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Interesting Observations - Dynamic Compression Testing

Bicyclops said:
A leak down test is the method specified in AC 43.13 and is especially helpful in determining where the pressure loss is occurring. Pulling a cylinder if it isn't above 60/80 as Lycoming long recommended (Service Instruction No. 1191A) may be unnecessary, expensive, and potentially cause bearing problems if the torque isn't done correctly on re-install. Continental published a service bulletin (SB03-3) addressing low results in compression tests. They have data showing 100% of rated horsepower produced by test engines with leak down readings in the low 40s. The main thrust of their SB is to identify the source of the leakage via sound and verify valve condition via borescope. If it is valves, something needs to be done, but moderate ring leakage doesn't keep an engine from making power, though the oil consumption is likely to be high. A dynamic or pump up test comes closer to showing if a cylinder is unusable because of ring leakage. Compression, combustion, and power will happen fast enough that leakage you would see in a differential pressure test may be pretty much moot. If it will pump up to some reasonable value, and if it's similar to the opposing cylinder, operating the engine is probably safe.

Ed
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Just a data point as to how bad it can get, years ago, when I purchased my first airplane (1957 C-172), I learned a lot. The purchase was from a family acquaintance, who was an A&P. Known the guy for a long time. Airplane came with a fresh annual, so no pre buy (lesson #1). Long story short, we had many issue with the airplane and decided to do a mid year inspection. Compression test yielded 68, 20, 0, 40, 0, 20 (O-300).

Decided to rebuild the engine early, it was mid-high time. Sent the cylinders to Gibson. Get a call back saying the bad news is three cylinders are cracked and can't be welded. The good news was that three cylinders were cracked but could be welded.

We had flown the airplane based on an A&P owner's sign off which was obviously pencil whipped. (lesson #2)

Point of the story is that the engine still ran, even with those abysmal numbers...

Lesson #3 was that you should always be willing to walk away from a purchase.
 
(Small) Drift Warning.

I'll ask the hive mind here. There was a documented demonstration where one of the OEMs or big OH shops ran an engine with no piston rings. The reported power loss was negligible. Does anyone remember where that was documented? My best guess is the old Sky Ranch Engineering Manual. Anybody else ever recall reading this/where you read it?
 
Freemasm said:
(Small) Drift Warning.

I'll ask the hive mind here. There was a documented demonstration where one of the OEMs or big OH shops ran an engine with no piston rings. The reported power loss was negligible. Does anyone remember where that was documented? My best guess is the old Sky Ranch Engineering Manual. Anybody else ever recall reading this/where you read it?
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Probably didn't need a separate smoke system... 😀
 
Not just one person. I was shown a set of dynamic compression gauges for a radial engine a while ago. 9 gauges total, with short hoses to connect them to each spark plug hole. I was told you screw themmin, and kick the starter for as many turns of the prop as there are cylinders. Presumably there is a book somewhere that says what the readings should be, and what the procedure is for using them.

That's what prompted me to build my own set, with recording gauges and hoses from Princess Auto (Harbour Freight for Canadians). So far I've always used them in parallel with a conventional leakdown test.
 
Time to resurrect this thread. Today, I did a dynamic and differential test on my 1036-hour-since-overhaul IO-360-Axx (8.7:1). I overhauled this engine in 2008 and sent the original, 2100 hr-since-new cylinders to ECI to get the nickel-carbide treatment. They are getting tired with the hours they have, with compressions and oil consumption not as good as earlier. With a cold engine, I got the following results.

Dynamic (cyl 1-4, PSIG):
205, 175, 195, 190

Differential (x/80 PSIG):
63, 64, 72, 65

Dynamic test was with 5 compression strokes each. I was quite surprised at the high values from an 8.7:1 engine. Apparently the cam timing is very conservative, as you'd expect with a 2700 RPM redline. This is with a Skytec LS starter and 4-yr-old PC680 battery and charger connected. Throttle WOT.

I was also surprised at the low correlation between the two testing methods: Cyl 1 had the highest dynamic pressure but also the highest leakage. All cyls had a slight trace of oil at the bottom of the bore, as typical in these engines.
 
hgerhardt said:
Dynamic (cyl 1-4, PSIG):
205, 175, 195, 190
Differential (x/80 PSIG):
63, 64, 72, 65
I was also surprised at the low correlation between the two testing methods: Cyl 1 had the highest dynamic pressure but also the highest leakage. All cyls had a slight trace of oil at the bottom of the bore, as typical in these engines.
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It would be interesting to measure the intake lift between 205 & 175 cyl's.
Would a 30# spread indicate a failing cam lobe? What else would cause this spread?
 
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The dynamic compression test is problematic because it depends heavily on the cranking rate. When you rapidly compress the air in the cylinder the air heats up and this heating increases the pressure. It's common to see compression pressures that are higher than the compression ratio x atmospheric pressure.

It's possible to use as a comparison of cylinders on a single engine, but make sure that the cranking rate stays constant.
 
gasman said:
It would be interesting to measure the intake lift between 205 & 175 cyl's.
Would a 30# spread indicate a failing cam lobe? What else would cause this spread?
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Leakage

Lift has no significant impact on pressure at the low testing rpms, though throttle position does. Intake valve closing angle though could influence pressure. The later the valve closes, the lower the pressure will be. However, A worn cam lobe should in theory close the valve earlier, raising pressure. That is if the lobe’s ramps wear like the tips do and not sure they do. I have a worn cam from lifter spalling that i just pulled and could measure.

The poster didn’t tell us where the air was escaping on the static test, so we don’t know if there is constant leaking past the valves. These tests can be misleading. Steady state they seal fine, but with loose guides, the valves may rock and poorly seal once the rpms come up. With a dynamic test, you are observing the ring wall interface quality across the whole cylinder, which could be different across its area. You also subject the rings to movement in the lands that can change effectiveness. Well all see that when we rock the engine while doing a static test.
 
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BrandonDixon said:
The dynamic compression test is problematic because it depends heavily on the cranking rate. When you rapidly compress the air in the cylinder the air heats up and this heating increases the pressure. It's common to see compression pressures that are higher than the compression ratio x atmospheric pressure.

It's possible to use as a comparison of cylinders on a single engine, but make sure that the cranking rate stays constant.
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But that has nothing to do with comparative testing. The dynamic test is not intended to match the theoretical CR. It is designed to identify weak cylinders among the group and to compare to known good results from exact replica engines (same displacement, same CR and same cam profile). Sadly, we should know a good pressure range for a healthy io 360 with 85.5:1 pistons, but the industry doesn’t provide that data because the industry don’t use that testing standard.:(
 
gasman said:
It would be interesting to measure the intake lift between 205 & 175 cyl's.
Would a 30# spread indicate a failing cam lobe? What else would cause this spread?
Click to expand...
Interesting theory and easy enough to check.
BrandonDixon said:
........It's possible to use as a comparison of cylinders on a single engine, but make sure that the cranking rate stays constant.
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Exactly. Since we all have such different setups with starters and batteries etc, hardly any RV's are alike. So the dynamic test is only good for comparing cylinders of one engine.
lr172 said:
Leakage

Lift has no significant impact on pressure at the low testing rpms, though throttle position does. Intake valve closing angle though could influence pressure. The later the valve closes, the lower the pressure will be. However, A worn cam lobe should in theory close the valve earlier, raising pressure. That is if the lobe’s ramps wear like the tips do and not sure they do. I have a worn cam from lifter spalling that i just pulled and could measure.

The poster didn’t tell us where the air was escaping on the static test, so we don’t know if there is constant leaking past the valves. These tests can be misleading. Steady state they seal fine, but with loose guides, the valves may rock and poorly seal once the rpms come up. With a dynamic test, you are observing the ring wall interface quality across the whole cylinder, which could be different across its area. You also subject the rings to movement in the lands that can change effectiveness. Well all see that when we rock the engine while doing a static test.
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I'll compare valve lifts later this week. And all the air escaped past the rings. Valves seal well.

Remember these are Cermi-Nil cylinders with over 1000 hours, and they're going downhill. 150 hours ago, I was still getting 70's in differential compression.
 
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