Re: Right way to do Leak Down Test?
[Re: Smoparmike]
#2846479
11/15/20 11:20 AM
11/15/20 11:20 AM
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Joined: Jan 2003
Posts: 8,028 Tulsa OK
Bad340fish
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Tulsa OK
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Some people might say warm, I do everything cold. Cold is easy to be consistent. When you start a test warm it is cooling off every minute you work on it.
68 Barracuda Formula S 340
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Re: Right way to do Leak Down Test?
[Re: Smoparmike]
#2846523
11/15/20 12:14 PM
11/15/20 12:14 PM
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Joined: Jun 2006
Posts: 5,163 CT
GTX MATT
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I do everything cold as well.
Put the piston at TDC, true TDC. If you get it at true TDC the crank will not turn. But you may need to set up each cylinder 2-3 times to get it there. If its not there it will turn the crank. DO NOT attempt to put a breaker bar on the crank and hold it, you will get hurt. You would be amazed at the amount of force 100 PSI on a 440 piston generates (it is enough to drive through a well adjusted E brake about 1 foot every few minutes)
Also, don't expect 2-3% leak down numbers. I was never a huge believer in leak down tests, and as time goes on the results I've seen have pushed me more away from them. The best results I ever got was 2% "Hero" results on a late model engine that made 125 PSI cranking compression (150-160 would be ideal) and did not run that great.
Last edited by GTX MATT; 11/15/20 12:27 PM.
Now I need to pin those needles, got to feel that heat Hear my motor screamin while I'm tearin up the street
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Re: Right way to do Leak Down Test?
[Re: Bad340fish]
#2846625
11/15/20 03:19 PM
11/15/20 03:19 PM
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Joined: Oct 2007
Posts: 4,209 New York
polyspheric
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Cold is easy to be consistent.
If it's in the same heated garage. Ambient isn't consistent.
Boffin Emeritus
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Re: Right way to do Leak Down Test?
[Re: Smoparmike]
#2846651
11/15/20 04:04 PM
11/15/20 04:04 PM
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Joined: Aug 2003
Posts: 43,230 Bend,OR USA
Cab_Burge
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I do them both ways, dead cold after sitting overnight in the shop and then warmed up to at least 180F. I do it at TDC usually but you can test with the pistons down in the cylinders like piston engine aircraft mechanics due They remove all the plugs and use the prop to bring the piston up far enough so both valves are closed and due the test there Some will do it at TDC also but not all of them do that I use a aircraft gauge now, two gauges and reset the first gauge pressure so it has 80 lbs. pressure with air going into the cylinder being tested at TDC and read the pressure lost with the second gauge going to the cylinders with 80 lbs. on the first gauge
Last edited by Cab_Burge; 11/15/20 04:04 PM.
Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)
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Re: Right way to do Leak Down Test?
[Re: Cab_Burge]
#2846698
11/15/20 05:52 PM
11/15/20 05:52 PM
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Joined: Jan 2003
Posts: 25,886 Rio Linda, CA
John_Kunkel
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As an aircraft mechanic I've done lots of these, the publication AC 43.13-1b outlines the correct procedure. The test is done on a warm engine and, to start the test, air is introduced into the cylinder while the piston approaches TDC and is finalized with the piston at TDC. The airplane has an advantage because of the prop, you can grab a prop blade and manipulate the piston position instead of just parking it at TDC. This assures that the ring is fully seated against to bottom of the ring land.
The INTERNET, the MISinformation superhighway
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Re: Right way to do Leak Down Test?
[Re: polyspheric]
#2846879
11/16/20 08:08 AM
11/16/20 08:08 AM
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Joined: Jan 2003
Posts: 8,028 Tulsa OK
Bad340fish
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Cold is easy to be consistent.
If it's in the same heated garage. Ambient isn't consistent. Agreed, but the temp you start the test at will likely be the same temp you finish with, unless you are super slow lol.
68 Barracuda Formula S 340
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Re: Right way to do Leak Down Test?
[Re: John_Kunkel]
#2846880
11/16/20 08:10 AM
11/16/20 08:10 AM
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Joined: Jan 2003
Posts: 8,028 Tulsa OK
Bad340fish
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As an aircraft mechanic I've done lots of these, the publication AC 43.13-1b outlines the correct procedure. The test is done on a warm engine and, to start the test, air is introduced into the cylinder while the piston approaches TDC and is finalized with the piston at TDC. The airplane has an advantage because of the prop, you can grab a prop blade and manipulate the piston position instead of just parking it at TDC. This assures that the ring is fully seated against to bottom of the ring land.
That is interesting, and makes total sense. If you have a balancer that is marked every 90 agrees and you do your leak down test following the firing order you should be good on this aspect as long as you don't pass the mark and back it up.
68 Barracuda Formula S 340
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Re: Right way to do Leak Down Test?
[Re: Cab_Burge]
#2847099
11/16/20 03:47 PM
11/16/20 03:47 PM
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Joined: Jan 2003
Posts: 25,886 Rio Linda, CA
John_Kunkel
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From my experience, rocking back and forth from TDC a little does improve the test results.
The INTERNET, the MISinformation superhighway
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Re: Right way to do Leak Down Test?
[Re: John_Kunkel]
#2847105
11/16/20 03:54 PM
11/16/20 03:54 PM
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Joined: May 2019
Posts: 713 Lake Villa Il
INTMD8
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Lake Villa Il
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One thing I've noticed is tapping open a valve when the cylinder is under pressure (soft mallet) can lower the leakdown percentage as the pressure shock squares the pistons/rings in the bore.
69 Charger. 438ci Gen2 hemi. Flex fuel. Holley HP efi. 650rwhp @7250 510rwtq @5700
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Re: Right way to do Leak Down Test?
[Re: John_Kunkel]
#2847107
11/16/20 03:57 PM
11/16/20 03:57 PM
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Joined: Aug 2003
Posts: 43,230 Bend,OR USA
Cab_Burge
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From my experience, rocking back and forth from TDC a little does improve the test results. I've seen 3 to 6% The piston and ring design has a lot to do with leak down results. All the stock aircraft pistons I've seen have tapered rings sides, (not square or parallel like car motors have) like some H.D. diesel motors do, correct John ?
Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)
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Re: Right way to do Leak Down Test?
[Re: Greenwood]
#2847378
11/17/20 04:03 AM
11/17/20 04:03 AM
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Joined: Aug 2003
Posts: 43,230 Bend,OR USA
Cab_Burge
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I look for 15% or worst on my race motors and similar on hot street motors. That would be with 65 lbs. to the cylinder being tested with 80 lbs. out put pressure on my test gauge pressure
Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)
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Re: Right way to do Leak Down Test?
[Re: Greenwood]
#2848452
11/19/20 03:28 AM
11/19/20 03:28 AM
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Joined: Aug 2003
Posts: 43,230 Bend,OR USA
Cab_Burge
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I haven't done a compression tests in more years than I can remember, 20 yrs+ I do remember doing them both cold and hot and adding oil into the weak cylinders to see if that helped the rings seal better, if it didn't increase more than a little bit I was taught it was not the rings leaking, it was usually the valve train causing those issues. Using a leak down meter you can feel the air leaking around the rings by blocking off one breather and holding your hand over the other one to see if it builds pressure up in the crankcase or not and you can hear it leak by the valves on the carb side and the exhaust sides if it is bad valve sealing I do use a brass hammer to pop open the valves when doing a leak down test that I hear leakage from the valves, sometimes it will help and on others not very much I use them as tools to help me under stand what condition the heads, blocks and rings are in
Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)
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