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How to compression check with engine out

 
Old 12-20-2018, 12:50 PM
  #16  
backitoff
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Ok thanks for the heads up regarding the belt.

Based on the information you guys have given so far, It seems like a ‘Leak down’ rather than compression test would be the way to go?
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Old 12-20-2018, 12:52 PM
  #17  
Speedtoys
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Originally Posted by SwayBar View Post
We are talking leakdown numbers, not compression results.


I've seen plenty of older engines record 100/100.


You really have no idea of what you're talking about, and yet, you keep posting like you do.

Why don't you do some research on automotive leakdown numbers and only then come back and report your findings.
I do them on my own cars.

And the planes that I fly..and I track the values over the years every annual inspection.

Ive not suggested you dont know what you're talking about, why dont you try explaining why I'm wrong.

Im not going out of my way to call you stupid....
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Old 12-20-2018, 01:40 PM
  #18  
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Originally Posted by Speedtoys View Post
I do them on my own cars.

And the planes that I fly..and I track the values over the years every annual inspection.

Ive not suggested you dont know what you're talking about, why dont you try explaining why I'm wrong.

Im not going out of my way to call you stupid....
Firstly, the tolerances on aircraft engines are WAY looser, by design, than automotive engines. Therefore aircraft engine specs do not apply to autos, and vice versa.

Here's one example of typical leakdown numbers for a healthy auto engine:

https://rennlist.com/forums/993-foru...n-numbers.html
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Old 12-20-2018, 02:18 PM
  #19  
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I performed my leak down testing with the engine fully warmed up and measured about 2% on each cylinder. As I understand anything up to about 5% is considered good and numbers north of 10% are worry bead time. What difference a cold engine will make to those numbers I do not know but the leakage can be expected to be more.
The thread I generated can be found here for anyone interested:

https://rennlist.com/forums/928-foru...n-testing.html

No idea what to expect on an aircraft engine. I have also wondered if throwing a thimble full of oil into the bore might be a way to "fudge" poor numbers thus why I prefer a warm engine with engine oil in the bores to give the most reliable results. As with a compression test, consistent results being more important than the absolute values.
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Old 12-20-2018, 03:11 PM
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Fred: I Sent you a PM, I think I see the difference between whats being stated here.

Its a tooling issue.
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Old 12-21-2018, 09:43 AM
  #21  
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Originally Posted by Speedtoys View Post
Fred: I Sent you a PM, I think I see the difference between whats being stated here.

Its a tooling issue.
Jeff,

This subject is actually quite an interesting point. What the typical leak down rig is measuring indirectly is air flow. Between the two gauges there is a restriction orifice and the pressure drop across said item is an indirect measure of the flow.

For this to be meaningful there ought to be a standard specification for the orifice dimensions so that all test rigs measure the same. Whether or not this happens in the real world I do not know but I suspect it does otherwise the leak down results would be meaningless.

The only spec I have come across [ironically] was an FAA spec that calls for an internal diameter of 40 thou [1.0 mm] and a quarter of an inch long [6.4mm]. Whether or not this is a "globally accepted standard" I do not know. If it was not, talking about leak down rates as a percentage would be meaningless. Somebody somewhere must have reasoned out these dimensions to give a flow that will create sufficient pressure differential to be meaningfully measured. For aircraft engines following this protocol the idea is to set the inlet pressure at 80 psig and a resultant downstream pressure of 60 psig is considered the minimum that can be considered acceptable [for aircraft piston engines]. This case would thus be considered to be 25% leak down so not hard to understand where you got such numbers from.

Car engines are [presumably] made to much tighter clearances as has been covered earlier and more so the likes of high performance Porsche units so leak down values like the 2% mine tested at are quite likely indicative of a very healthy motor sealing wise. Anything beyond 10% would have me concerned as to why I was seeing that number. I took a look at my rig and there was nothing to indicate what the dimensions of the installed orifice are or what spec it is built to but I suspect they may well be in line with the FAA spec unless someone knows otherwise.

I then did some quick calcs and concluded that to get a pressure drop of 1.5 psi with an inlet pressure of 80 psig across an orifice of that FAA specified size the air flow rate [leakage] would be in the region 350 litres of air per hour at pressure. Whether or not that makes sense remains to be seen but it sounds kind of reasonable to me given it equates to about 4 litres every 10 seconds at atmospheric pressure or about 0.5 litres per second in order of magnitude terms.
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