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leak down?

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    IAFF 1014 another72's Avatar
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    Default leak down?

    This may be a dumb question but i dont have much of an ego and would appriciate the knowledge. I was curious as to the process of a leak down test and how it differs from a compression test. Thank you

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    gn7
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    Quote Originally Posted by another72 View Post
    This may be a dumb question but i dont have much of an ego and would appriciate the knowledge. I was curious as to the process of a leak down test and how it differs from a compression test. Thank you
    We never answer dumb questions here, but we are pretty big on dumb answers. With a leak down test you fill the cyl with air from a compressor, and measure the rate the cylinder leaks down in %. More than anything, you really end up just listening to the carb (leaking intake valves), exhaust (leaky exhaust valves)and crankcase breathers (leaking past rings) for air to find out where it is leaking. Leak down measurements can be a little misleading if you aren't accustom to doing it. And as far as the ring seal goes, it really only means something at whatever place the piston is at when your test. Which means you need to test at different places in the bore.



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    Last edited by gn7; 01-15-2010 at 05:06 PM.

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    I'm No Expert Shaun's Avatar
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    Ideally the engine should be nice and warm too right?

    I remember when i was doing a leak down on my old build when i was hunting down the cause of the smoking it would rotate the motor over for me if i hooked up the gauge on that cylinders compression stroke

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    gn7
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    Yes the motor SHOULD be to operating temp, idealy, what a pain. And if it leaks 10% and you hoping for 5 are you going to take it apart. I just use it to tell if it is getting worse, and where.



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    Senior Member jetboatperformance's Avatar
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    Very Handy and usefull diag tool and not real expensive , dont remember if ours is Snap-on or Matco Tom

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    steelcomp was here
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    Biggest problem with a leak down test is it tests under static conditions, and I don't know of any engines that run standing still...

    True story...a jet boat racer I know, bck in the 80's, was right on the edge of a record. He and his engine builder did a leak down after their last run and found one cyl. down significantly from the rest due to a leaking valve. They figured the engine HAD to be down on power, so they loaded up and drove 2 1/2 hrs back to the shop, pulled the head, thrashed all night and fixed the leaking valve and were back at the track for Sunday. Boat didn't pick up a tenth.
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    Quote Originally Posted by another72 View Post
    This may be a dumb question but i dont have much of an ego and would appriciate the knowledge. I was curious as to the process of a leak down test and how it differs from a compression test. Thank you
    I have just always set the cylinder at TDC:



    At the beginning of each season I do a leak down and compression test. Typically after each weekend, if I've run a bottle or two of Nos through the engine I'll run an another test.

    The process starts with having the tool and an air compressor that can put out over a 100 psi (preferably). The test does work best if you have warmed up the engine. After warm-up put the cylinder you want to test at TDC pull the spark plug out and insert the hose/fitting from the tool you have the tool hooked up to your compressor and then you open the regulator and take to pressure up in the cylinder.

    You can expect different engines to have a different range of leak. The 565 has had the same zero-gap rings in it since 2000 it has had at least 4 hone jobs and is very sloppy right now on piston to wall clearance. It leak zero on most and less than 2% on the others. If it leaks over 4% on a cylinder there's a problem.

    Non-zero gap engines can leak 2-8% when warm. You can hear it passing the rings or valves so you can identify where the problem is.

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    A Leak down is like anything else, it's relative, you build a new engine, it leaks 2% on the engine stand, warm it, get another reading, put those in your book of records, and you can see when it's time to look at it, anything over 15% had me a little concerned, 20% and it was coming out, but our engines were torn down every week and gone thru, all aluminum was zyglo'd, all other dye penetrant or magnetic inspection, it gets expensive, but not as bad as repairing a windowed block broken or dead crank and more....Clark Inspection did all ours, and let me look at the parts to decide when a crack was to bad to chance.

    Leak downs on a Dykes ring arent conclusive....by the way.

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    LP-25.com Infomaniac's Avatar
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    Since 99% of the folks here are asking about pleasure/performance engines. Someone with a full out race engine is not going to ask questions here.

    I prefer to use leakdown readings to establish a trend. Or a "change" in readings.

    The only time the actual reading itself is used to make a decision is when there is a significant change.

    If the engine is making good power, not blowing too much oil, responds to tune up changes etc. Then the readings are what they are. Plot them on a trend chart. Make a decision that when they deteriorate down to a specific value then tear it down. And unless any reading does not have a big change run it until then.

    Leak down and oil filter contents are primary things I look at to determine engine health.

    Oh if a valve is blowing by - You can "STAKE" the valve with air pressure in the cylinder and it's possible some carbon etc just might be expelled sealing the valve again. That is a whole lot easier to check than pull the head.
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    Quote Originally Posted by Infomaniac View Post
    Since 99% of the folks here are asking about pleasure/performance engines. Someone with a full out race engine is not going to ask questions here.

    I prefer to use leakdown readings to establish a trend. Or a "change" in readings.

    The only time the actual reading itself is used to make a decision is when there is a significant change.

    If the engine is making good power, not blowing too much oil, responds to tune up changes etc. Then the readings are what they are. Plot them on a trend chart. Make a decision that when they deteriorate down to a specific value then tear it down. And unless any reading does not have a big change run it until then.

    Leak down and oil filter contents are primary things I look at to determine engine health.

    Oh if a valve is blowing by - You can "STAKE" the valve with air pressure in the cylinder and it's possible some carbon etc just might be expelled sealing the valve again. That is a whole lot easier to check than pull the head.


    another helpful device is a Dwyer Flow meter, used to measure Crank Case air flow, same deal, establish a baseline, then keep track of the deal, if you hurt a ring going lean it'll show in blowby, you are measuring SCFM, "Standard Cubic Feet Per Min"....it'll also tell you if an oil ring is messed up and lapped at the expanders gap losing all tension, which I have seen a few times....no oil ring tension is a bitch,......just remember to plug off all but the hole to the meter, or it isnt going to be very accurate.

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    IAFF 1014 another72's Avatar
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    thanks for all the input guys. That is the time line for the pressure drop? ie. 10% drop in 1 minute?

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    Quote Originally Posted by another72 View Post
    thanks for all the input guys. That is the time line for the pressure drop? ie. 10% drop in 1 minute?
    It's immediate.

    The way the tester functions - you put in 100 lbs of air and see what it holds. There is a calibrated orifice between the gages that allows the air to leak from the cylinder if it can. So the air input to the gage cannot overpower the leakage.
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    Quote Originally Posted by another72 View Post
    thanks for all the input guys. That is the time line for the pressure drop? ie. 10% drop in 1 minute?
    In my picture above the gauge on the left is attached to the feed regulator the gauge on the right is connected to the cylinder you can see they are both reading 100 psi. If the piston had a hole in it the gauge on the right would be reading zero (100% leak) and air would be blowing through the crank case. If I hurt a valve the gauge on the right would be reading 85 psi ( or a 15% leak) and the air would be coming out of the intake manifold or the exhaust pipe.

    As stated by Info, I too use it to monitor trends on the engine, but it also lets you know what you hurt. Sometimes if the intake or exhaust is leaking I'' take a rubber mallet and whack the rocker arm a couple of times to reseat it in case there was something on the seat.

    Lastly, every engine is different and once you know your engine you can do the test cold if you know the trends they be the same but with different numbers.

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    Quote Originally Posted by another72 View Post
    thanks for all the input guys. That is the time line for the pressure drop? ie. 10% drop in 1 minute?


    as info says, it's virtually instant, you are measuring a pressure drop, any leakage on the down stream side shows up as differential pressure,

    you have 100 psi going in, 95 psi on the out gauge, you have 5% leakage

    on the Dwyer, it isnt a Static Gage not that it makes much difference, it will still give insite as to the engines condition, when you reach over and HIT the throttle on the NEW engine, you'll note a jump in flow, (blowby) at any given throttle setting, the gauge will show how much virtual blowby you have....any signifigant changes , means a signifigant internal issue has changed.

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