orifice and reading the right pressure gage with the left pressure gage at 80 psi.
So how do you know whether to use one with a .040 orifice vs one that is .060? Well it clearly states on the Eastern Technology?s website that with cylinder bores less than 5.00? you use the .040 orifice tester and over 5.00? you use the one with the .060 orifice.
http://www.easterntech.com/e2m.html
The bore on the IO-360 on my RV-8 is 5.125.? This is why I have the E2M-1000. I bought it with Master Orifice feature because if I ever owned an airplane with a Continental, it would save me from having to buy another tester. The bore on our TCM IO-550B is 5.25.? Perfect.
Now that I know how to use the Master Orifice (MO) feature I decided to give it a try the other day. I didn?t really know how to use it when we did our own compression check. So I checked the use of the MO and I got a minimum acceptable pressure of about 68 psi! What! How can that be? My friend Jon and I discussed it and we assumed there was something wrong with my tester. It should be in the low 40s.
Today, I went to check on the Bonanza, it's getting an annual inspection, and see what kind of progress Bob, the shop owner was making. It was cool to see six shiny and bright, brand new cylinders on the table! I saw his compression test results on the squawk sheet and some were in the 20s. Yikes! How can that be? When we did it our compression check a couple of weeks ago, most of ours were in the 60s and even 70s. So I asked him about it and a discussion of .040 and .060 testers ensued. I told him I had the right one with .060 orifice and when I checked it I got a minimum pressure of 68 psi. He told me that I was using the wrong one, that TCM specifies the use of the .040 tester. But wait, the bore is over 5.00.? What gives? Bob had several different testers in the shop and we checked the operation of the MO on his E2M and the E2M-1000. The E2M free-flowed at about 42 psi like we expected and his E2M-1000 free-flowed 68 psi just like mine!
OK, now I?m confused. I had the studied the appropriate SB and there is no mention of what size orifice tester to use. Was I wrong to use the tester designed for bores over 5.00??
http://www.easterntech.com/servicebulletin.pdf See Part B.
So once again I spoke with my friend Jon, about my conversation with Bob, the results from our simple experiment of testing the MO on the two testers, and the SB above. Jon said that, a while ago Continental had incorporated a bunch of their old Service Bulletins and Instructions into this manual:
http://sydney.edu.au/engineering/aeromech/AERO3460/Additional Documents/IO-550-manual.pdf
I went digging and lo and behold on page 6-57 is this excerpt:
"The Eastern Technology web site (eastertech.com) indicates a Model
E2M-1000, with a 0.060 Master Orifice should be used on cylinders with
a bore greater than 5.0 inches however, approved type certificate data
pertaining to engines in this manual were approved by the FAA using a
Model E2M Differential Pressure Tester with a 0.040 inch Master Orifice.
? Eastern Technology Corporation Model E2M Cylinder Differential Pressure Tester
(Figure 6-27). This Differential Pressure Tester incorporates a 0.040 inch Master
Orifice Tool"
Go figure. Bob was right. It is indeed specified.
OK back to the actual compression check. If you hook up either tester into a cylinder with no leaks, both testers would indicate a pressure of 80/80 psi. However if you hook up each tester to a cylinder with some leakage, the tester with the larger orifice will flow more air and will consequently indicate a higher pressure than the tester with the smaller orifice which is flowing less air and will indicate a lower pressure. And there you have it. My guess is that James, the pre-buy guy, used the proper tester, the shop in Waco used the wrong tester, we used the wrong tester, and Bob used the correct tester.
So what about the the IO-360 on my RV-8? I don?t know. My bet is that I would get lower compression results with the .040 tester than with the .060 tester. This could explain why my compressions have always been in the high 70s.
Have fun interpreting all of this!"
So why 6 new cylinders? In addition to the compression checks that have been done since the pre-buy, borescope revealed two cylinders with significant exhaust valve discoloration and two more with some discoloration indicating burned valves. This is not surprising since the engine has 1100 hours SMOH and the GAMI spread was almost 2 gph. Who knows how the engine has been operated over the last 23 years! We bought the airplane at a significant discount knowing the condition of the cylinders.
"Hi Guys,
It seems like since we started with the pre-buy inspection, we have been getting very different results each time we have had the differential compression tests done. As you know, when the pre-buy was done we had a couple of cylinders that failed. Later, when we were getting the spar checked in Waco, we had another compression check and it was miraculously better. We even did our own compression check with excellent results. And finally Bob did a a compression check with very poor results. The mystery has now been solved.
Eastern Technology makes basically four different testers. Two do not have the Master Orifice feature and two do. They all have an internal orifice. The Master Orifice is used to establish the minimum acceptable pressure for that day's atmospheric conditions. The models are:
E2A: No master orifice, internal orifice of .040
E2A-1000: No master orifice, internal orifice of .060
E2M: Yes master orifice, internal orifice of .040
E2M-1000: Yes master orifice, internal orifice of .060 This is the one I have.
Lycoming does not specify the use of a master orifice to establish minimum acceptable compression. Continental does. This minimum is typically about 40-45 psi and is determined with free flowing air without hooking up the tester to a cylinder by opening the master orifice and reading the right pressure gage with the left pressure gage at 80 psi.
So how do you know whether to use one with a .040 orifice vs one that is .060? Well it clearly states on the Eastern Technology?s website that with cylinder bores less than 5.00? you use the .040 orifice tester and over 5.00? you use the one with the .060 orifice.
http://www.easterntech.com/e2m.html
The bore on the IO-360 on my RV-8 is 5.125.? This is why I have the E2M-1000. I bought it with Master Orifice feature because if I ever owned an airplane with a Continental, it would save me from having to buy another tester. The bore on our TCM IO-550B is 5.25.? Perfect.
Now that I know how to use the Master Orifice (MO) feature I decided to give it a try the other day. I didn?t really know how to use it when we did our own compression check. So I checked the use of the MO and I got a minimum acceptable pressure of about 68 psi! What! How can that be? My friend Jon and I discussed it and we assumed there was something wrong with my tester. It should be in the low 40s.
Today, I went to check on the Bonanza, it's getting an annual inspection, and see what kind of progress Bob, the shop owner was making. It was cool to see six shiny and bright, brand new cylinders on the table! I saw his compression test results on the squawk sheet and some were in the 20s. Yikes! How can that be? When we did it our compression check a couple of weeks ago, most of ours were in the 60s and even 70s. So I asked him about it and a discussion of .040 and .060 testers ensued. I told him I had the right one with .060 orifice and when I checked it I got a minimum pressure of 68 psi. He told me that I was using the wrong one, that TCM specifies the use of the .040 tester. But wait, the bore is over 5.00.? What gives? Bob had several different testers in the shop and we checked the operation of the MO on his E2M and the E2M-1000. The E2M free-flowed at about 42 psi like we expected and his E2M-1000 free-flowed 68 psi just like mine!
OK, now I?m confused. I had the studied the appropriate SB and there is no mention of what size orifice tester to use. Was I wrong to use the tester designed for bores over 5.00??
http://www.easterntech.com/servicebulletin.pdf See Part B.
So once again I spoke with my friend Jon, about my conversation with Bob, the results from our simple experiment of testing the MO on the two testers, and the SB above. Jon said that, a while ago Continental had incorporated a bunch of their old Service Bulletins and Instructions into this manual:
http://sydney.edu.au/engineering/aeromech/AERO3460/Additional Documents/IO-550-manual.pdf
I went digging and lo and behold on page 6-57 is this excerpt:
"The Eastern Technology web site (eastertech.com) indicates a Model
E2M-1000, with a 0.060 Master Orifice should be used on cylinders with
a bore greater than 5.0 inches however, approved type certificate data
pertaining to engines in this manual were approved by the FAA using a
Model E2M Differential Pressure Tester with a 0.040 inch Master Orifice.
? Eastern Technology Corporation Model E2M Cylinder Differential Pressure Tester
(Figure 6-27). This Differential Pressure Tester incorporates a 0.040 inch Master
Orifice Tool"
Go figure. Bob was right. It is indeed specified.
OK back to the actual compression check. If you hook up either tester into a cylinder with no leaks, both testers would indicate a pressure of 80/80 psi. However if you hook up each tester to a cylinder with some leakage, the tester with the larger orifice will flow more air and will consequently indicate a higher pressure than the tester with the smaller orifice which is flowing less air and will indicate a lower pressure. And there you have it. My guess is that James, the pre-buy guy, used the proper tester, the shop in Waco used the wrong tester, we used the wrong tester, and Bob used the correct tester.
So what about the the IO-360 on my RV-8? I don?t know. My bet is that I would get lower compression results with the .040 tester than with the .060 tester. This could explain why my compressions have always been in the high 70s.
Have fun interpreting all of this!"
So why 6 new cylinders? In addition to the compression checks that have been done since the pre-buy, borescope revealed two cylinders with significant exhaust valve discoloration and two more with some discoloration indicating burned valves. This is not surprising since the engine has 1100 hours SMOH and the GAMI spread was almost 2 gph. Who knows how the engine has been operated over the last 23 years! We bought the airplane at a significant discount knowing the condition of the cylinders.