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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
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Last edited by logician; 06-22-2003 at 01:49 AM..
Old 06-22-2003, 12:47 AM
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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
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Last edited by logician; 06-22-2003 at 10:42 AM..
Old 06-22-2003, 01:37 AM
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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
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Old 06-23-2003, 05:23 AM
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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
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Old 06-24-2003, 02:25 PM
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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
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Last edited by logician; 06-26-2003 at 11:51 AM..
Old 06-26-2003, 11:39 AM
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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.
Old 10-22-2003, 06:04 AM
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Old 10-22-2003, 08:23 AM
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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????
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Old 10-23-2003, 11:40 AM
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Old 10-23-2003, 12:02 PM
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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!!!!
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Old 10-23-2003, 12:13 PM
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Quote:
Originally posted by logician
Does air-cooling vs. water cooling affect leakage, assuming the measurment was done correctly with the engine warmed up.
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.
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Old 10-23-2003, 02:44 PM
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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
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Old 06-07-2011, 10:46 PM
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Sorry - the column formatting got a bit screwed up in the posting.
Alan
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Old 06-07-2011, 10:48 PM
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Quote:
Originally Posted by jgparker View Post
My question is what is the typical orifice size between gauge one and gauge two.

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.
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Old 07-26-2012, 06:45 AM
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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
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Old 07-26-2012, 12:30 PM
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Quote:
Originally Posted by Alan L View Post
The 0.04" is an aircraft standard - typically big pistons. In my view it is a bit large for our engines.
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.
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Old 07-26-2012, 12:53 PM
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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

Last edited by bodybyfisher; 09-09-2013 at 05:53 PM.. Reason: grammar
Old 09-09-2013, 05:51 PM
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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.
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Old 09-10-2013, 04:50 AM
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Quote:
Originally Posted by jgparker View Post
Does anyone know the specs on the restriction between gauge 1 and gauge 2 on this type of tester?
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)

Last edited by otto_kretschmer; 09-13-2013 at 08:03 PM..
Old 09-13-2013, 07:54 PM
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Quote:
Originally Posted by docrodg View Post
Measuring an engine with a .6 orifice will yield different results than with a .4 orifice. 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.
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.

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Old 09-14-2013, 04:33 AM
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