leak down tester ..how does it work
Le Mans Master
Joined: Apr 2005
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From: Seattle Area WA
Bob, you have two gages, a supply line from your compressor and an inlet to your combustion chamber via the spark plug hole. One gage reads the supply pressure, say 100 psi while the other gage reads what is staying in the cylinder say 95-96 psi for a fresh engine. IMHO 10% max is typical acceptible leakage for a driver.
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Race Director
Joined: Sep 2004
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From: Arlington Va Current ride 04 vert, previous vettes: 69 vert, 77 resto mod
ok so i hook it up and set the pressure.....the pressure going in is constant......i set the leak down gauge to 0 and it stays there......is this measured over time? and how does it go down if there is constant pressure?
Tech Contributor
Joined: Jun 2004
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From: I tend to be leery of any guy who doesn't own a chainsaw or a handgun.
Bob, you have two gages, a supply line from your compressor and an inlet to your combustion chamber via the spark plug hole. One gage reads the supply pressure, say 100 psi while the other gage reads what is staying in the cylinder say 95-96 psi for a fresh engine. IMHO 10% max is typical acceptible leakage for a driver.
Like I said, I'm not familiar with the workings of this, but I am interested in a few details.
Race Director
Joined: Sep 1999
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From: Plano TX
There's a constant supply of air at 100 PSI. The gauge has an orifice (0.040", if memory serves). Second gauge measure pressure in the cylinder. Less than 10% leak is good. This means you should read 90 PSI in the cylinder. Some leakdown testers have the second gauge showing leakge in percent, but it's just numbering. i.e. instead of 90 PSI it says "10%", etc.
Large or small bore should not matter much. A quality engine will read 5% or so. Less with zero-gap rings. Up to 10% is fine. Once it starts creeping up towards 20% it's time to start putting money away for the rebuild.
BTW, I think I hold the record for the highest leakage on an engine that still ran: 92%!
Large or small bore should not matter much. A quality engine will read 5% or so. Less with zero-gap rings. Up to 10% is fine. Once it starts creeping up towards 20% it's time to start putting money away for the rebuild.
BTW, I think I hold the record for the highest leakage on an engine that still ran: 92%!
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Thread Starter
Race Director
Joined: Sep 2004
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From: Arlington Va Current ride 04 vert, previous vettes: 69 vert, 77 resto mod
ok just cranked it up to 80 psi and it makes no difference.....on mine you have a gauge that you "zero" out on by controlling the air coming in and on mine when its at "zero" i only have 18 PSI coming in??? even though i have much more pressure on the other side....
any thoughts?
i also want to use this to see if my harmonic balancer has slipped.....i figure that if i hand crank the engine over at some point the exhaust valve will start to open and that should be somewhere towards the bottom of the power stroke i would guess...i can put a degree tape on the harmonic balancer and see exactly when it starts to open....and the leak down tester should show that quite obviously.....any thoughts on my plan? and when will the exhaust valve start to open in terms of degree of rotation?
any thoughts?
i also want to use this to see if my harmonic balancer has slipped.....i figure that if i hand crank the engine over at some point the exhaust valve will start to open and that should be somewhere towards the bottom of the power stroke i would guess...i can put a degree tape on the harmonic balancer and see exactly when it starts to open....and the leak down tester should show that quite obviously.....any thoughts on my plan? and when will the exhaust valve start to open in terms of degree of rotation?
Le Mans Master
Joined: Apr 2005
Posts: 6,346
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From: Seattle Area WA
Rather than try to write directions try this link or do some google searches. http://www.tmossporting.com/tabid/337/Default.aspx
There are some more links that may help you since I don't know the configuration of your tester. You should be able to determine if the ring has slipped once the piston is blown down the cylinder to the bottom.
Why are there so many concerns on this board with damper rings slipping? If it hasn't slipped, pin it in place.
There are some more links that may help you since I don't know the configuration of your tester. You should be able to determine if the ring has slipped once the piston is blown down the cylinder to the bottom.
Why are there so many concerns on this board with damper rings slipping? If it hasn't slipped, pin it in place.
Le Mans Master
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From: Gladstone MO
C3 of Year Finalist (appearance mods) 2019
A leak down test is the percentage of air the cylinder will hold. If you give it 100psi, at TDC with both valves closed, and it holds 80psi on the second gauge, then you've got 20% leaking past something. You'll be able to hear it at the carb,tailpipe, or crankcase breather. THe trick is getting it exactly at TDC or else the air pressure will turn the engine, negating that cylinder's test, you'll need to rotate it back to TDC and go again. It's PITA on a car, but it's a standard test for small piston engine planes. On those you hold the prop 
Le Mans Master
Joined: Oct 2006
Posts: 7,123
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From: Gladstone MO
C3 of Year Finalist (appearance mods) 2019
Thread Starter
Race Director
Joined: Sep 2004
Posts: 11,874
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From: Arlington Va Current ride 04 vert, previous vettes: 69 vert, 77 resto mod
i will do some google searches tomorrow and learn more about this.....anyone know when the exhaust valves open in terms of degrees of crank rotation? or how i can relate a cam card to this?
Thread Starter
Race Director
Joined: Sep 2004
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From: Arlington Va Current ride 04 vert, previous vettes: 69 vert, 77 resto mod
i appreciate everyones thoughts .... i am at about 10* BTDC and the piston is showing no signs of moving so maybe i am close to TDC......i have no reason to think my harmonic balance has slipped i just want to have fully thought out all possibilities as i am checking this out.
Thread Starter
Race Director
Joined: Sep 2004
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From: Arlington Va Current ride 04 vert, previous vettes: 69 vert, 77 resto mod
Melting Slicks
Joined: Sep 2004
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Take the distributor cap off.
Turn the engine over so the rotor is pointing toward the cylinder wire for the cylinder you're testing. That will put it as close to TDC as you need. Close enough so that both intake and exhaust valves will be closed.
Use a breaker bar or some method to keep the engine from cranking when you apply 100 psi to one cylinder.
Now the leakdown tester.
A compressor supplies 100 psi to one side/guage of the tester. There is a small orifice that allows a slow leak of air to the other side/guage of the tester. This second gauge is in eqilibrium with the cylinder it's connected to. So the pressure on the second guage will read how much air can go by the rings, valves, etc in comparison to what can enter through the orifice.
Simple.
Gary
Turn the engine over so the rotor is pointing toward the cylinder wire for the cylinder you're testing. That will put it as close to TDC as you need. Close enough so that both intake and exhaust valves will be closed.
Use a breaker bar or some method to keep the engine from cranking when you apply 100 psi to one cylinder.
Now the leakdown tester.
A compressor supplies 100 psi to one side/guage of the tester. There is a small orifice that allows a slow leak of air to the other side/guage of the tester. This second gauge is in eqilibrium with the cylinder it's connected to. So the pressure on the second guage will read how much air can go by the rings, valves, etc in comparison to what can enter through the orifice.
Simple.
Gary
Le Mans Master
Joined: Apr 1999
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From: Raleigh North Carolina
Obviously, I am missing something here.
Probably in the wording.
Tech Contributor
Joined: Jun 2004
Posts: 20,847
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From: I tend to be leery of any guy who doesn't own a chainsaw or a handgun.
There's a constant supply of air at 100 PSI. The gauge has an orifice (0.040", if memory serves). Second gauge measure pressure in the cylinder. Less than 10% leak is good. The second pressure gauge only tells you the percentage of the baseline pressure, not the percentage of leakage. Pressure and volume are two different things. This means you should read 90 PSI in the cylinder. Some leakdown testers have the second gauge showing leakge in percent, but it's just numbering. i.e. instead of 90 PSI it says "10%", etc.
Large or small bore should not matter much. I still disagree, and the tester description in the link offered in a prior post above confirms that. After all (disregarding a broken cylinder wall or holed piston), the only leakage paths are by the rings (larger bores mean bigger ring/cylinder interface circumference) or the head gasket (same bore size impact), or the valve/seats. A quality engine will read 5% or so. Less with zero-gap rings. Up to 10% is fine. Once it starts creeping up towards 20% it's time to start putting money away for the rebuild.
BTW, I think I hold the record for the highest leakage on an engine that still ran: 92%!
Large or small bore should not matter much. I still disagree, and the tester description in the link offered in a prior post above confirms that. After all (disregarding a broken cylinder wall or holed piston), the only leakage paths are by the rings (larger bores mean bigger ring/cylinder interface circumference) or the head gasket (same bore size impact), or the valve/seats. A quality engine will read 5% or so. Less with zero-gap rings. Up to 10% is fine. Once it starts creeping up towards 20% it's time to start putting money away for the rebuild.
BTW, I think I hold the record for the highest leakage on an engine that still ran: 92%!
Le Mans Master
Joined: Apr 2005
Posts: 6,346
Likes: 418
From: Seattle Area WA
Tom,
Maybe in the technique of how it is done. The three allen head countersunk bolts, spaced 120 degrees apart only prevents the possible slipping of the outer ring, not the function of the rubber damping. The bolts are not torqued down, just held ever so slightly snug in place with the use of nylocs. I don't think this is anything unheard of, I learned it working for an engine builder 25+ years ago who was buddies with guys like Keith Black, Nick Arias, Bill Hunt, Henry Velasco and Jack Engle, all a bunch of old timers today. Maybe this is considered old school today, but we would pin any 8" Hi Perf GM Nodular iron damper going onto an engine that would see 6,500 and above on a regular basis such as Nitrous and blown motors and I still do it today without ever having had a problem. I ran my blown 496 race boat for years this way along with my Nitrous 383 vette without any issues. I'm sure this was before we had all these other options available today.
Last edited by Scott Marzahl; Apr 24, 2007 at 02:25 PM.