leak down test
 

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

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.

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

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

????

Explain what you mean please?

-Wayne

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,

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

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,

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.

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

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

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

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

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

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

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.

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:
http://forums.pelicanparts.com/uploads/Leakdown.JPG

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.

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

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

Hi Wayne,

> From what you say, the only thing I can gather is that the
> gauge we sell has two different gauges on it?

Yes exactly. The cylinder pressure gauge is at full scale at 36psi, and is marked in reverse, with 100 marked at zero scale, and 0 marked at full scale. Presumably, this is the same gauge that is used for the single gauge tester from that same manufacturer. The calibration gauge reads full scale at 100psi and is labled normally. Look at page 15 of your engine rebuild book for a picture of the instrument that PP sells. You can see how the gauges are marked differently. However, you actually have to use the gauge to see that their ranges are different.

> 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...

Often in engineering, it's easier to make a small delta to something you already have. Then there is also specsmanship. Have you ever head the story about how railroad gauge evolved? It goes all the way back to the Romans...

I surmise that what the manufacturer did was simply add a second gauge to the existing single gauge instrument that they already had in production. The reason that they added a 100psi gauge instead of a 36psi gauge is so that it would appear to match the specs of the competing two gauge instruments. It fooled me. A reason for them not to use the same gauge for both is that the gauges would then have to be very accurate, and that would be more expensive. Their existing gauges are probably not that accurate, but because you aren't comparing their values, their accuracy is not very important.

Regarding why the single gauge instrument would have been designed with a 36psi gauge, I bet its a common gauge for tire pressure. BTW, I don't really know if that gauge is 36psi, I just know that the calibration gauge reports 36psi when the cylinder pressure gauge is at full scale. The gauge might very well be a 45psi gauge.

> Juan, you've done a great job researching this, but I think
> there are still some details missing.

I would like to understand what is missing. At this point I'm pretty convinced that the acceptable leakdown percentage is specific to the orifice size and the specs of a particular engine design. A larger engine is going to have a larger leakdown percentage at equivilent health. So while a 10% leakdown figure might be OK for a large auto engine, it would probably be very bad for a small motorcycle engine.

The reason for this is very simple. What a 10% leakdown figure really means is that the leak is (approximately) 10% of the size of the orifice. This falls out of the analysis given earlier. So if the orifice is approximately .8 sqmm in area, then the total leakage area is approximately .08mm.

If in fact, it doesn't work the way that I explain, then I would really like to understand why this explanation is wrong, and how it really does work. What is missing?

> Obviously if the orifice is sufficiently huge, then the two
> gauges will read the same.

Yes, I agree. That's exactly what the analysis says. So here's the equation for cylinder pressure (P) as a function of orifice area (O) and leakage area (L):

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

If the orifice (O) is huge, then L/O is very small. So:

> P = 100 / (1 + .00001) = 99.99999

So as you say, the cylinder pressure would be very close to the calibration pressure of 100psi when the orifice is large.

-Juan

Hi,

> What a 10% leakdown figure really means is that the leak is
> (approximately) 10% of the size of the orifice. This falls out of
> the analysis given earlier.

For the record, here is why I believe the previous statement is true. Starting with the formula for leakage (L) as a function of cylinder pressure:

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

Now the leakdown percentage at pressure P and calibration pressure 100psi is defined as:

> percentage = (100-P)

We substitue this into the formula for leakage area, and get:

> L/O = percentage/P

So for P equal to 90psi, leakdown percentage would be 10%:

> L/O = 10/90 = .11

Thus for leakdown of 10%, the equivilent leakage area is 11% of the orifice area. Here's a table that translates between leakdown % and orifice %. You can see that for smaller leakdown values, the orifice and leakdown percents are very close.

Leak
down Orifice
---- --------
05% 05%
10% 11%
20% 25%
30% 43%

That is why I say that the leakdown percentage should be intepreted as (approximately) giving the leakage area as a percent of the orifice area. If all leakdown instruments standardize on the same size orifice (.04" seems to be common), then the leakdown % is actually an absolute measure of the size of the leak. It is not relative to any aspect of the engine.

Forget all those other intepretations of leakdown percentage as somehow a percentage of gas escaping (relative to what?). This interpretation is much more useful and direct. With this understanding it is easy to see why a smaller engine should have a smaller leakdown % measurement.

-Juan

Sorry I was in error on my posting..I went and looked it up and found that I have used .040" orifice not the #40 I said. I do like the analysis
of the system...good reading. As far as the size of engines..I have used the same tester on aircraft engines from 200 ci to 1340 ci. Some manufacturers allow more leakage than others. Must have something to do with the style of rings..etc.
Thanks
Steve

This is indeed a complicated question, if you look at the details. I'll reiterate again that the test is just one indicator, and shouldn't be an end-all answer to the health of your engine.

Orifice size would make a difference on the readings with respect to engine cylinder size. Let's assume that the engine is a huge locomotive engine with a huge cylinder size. That cylinder is going to act as a pressurized 'capacitor' in the dynamic system. The cylinder acts as a storage mechanism.

On the other hand, running a compression test on a model airplane engine would have a small cylinder size, and thus no capacitive effects. What does this mean? For an orifice size that is adjusted with respect to the size of the cylinder, nothing. For a fixed orifice size, then the readings will vary, as you will need to adjust the flow rate to compensate for the extra time it would take to reach steady state. I'm not explaining this well.

The bottomline is that the leakdown tester is useful for determining big problems (like 100% leakage), or big variations among cylinders. Like the compression tester, it is affected by many factors. As I state in the book, an engine with a leakdown test of 30% or more is probably experiencing problems. I also say not to use a single test by itself, but to go through the many tests and indicators in the book to figure out the health of your engine. It's not uncommon for engines that have been stored for many years to have bad leakdown numbers, but then leap to life when run for a short while. Another thing to remember is that the 911 engine is air cooled, and expands and contracts quite a bit when going from cold to hot. These clearances affect leak down testing too. Numbers achieved at cold may not have any relevance to the numbers measured when the engine is warm.

Bottomline? Go through the checklist and gather an aggregate picture of whether your engine needs a rebuild or not. I have also taken a closer look at the tool, and I feel that it does exactly what it is supposed to do.

-Wayne

Hi Wayne,

> Orifice size would make a difference on the readings with
> respect to engine cylinder size. Let's assume that the engine
> is a huge locomotive engine with a huge cylinder size. That
> cylinder is going to act as a pressurized 'capacitor' in the
> dynamic system. The cylinder acts as a storage mechanism.
>
> On the other hand, running a compression test on a model
> airplane engine would have a small cylinder size, and thus no
> capacitive effects. What does this mean? For an orifice size
> that is adjusted with respect to the size of the cylinder,
> nothing. For a fixed orifice size, then the readings will vary,
> as you will need to adjust the flow rate to compensate for
> the extra time it would take to reach steady state. I'm not
> explaining this well.

In the interest of demystifying leak-down tests, I'd like to respond to your statement about "capacitive effects". The leak-down measurement is in fact static, so any such capacitive effects have no effect on the leak-down measurment or orifice size.

The leakdown test is measuring how much air blows by the rings and valves, and that is measured with the pressure in the cylinder at a steady and unchanging value. You are right that "capacitive effects" would affect how long it took for the pressure to reach that steady value, as the orifice restricts how quickly the pressure in the cylinder can change. But in practice, with the volumes and orifice sizes we are dealing with, that happens in fractions of a second, and so when you take the measurement, the pressure is static.

A good way to convince yourself that the volume of the cylinder won't affect the leak-down measurement is to consider whether the leakdown measurement would change if [by magic] you increased the stroke of your Porsche engine from 70.4mm to 704mm, without changing the bore. In this hypothetical situation, the volume of the cylinder would be increased by a factor of 10, thus your "capacitance" would be increased by a factor ot 10 also. However, the amount of leakage surface -- the circumference of the valves, and the circumference of the rings would not have changed, so the amount of air blowing by the rings and valves would not change. It is easy to understand that in this case, the leakdown measurement would not be affected, despite the increase in "capacitive effects".

Regarding how engine size affects leak-down measurement, I agree that size should affect the measurement. But as explained above, it is not because of any "capacitive effects". All other things being equal, a smaller engine should have a lower leakdown measurement than a larger engine because the rings and valves are smaller, so there is less length of sealing surfaces that can leak. You can see that since the size of the rings and valves are proportional to the bore of the engine, the leakdown measurement for different size engines (all other things being equal) will be proportional to the bore.

Regarding how engine type affects leakdown, that is an interesting question that I don't understand. Does the metal used for the cylinder and rings affect how much air leaks by? Does air-cooling vs. water cooling affect leakage, assuming the measurment was done correctly with the engine warmed up. How about the gap in the rings, or the springiness? What about 2 valve vs 4 valve? Perhaps the answers to these questions might give a reason why Porsche engines might have a different leak-down spec than other engines. Don't know. I'd like to learn more.

BTW, I think what you call "capacitive effects" might affect a compression test, i.e. a test that measures the peak pressure in the cylinder when you turn the engine over with the starter. Unlike the leakdown test, that compression test is dynamic, since the pressure in the cylinder is being cycled. No orifice is involved in that measurement. I don't know much about compression tests, perhaps that could be the subject of a future thread.

-Juan

Well, my apologies in advance.

I used my brand new tester last night and, oh boy........#$#@$#$#.

Without going into details, I finally had to bypass the air regulator as it never stays the same pressure. I totally opened it and used the compressor in-line pressure/filter/regulator.

Juan,

What is also strange is that, assuming I am using the gauge correctly, the gauge has a built in 5% loss from the 0% setting to the time you hook up the quick-connections. Try this:

1. Set the gauge to 0%.
2. Using your finger, block the orifice.
3. Connect the hose to the gauge.
4. Gauge changes from 0% to 5%.

WHAT GIVES???????????

Theoretically, you would get 0% leak.

$70 at pelican.

Ok,

so my question is; should a leakdown test be performed when the engine is cold? I know that a compression test done cold versus warm will be completely different so wouldn't that be the same for a leakdown test?

Am I missing something here????

hot, with the rings oiled.

So if I did it stone cold then my results were skewed?

Also, do you use 100psi or less for your testing?

I thought something might be up; my compression numbers were even and within specs for being at 5200 ft of altitude, but my leakdown numbers were weird.

I will warm the car up tonight and retest.

THANKS John, your help is ALLWAYS appreciated!!!!

I believe as wayne stated, air cooler motors rely heavily on their clearences because they expand significantly. Water cooled motors control their expansion better with cooler running heads and cylinders.

OK, this is an old thread - but still good for information searches on leak down. So I will update it a bit with my latest experiment.
Am a bit concerned whether my track machine (930) is down on power - trying to identify a possible source. Did leakdown test first - got the following (first column)
#1 98 % 100 psi 92%
#2 96 105 86
#3 98 120 98
#4 100 110 98
#5 98 120 97
#6 100 120 98
These numbers were similar to a check a year or so ago on my same home made tester - 0.04" orifice.
Still not happy, I went looking for trouble and hooked up a comp tester. Second row of numbers.
Could not rationalise the numbers on 1&2. Thought about it and concluded the 0.04" orifice is a bit big for our engines (ie not sensitive enough). Is used for aircraft motors as a standard, and they have big donks.
So I filled my 0.04 in with solder and re drilled to 0.03" - about 60% surface area to previous - ie increased restriction by about 40% - this way the leaks will show up better - column 3.
So now the leak numbers confirm the comp numbers. Not sure if I am happier about this - prefer the first leak numbers. But in reality 0.04" diameter is hiding the issues for our size engines, until things get quite bad. If you want more sensitivity go for a smaller orifice.
#1&2 both have a bit of E valve leak, and cylinder leak.
Now I have to decide what to do. But in future I am sticking with 0.03" on my leakdown tool.
Alan

Sorry - the column formatting got a bit screwed up in the posting.
Alan

0.040 in (1.0 mm) orifice diameter, 0.250 in (6.4 mm) long, 60-degree approach angle. This applies to engines with piston diameters of less than 5 inches.

It does make a difference what the orifice diameter is.

I've seen posts where one owner took his car for a leakdown test to one garage, got some leakdown numbers and then took the same car to another garage and got different leakdown test numbers. All things being equal, chances are very good that both leakdown testers did not have the "standard" orifice size, or one did and the other didn't. A different orifice size will give you different leakdown numbers.

I've gone the Eastern Technology E2M leakdown tester route. That unit has a Master Orifice Valve to test the unit for accuracy. And at about $110, it isn't overly expensive.

The 0.04" is an aircraft standard - typically big pistons. In my view it is a bit large for our engines. As you close the hole up, you get more determinative/sensitive numbers. For my race car I consider this important - pick up any issues early, rather than satisfy yourself all numbers are 95% or better.
I closed my orifice down to 0.03" from memory - it is another posting somewhere on this topic. (You can fill with solder and re drill with jet drill).
This made the unit much better at picking out differences between cylinders.
At 0.04", by the time you get below 90%, you really have noticeable blow by out somewhere - you could pick it up without the tester.
Alan

At least it is a standard and the only way you can compare numbers from one tester to another is if they are standardized.

BTW, The 0.040" orifice standard if for 5" and smaller pistons. 993 pistons are 4". For pistons larger than 5", the spec is 0.060". Originally, they used displacement instead of piston diameter, but I guess they figured that one out fast enough.

As for many of the leak down testers advertized, I do not see any specs listed.

I hope this is not considered off topic. I acquired a 97 Eldorado with a Northstar that had a misfire in the #8 cylinder. A supposedly reputable shop did a leak down test and found a 78% leakdown on that cylinder. Note that the misfire went away above idle and at highway speeds the misfire light went out. The shop stated that the 78% leakdown was caused by one of the intake valves as they reportedly heard air in the intake.

Let me interject that while the seller of the car was adamant that the shop was "beyond reproach" the test was done by trainee and verified by an experienced tech.

I made a huge mistake by not doing the test myself but the seller an engineer was aggressive and in my face regarding the problem and leakdown test and I neglected to proceed properly.

I pulled the cylinder head very carefully, so as to not disturb the valves and left the cams in place turned it upside down and filled the combustion chamber with alcohol and there was NO LEAK none! I took the head to the machine shop and they also were unable to observe a leak at the intake valves using soapy water and compressed air....

A 78% leak is huge.....I can only think the shop had a bad gage set or the engine was not at TDC for #8.

I am a big fan of "cause and effect" and given that I did not find the cause of the 78% leak I have not found the problem. As a result I am VERY reluctant to reinstall this head.

Keep in mind that a problem that causes a 78% leak probably wont go away above idle.....

I am curious what you guys think about this so far.....you are a bunch of deep thinkers I like that....

I have a theory that I will share with you based on what I found on the exhaust lifter bore but I dont want to skew your thinking yet... thanks, mike

A leakdown tester uses the orifice as a measuring element much like a flowbench or Frazier machine used in the paper industry. The gages are measuring the pressure differential across the orifice. Since the engine is also connected the flow thru the orifice is the same as that thru the engine. If you use a larger orifice you get less pressure drop so a less sensitive reading - up to a point where you get no reading. Too small an orifice and you get way too much pressure drop as airflow can't keep up with leakage in the cylinder. Measuring an engine with a .6 orifice will yield different results than with a .4 orifice. The trick is finding the right size. The aircraft industry must use a .4 orifice as that is regulated to ensure "apples to apples" comparison across tests. The car industry does not have such a requirement.

This is kind of like an analog electrical voltmeter where the gage needle is moved by passing the tested voltage thru a resistor to convert the voltage into amperes that cause the needle to move. If you use a smaller resistor in the meter then your readings would be different as you are converting the same voltage into more amps and the scale is calibrated for the original resistor.

I made my own leak down tester and I drilled a .030" hole for the orifice


(jeez, this thread is ten years old, maybe I used a .040 drill, its been a while)

Which is why we might be seeing different numbers from to-be owners who take the car to have a PPI performed at two different shops, or owners who just have another shop perform a leakdown after seeing some bad numbers from the first shop.

Operator error might be another reason.

I bought an el-cheapo Harbor Freight unit. What size orifice it has does not really matter when doing comparative measurements to see how things change, but if does matter when trying to compare numbers from different shops using different testers.

I recently bought an Eastern Technology E2M Differential Cylinder Pressure Tester that also has a built-in master orifice as a "check". It has the standard 0.040" orifice.

http://edelweiss.smugmug.com/Categor...tv/1/O/E2M.jpg