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Old 02-17-2019, 10:32 PM   #1
PDXREALTOR
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Default New built motor leak down target

What's the target and max leak down number for new a built motor before its started and, after it has the cam breakin and first 100miles on it?

Whats good, and, what's the limit at which there's a problem and it needs to be dealt with?
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Old 02-18-2019, 01:39 PM   #2
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Are your numbers high? I would imagine less than 5%
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Old 02-18-2019, 02:42 PM   #3
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Are your numbers high? I would imagine less than 5%
Haven't tested them yet (since running it). I got to reading and, there's only a couple of sources I found that I consider reliable so I wanted to ask.

One of them is a Cobb document - states 7% is ideal after the break in of 1k miles. Cobb https://www.cobbtuning.com/support/h...-engine-v1-03/

Another one was a post on this forum from a builder who stated as much as 15% was in the 'good' range. AR Fab



Seems like everyone has their opinions and I wanted to perhaps get others input based on actual test results on motors from places like IAG, Outfront, etc. Or, some input from guys who are building motors, or dealing with them, on a reg. basis.
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Old 02-18-2019, 04:29 PM   #4
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One of them is a Cobb document - states 7% is ideal after the break in of 1k miles.
^love it
A tuning company states a leak down spec and DOESN'T specify what pressure to do the damn test at!!!!!!!!!!!!!!!!!!

ASININE

Do a leakdown test at 80psi and you are going to get a MASSIVELY larger leakdown percentage than if you test the SAME motor at 150psi.
Won't even be in the same ballpark.

Anyone who states a percentage without saying the pressure to do the test at is someone you should not listen to.
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Old 02-18-2019, 04:31 PM   #5
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The few I've done, I do at 75 and 100psi. I've not seen a difference but I always do both because it just makes sense.
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Old 02-19-2019, 06:10 AM   #6
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The few I've done, I do at 75 and 100psi. I've not seen a difference but I always do both because it just makes sense.
there HAS to be a difference
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Old 02-19-2019, 12:24 PM   #7
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Leakdown is a percentage, not an absolute number. Why should it be so "massively different?"
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Old 02-19-2019, 01:19 PM   #8
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there HAS to be a difference
There was not.
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Old 02-19-2019, 02:34 PM   #9
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Leakdown is a percentage, not an absolute number. Why should it be so "massively different?"
A leakdown test forces air out any openings right? I hope we can agree on that. Let's pretend the 'openings' are a hole.
The more pressure you put to the same opening the more resistance it will have to flow the same amount, and it's flow capability has a max limit as well, that's the whole premise.

Put 5 psi of pressure to a space with a tiny little hole in it. That little hole will easily flow out most of what you are putting in, since you aren't putting crap in at 5 psi. So it will "bleed off" a solid 50-75% of what you put in to maintain a measly 5 psi.

Now hit it with 50 psi. That same size hole sure as hell isn't going to get rid of 50-75% of what it takes to maintain 50psi. It will be more like 15% now.

Now hit it with 150 psi. That same hole will rapidly reach it's max flow, and now it's only going to leak down 5% or so.

Now make it 1000 psi. Leakdown will be a fraction of 1% at most.

Think about it. What's a typical leakdown result?
10% right

If your engine leaked down 10% of it's combustion gases into the crankcase EVERY cycle your engine would erupt out of every seal and gasket it has. It would have to evacuate thousands of cubic feet of gases per minute from it's case.

If you have an actual leakdown gauge it has your two dials with a regulator. You can watch exactly what I am describing by starting at zero psi and turn your regulator up.
As your regulated pressure goes clockwise the leakdown will go counter clockwise.
Try it
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Old 02-19-2019, 02:35 PM   #10
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There was not.
Your gauge is broken then.
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Old 02-19-2019, 02:40 PM   #11
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A leakdown test forces air out any openings right? I hope we can agree on that. Let's pretend the 'openings' are a hole.
The more pressure you put to the same opening the more resistance it will have to flow the same amount, and it's flow capability has a max limit as well, that's the whole premise.

Put 5 psi of pressure to a space with a tiny little hole in it. That little hole will easily flow out most of what you are putting in, since you aren't putting crap in at 5 psi. So it will "bleed off" a solid 50-75% of what you put in to maintain a measly 5 psi.

Now hit it with 50 psi. That same size hole sure as hell isn't going to get rid of 50-75% of what it takes to maintain 50psi. It will be more like 15% now.

Now hit it with 150 psi. That same hole will rapidly reach it's max flow, and now it's only going to leak down 5% or so.

Now make it 1000 psi. Leakdown will be a fraction of 1% at most.

Think about it. What's a typical leakdown result?
10% right

If your engine leaked down 10% of it's combustion gases into the crankcase EVERY cycle your engine would erupt out of every seal and gasket it has. It would have to evacuate thousands of cubic feet of gases per minute from it's case.

If you have an actual leakdown gauge it has your two dials with a regulator. You can watch exactly what I am describing by starting at zero psi and turn your regulator up.
As your regulated pressure goes clockwise the leakdown will go counter clockwise.
Try it
I understand now, what you are saying I think. The numbers will read different, of course. However, the percentage of leakdown (percentage being key word) remains the same or similar.

I have an otc brand gauge. It simply has 0-100psi on both gauges. Test at 75 psi of course yields a different number on the right gauge than a test at 100psi. But it's the percent difference that matters. Not the actual numbers on the gauge.

Edit to add: I have some pictures of my gauge in action if I'm not being clear enough

Now.... Anyone have some input on the OP?
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Old 02-19-2019, 03:29 PM   #12
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Originally Posted by PDXREALTOR View Post
Another one was a post on this forum from a builder who stated as much as 15% was in the 'good' range. AR Fab
For those that won't actually read what was stated, Howard said "A decent condition engine will be no more then 20% / Good condition should be between 10-15% / very good should be below 10% / or you can get one of ours at 2-3%"

I would agree that 2-3% is ideal. We did leakdowns with nitrogen tanks, and had the coolant systems connected with vertical columns of water to the coolant system for visual indicator of headgasket health.
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Old 02-19-2019, 03:35 PM   #13
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I would think the cylinders having equal results would come into play as well. One being 10% and three being 4% wouldnt be ideal. Correct?
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Old 02-19-2019, 11:19 PM   #14
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So - RTV - here is my gauge. You can see where the confusion is. My gauge reads the same percent despite taking the readings at different PSI. Sorry for the rotated pictures.







Last edited by PDXREALTOR; 02-20-2019 at 05:39 PM.
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Old 02-20-2019, 07:56 AM   #15
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^mine is different
the 2nd gauge reads in leakdown percent, not actual pressure

you only posted the pic of one pressure setting, but you are saying you don't get different results if you say, turned it down to just 50psi?

I think there may be more expensive gauges that have internal parts that account for the issue of different test pressures, but the type that just put 'x' psi to the cylinder definitely produce different leakdown percentages as you adjust test pressure.
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Old 02-20-2019, 09:05 AM   #16
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Quote:
Originally Posted by rtv900 View Post
A leakdown test forces air out any openings right? I hope we can agree on that. Let's pretend the 'openings' are a hole.
The more pressure you put to the same opening the more resistance it will have to flow the same amount, and it's flow capability has a max limit as well, that's the whole premise.

Put 5 psi of pressure to a space with a tiny little hole in it. That little hole will easily flow out most of what you are putting in, since you aren't putting crap in at 5 psi. So it will "bleed off" a solid 50-75% of what you put in to maintain a measly 5 psi.

Now hit it with 50 psi. That same size hole sure as hell isn't going to get rid of 50-75% of what it takes to maintain 50psi. It will be more like 15% now.

Now hit it with 150 psi. That same hole will rapidly reach it's max flow, and now it's only going to leak down 5% or so.

Now make it 1000 psi. Leakdown will be a fraction of 1% at most.

Think about it. What's a typical leakdown result?
10% right

If your engine leaked down 10% of it's combustion gases into the crankcase EVERY cycle your engine would erupt out of every seal and gasket it has. It would have to evacuate thousands of cubic feet of gases per minute from it's case.

If you have an actual leakdown gauge it has your two dials with a regulator. You can watch exactly what I am describing by starting at zero psi and turn your regulator up.
As your regulated pressure goes clockwise the leakdown will go counter clockwise.
Try it
I'll preface this by saying I am no scientist and I don't have any background with air movement in relation to pressure/flow.

When you pressurize air are you not increasing the potential energy of said air?

Would that energy not increase the velocity at which the air can escape from the leak points?

Air at atmospheric pressure does not escape a container as quickly as air at atmospheric +100 psi so wouldn't the velocity of the air escaping the cylinder at 75 psi be lower than the air escaping the cylinder at 100 psi?

Based on my assumptions, wouldn't this mean that a higher volume of air would escape at a higher test pressure meaning that the leakdown pressure could theoretically remain the same?
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Old 02-20-2019, 09:51 AM   #17
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Originally Posted by Turpid Porpoise View Post

Based on my assumptions, wouldn't this mean that a higher volume of air would escape at a higher test pressure meaning that the leakdown pressure could theoretically remain the same?
no because the higher flow through something the more resistance there is and the more turbulent it becomes, reducing its ability to keep flowing.

plus, in the case of something like a piston (which is why they work at real operating speeds and thousands of psi) I'm pretty sure once the velocity pushing through a gap reaches the speed of sound it won't increase past that point no matter how much pressure you put behind it

I know that sounds crazy, but it really gets to that point in a combustion event and it happens so fast that practically nothing gets past
Plus, it has to squeek through one gap, snake around to the next one, squeek through, etc.

That's why at something like 5 psi, you'd probably have 95% leakdown. It's such a minimal volume that air can easily follow that path out with no resistance.
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Old 02-20-2019, 10:06 AM   #18
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That actually makes sense.

Here’s what I can offer to this conversation to make it a somewhat useful resource going forward and hopefully simplify this for anyone who doesnt fully understand the physics.

I get my motor back from IAG in a few weeks and I can do a leakdown just as you described at multiple test pressures to see how it affects leakdown percentage. Ill take a video with no cuts or take multiple pictures as I go, whichever is more transparent.
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Old 02-20-2019, 05:43 PM   #19
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Originally Posted by rtv900 View Post
^mine is different
the 2nd gauge reads in leakdown percent, not actual pressure

you only posted the pic of one pressure setting, but you are saying you don't get different results if you say, turned it down to just 50psi?

I think there may be more expensive gauges that have internal parts that account for the issue of different test pressures, but the type that just put 'x' psi to the cylinder definitely produce different leakdown percentages as you adjust test pressure.
I edited so the third picture should now be there now. I get different numbers on the gauge, but the percentage remains the same in this specific case.

With this type of gauge I'm not sure what a higher or lower leak down percentage would mean at a test pressure of 75 psi vs. 100 psi.

I would think more pressure in could result in a higher leak % but in all the reading I've done the last few days it seems more pressure could actually result in a higher reading. *shrug*
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Old 02-20-2019, 05:49 PM   #20
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Originally Posted by Turpid Porpoise View Post
That actually makes sense.

Here’s what I can offer to this conversation to make it a somewhat useful resource going forward and hopefully simplify this for anyone who doesnt fully understand the physics.

I get my motor back from IAG in a few weeks and I can do a leakdown just as you described at multiple test pressures to see how it affects leakdown percentage. Ill take a video with no cuts or take multiple pictures as I go, whichever is more transparent.
I think it's obvious there are two types of gauges and they both work differently.

The OTC gauge simply puts air in via an air compressor on one gauge, and lets it out into the engine on the second gauge. While the chart provided on the box makes it easy for lazy people you could really take a test at any pressure and simply calculate the percent difference between the two guage readings.

I'd love to see what your new IAG motor leaks at, both new and after a few hundred miles.

I'm getting ready to do mine, hopefully this weekend, at the 500 mile mark.
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Old 02-20-2019, 06:44 PM   #21
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So what exactly are you hoping to learn from a leak down test on a new motor with no run time?
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Old 02-20-2019, 07:04 PM   #22
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So what exactly are you hoping to learn from a leak down test on a new motor with no run time?
I'm just curious how many have solid leak down numbers from the get go, and how much if any difference there is after some break in miles.

I know this will vary from motor to motor. I just wanted some input based on members real world experience.

Try to, through others, put myself in the mind of a guy whose built 1000 motors. Ok several guys who have built 1000 motors.
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Old 02-20-2019, 07:55 PM   #23
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So what exactly are you hoping to learn from a leak down test on a new motor with no run time?
If this was directed at me, my motor has miles on it but the heads are having the valvetrain replaced with upgraded components. Really I just want to see what it will be straight from IAG, what it reads about 10,000 miles later and how input pressure may affect leakdown results.
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Old 02-20-2019, 08:17 PM   #24
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Originally Posted by Turpid Porpoise View Post
If this was directed at me, my motor has miles on it but the heads are having the valvetrain replaced with upgraded components. Really I just want to see what it will be straight from IAG, what it reads about 10,000 miles later and how input pressure may affect leakdown results.
If the SB has miles and you aren't touching those internals that makes sense.

What I am missing is the value in a leak down test before you have the rings properly seated? (OP)
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Old 02-20-2019, 08:42 PM   #25
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So - RTV - here is my gauge. You can see where the confusion is. My gauge reads the same percent despite taking the readings at different PSI. Sorry for the rotated pictures.
Ignoring their chart, where the rates aren't the same, you have 6.67% on 75 psi and 8.00% on 100 psi.
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