leak down test

[ken martin]

I'm rigging up a way to attempt to pressurize the cylinders and need to know how much pressure I should use?
Thanks

[JB]

Ken, on a normal leak down tested, if you use 100psi in, then the leakage can be read directly as a percentage. By that I mean if you have 100psi inlet pressure and can hold a cylinder pressure of 95psi, then you have 5% leakage in that cylinder.

[Wayne 962]

We sell leakdown testers for about $70...

http://www.pelicanparts.com/cgi-bin/autocat/load_page.cgi?page_number=15&bookmark=0&model=DWKS&currsection=toolsA

-Wayne

[jgparker]

Does anyone know the specs on the restriction between gauge 1 and gauge 2 on this type of tester?

[Wayne 962]

????

Explain what you mean please?

-Wayne

[jgparker]

My question is what is the typical orifice size between gauge one and gauge two. This orifice will fix the airflow resistance between the two chambers, which will set the percentage drop for a given leakage in the engine. The percentage leak-down is an arbitrary scale based on this orifice size. I'm just trying to fix the scale in real units.

Yes, I'm trying to make my own leak-down tester.

Thanks,

[dad911]

Try this link: http://216.239.53.104/search?q=cache:A0eL7ZImmEUC:wabisabi.swarthmore.ed u:9673/sohc4.net/library/Engine/tech009.html+leak+down+orifice&hl=en&ie=UTF-8

[jgparker]

Thanks Alan, you're the best. So the answer is a #60 drill. You don't know how grateful I am to get a helpful response. There is a little too much of the other type going around.

Thanks,

[snowman]

The pressure is abritrary. completely. The first guage becomes the referance, the second the measurement guage. I don't think the hole size is critical as long as the compressor can keep up. It is then the percentage loss vs the referance guage. 100 lbs is convienent for calculation purposes, but may be to high to keep the piston on TDC as the motor will constantly keep trying to turn over. You can also do this with a single guage. The referance is set with NO leakage, ie at full scale. then it is connected to the cylinder and the leakage is measured as percentage of full scale. This is slightly less accurate but still ok. I think this is the way the snap on guage works.

[steve911sc]

I am a new guy, just regestered...I have been an aircraft mechanic for 25+ years...we rely on the differential compression test (leak down) as a great indicator of ring and valve condition. We use 80 psi as the engine mfg. suggest that. I have made a couple of the testers myself and have used a #40 drill as the oriface..snowman is right in his obsevations. On an airplane we can use the propeller to keep the cylinder at TDC..another great thing about the test is if you have a leak in the cylinder you can troubleshoot it further. sound out of the exhaust = exhaust valve, sound out of intake = intake valve...sound out of breather = rings.. do the test with a warm engine also.
Later

[logician]

Hi,

Orifice size does make a difference. Consider the following: if the pressure inside the cylinder under test is 50% of the calibration pressure (pressure applied to other side of orifice), then that means that the total of the "leakage area" around the rings and valves would be equal to the orifice size.

So if you used an instrument with twice as large an orifice, then the leakage area needed to get a 50% reading would be twice as large. Thus the second instrument would be much more optimistic if you only considered the % measurement without considering orifice size.

Wayne, do you know what is the orifice size of the leakdown testers that you sell? I'm interested because I bought one, and I am trying to interpret some measurments that I recently took.

-Juan

[Wayne 962]

Hmm, I'm not sure. Theoretically, if the orifice is sufficiently small, then it shouldn't make a difference either way. The reference air pressure source will be filling the engine cylinder until the pressure is so high that it start to lose pressure past the rings and the guides. In this same manner, the pressure applied to the leakdown tester will affect results as well. Higher pressure may result in higher leakdown.

Does anyone know of a tech page on leakdown testers?

-Wayne

[logician]

Hi,

> Theoretically, if the orifice is sufficiently small, then it shouldn't make a difference either way.

Doesn't seem right. Here's how to quantify the effect of orifice size. Consider the following four factors in the leakdown measurement:

> 100 -- calibration pressure on the other side of orifice in PSI
> O -- orifice area/restriction
> P -- pressure in cylinder
> L -- leakage area/restriction

You can compute the airflow through the leakage area by multiplying the leakage area times the pressure across the leak. The pressure across the leak is simply the pressure in the cylinder, so you get:

> P L = flow through leakage area

You can compute the airflow through the orifice by multiplying the orifice area times the pressure across the orifice. The pressure across the orifice is the difference between the 100psi calibration pressure and the cylinder pressure. So you get:

> (100-P) O = flow through orifice

The pressure inside the cylinder reaches an equilibrium when the airflow through the leakage area and the airflow through the orifice are equal. Thus:

> P L = (100-P) O

Rearranging things to get cylinder pressure (in psi) as a function of leakage area and orifice area, we get:

> P = 100 / (1 + L/O)

We can also get L/O as a function of cylinder pressure:

> L/O = (100-P)/P

So you see that the pressure that we measure is dependent on the ratio, L/O, of the leakage area to orifice area. Thus the orifice area does matter in the measurment.

So on a typical instrument, is the orifice larger than the leakage area or is it the other way around? Typically leakdowns on the order of 10% are considered reasonable. So in that case, the cylinder pressure would be 90psi, and we would get:

> L/O = (100-90)/90 = .11

So the leakage area is about one tenth the size of the orifice.

The thing that bugs me is that unless all instruments use the same orifice size, the % figure has very little meaning. So a 30% reading on one instrument could very well be equivilent to a 10% reading on another instrument, depending on their relative orifice sizes. It's like using photographic film without knowing the ASA speed of the film. How would you know what aperature/shutter speed to use?

If that's the case, then leakdown measurments would be virtually useless. Maybe this is one reason that leakdown measurments done by different shops seem to disagree a lot -- everybody's instrument has a different orifice size since there is no standardization.

Regarding the calibration pressure, I think I agree with Wayne that the calibration pressure might make a difference. You might need sufficient pressure to overcome the spring pressure on the rings, or to blow out some oil that is sealing a small leak. I could see how 100psi might be more relevent, as that is closer to the actual compression pressures than the 36psi that the PP instrument uses.

-Juan

[logician]

Hi Dad911,

> Try this link: http://216.239.53.104/search?q=cach...&hl=en&ie=UTF-8

Your link doesn't seem to work anymore. Can you post a more direct link that does work?

Thanks!
-Juan

[logician]

Hi,

> Does anyone know of a tech page on leakdown testers?

I've been doing Google searches, and I haven't really found anything authorative on the net. The only things I have found are 1) instructions for other manufactured instruments like Longacre, and 2) instructions for building your own instrument.

The instructions for manufactured instruments don't give specifications for orifice size. However, they all seem to use a 100psi calibration pressure.

The instructions for home made instruments all seem to recommend a .04" orifice through a block of material less than an inch thick. Nothing I've found says where they got that orifice size and length, nor have I found anything that says that this is the standard among all instruments.

The most authorative information I found was 1) in Porsche's MFI "Check Measure Adjust" book, page 11, where Porsche specifies a cylinder leakage of less than 10%, but no instructions for how to perform the test are given. And 2) in Bruce Anderson's book, page 46. Anderson says to use a 100psi calibration pressure, but does not cite any other specifications for the instrument. He also says that a well maintained Porsche engine should have less than 3 to 5% leakdown. Anderson doesn't really recommend a leakdown range except to say: "most people will tell you that 5 to 15% leakage is OK".

Wayne, in your rebuild book, page 16, you quote leakdown specs of 10% for a good engine, 20% for some wear, and 30%, needing rebuild. Can you say where you got those values from, and are they specific to Porsche 911 engines? Where they measured at 100psi?

Regarding the Pelican Parts tester that I used, it is designed to use a calibration pressure of only 36psi, instead of the more common 100psi specification. At first, I thought the instrument might be defective because of this low pressure requirement. However, after reading the instructions, I found that unlike the Longacre and other instruments which require >100psi compressor, the PP instrument can be used with a compressed air source as low as 45psi. This would only be possible if the instrument was intentionally designed to work with a lower, i.e. 36psi, calibration pressure. So I think the lower calibration pressure may be a feature to allow it to be used with lower pressure air sources, for example a tire pump.

So, now I am concerned that perhaps the measurements with the PP instrument might not be valid. First, given that the instrument uses an odd-ball pressure, I am suspicious that it's orifice size might be uncommon as well. Second, Anderson's published data that I would use for reference is measured at 100psi. Probably leakdown data from other Porsche mechanics would be measured at 100psi, if this is indeed the more common pressure used by most instruments.

-Juan

[logician]

Hi,

OK, so I've been studying the instructions for different leakdown testers, and I now understand that there are two basic types: single gauge, and two gauge. The two gauge ones are more accurate.

In the single gauge instruments, the gauge reads the the pressure on the cylinder side of the orifice. The way that you calibrate the single gauge instrument is that before connecting the instrument to the cylinder, and with the airflow to the cylinder blocked, you adjust the regulator valve until the gauge reads full scale, i.e. 100% (or 100psi if that's the calibration pressure). Then when you connect the gauge the the cylinder, some air flows through the instrument, and the pressure is reduced due to the restriction of the orifice. You can then read the reduced pressure from the gauge to determine leakdown. Some gauges are marked in reverse, so full scale reads 0, and zero pressure reads 100. That way the number that you read off the gauge is exactly the leakdown value.

So this sounds pretty nifty. However, the problem with these single gauge instruments is that the regulator isn't very accurate, and the pressure from the regulator changes a little when there is air flowing through the regulator, compared to when it was calibrated with zero airflow. The two gauge instruments solve this problem.

The two gauge instruments add a gauge to the other side of the orifice, the side that has the calibration pressure. That way you can tell if the pressure changes. The two gauges are identical, and typically full scale is 100psi. These instruments are calibrated with the instrument connected to the cylinder and with the air flowing. The idea is that with the air flowing, you set the calibration gauge to exactly 100, and then read the cylinder pressure on the other gauge. That way, even if the regulator changes with airflow, it won't matter, since the pressure was set with the air flowing. So the two gauge instruments are the most accurate, and this is important when you are trying to measure small leakdown values like the single digit leakdown values for 911 engines.

Now, the PP leakdown tester does have two gauges. However, it turns out that it is in fact only a single gauge tester to which the manufacturer has added a second, not very useful gauge. The problem is that the calibration gauge and the measurement gauge actually have different scales, so full scale on the measurement gauge is 36psi, while full scale on the calibration gauge is 100psi. Because the calibration scale is only at 1/3 of it's range when the measurement scale is at 100%, you can't use it to set the calibration pressure very precisely. One mark of the calibration gauge corresponds to a 3% leakdown amount -- not very accurate. In fact, the calibration procedure that the manufacturer gives for the instrument is the calibration procedure for the single gauge type instrument, not the procedure for the two gauge instrument. Thus the PP gauge has all of the inaccuracies of the single gauge instrument.

Given that realistic Porsche leakdown figures seem to be in the single digits, the PP instrument and other single gauge testers would appear not to be a very good choice. Unfortunately, the more accurate two gauge testers cost over $200. It would be great if PP could stock a more accurate two gauge instrument.

-Juan

Wayne, my apologies if I appear to be ragging on one of your products. That is not really my intent. I really respect your efforts to build a quality business and your work in developing this invaluable online community. My only intent is to explain fully what I am learning about how leakdown tests work, and how they might be best applied for 911 engines.

[dad911]

Now you guys have me curious. As a child I used to take my toys apart to see how they worked, so I have just taken apart my (nearly new) MAC Tools Leakdown tester.

Starting at the compressor side (input) there is a standard gauge w/regulator. This outputs to a 'spacer' which connects to a standard 'T" fitting. The top of the 'T' goes to a second gauge, and the other side goes to a fitting/hose that connects to the cylinder/spark plug hole.

The 'Spacer' is the only custom part. It is 1.75 inches long, and most of it is drilled out to about 1/4", but the end closest to the regulator is drilled so that a #60 drill bit (.040) is a 'snug' fit. It seems the intent is to have some distance (about 2.25") between the orifice and second gauge.

As for use: cylinder TDC, remove plug & screw in adapter/hose. Apply greater that 100# to input, and adjust regulator to read 100. second gauge reads pressure (%) of cylinder. For example, if output side gauge reads 95#, there is 5% leakage. Please excuse the 'quick' paint diagram below:

[snowman]

I think you are over analyzing an instrument that is not intended to give highly accurate leakage measurements. The reason is that highly accurate measuements are not really needed.

From what I have been told by at least one world class racer who made megabucks from driving in real races is this. If the leak down is less than 10% all is well. As leakdown goes beyond 10% performance starts to suffer slightly, and at 20% you can start to tell the difference, ie slower track times. And we are talking 100 ths of seconds in track times.

So just how improtant is an accurate number of less than 10%? Next given the accuracy restrictions of a given orfice, and the fact t hat one only has to tell if the leakdown is 10, 20 or 30 percent, is the instrument good enough?

PS for those smart asses that think I am saying I know everything, I am not, I am just parroting what I heard from someone who Is an expert. With a little sense anyone else can tell what he ment. As for his accuracy, I do not know, but he is very very rich and all the bucks came from his driving ability and his engne sense.

[logician]

Hi Alan,

Your picture is right on. Your input gauge measures what I call the "calibration pressure." Your output gauge measures the cylinder pressure. Thanks for reporting on the design of the SnapOn gauge! Question: how thick is the wall through which the orifice hole is drilled?

Also, can you tell us the results of leakdown measurments that you have done on 911 engines? In my case, using the PP gauge, my '73 911s got leakdown figures of 5-9%, using 36psi pressure, and with the accuracy of the gauge around 4%.

-Juan

[Wayne 962]

I have one of our leak down testers in my garage, but I haven't used it in a long while. I think Alan's diagram is spot-on, this is really a simple tool. In reality, since it is dyamically filling air into the cylinder, then by definition, it cannot have an extremely accurate reading. Such is the case with compression testers (see the Engine Rebuild Book, Chapter 1, for the list of 5 or factors that skew compression tests). From what you say, the only thing I can gather is that the gauge we sell has two different gauges on it? There must be a logical reason why the manufacturer did that, as I'm sure it would be cheaper to use two of the same...

Snowman, in rare form - I agree with you here 100%. It's a simple instrument designed to give a simple reading. Obviously if I apply 1000psi to a cylinder on a brand new engine, almost all of that pressure will leak out. Juan, you've done a great job researching this, but I think there are still some details missing. Obviously if the orifice is sufficiently huge, then the two gauges will read the same.

-Wayne