Need advice - Leak-down great, Compression test not so much

[AndrewCologne]

Originally Posted by winders

20% leak down is not acceptable for a 911 air-cooled engine…..

Just READ my lines above .... it depends on the incoming pressure. If you apply 7 or more bars then you almost everytime end up with values below 10% which gives you -nothing- but nice looking numbers below 10.

Or do you think Hazet as one of the best tools manufacturer just sets wrong areas on their gauges to green? I dont think so.

[winders]

At 50 psi or 100 psi, 20% leak down is too much for an air-cooled 911. The last time I did a leak down on my race engine, which was just a few weeks ago, I has the same leak down percentage at 50 psi and 100 psi......

[AndrewCologne]

Shure, because your setup (I guess) comes with two gauges where behind the first one an orifice keeps the internal prassure at a given level no matter what pressure comes in. Cheap Chinese ones internally even go down to 1 Bar. These even go up to 40% in their acceptable range. Watch the Sets shown in the www.

All depends on the actual pressure present in the combustion chamber.
Use a one-Manometer setup where you can set the an individual pressure at the output reaching the chamber and you will have diff. percentage results at diff. incoming pressures.

So a "20% is too much" answer is valid for your leak down test set and many others out there but not a rule of thumb in general.
Btw I personally also prefer a maximum here of 10% at 4 bar but ... thats my personal limit before I suggest an engine overhaul. But main focus is - as said - that rleak down esults within the cylinders are almost matching.

[LJ851]

Originally Posted by hcoles

We tried open and closed throttle, no change but we used open throttle.

This sounds fishy. I have never seen an engine produce the same compression numbers with the throttle closed.

[Walt Fricke]

I try to be careful about things where I am not entirely certain. Aircraft mechanics use these things where there are more consequences if you don't get it right, and OK an engine which didn't actually meet FAA or someone's specs. Maybe someone with that experience will chime in.

A quick look at testers which are for aircraft shows plenty with two dials (though many also have two valves - don't know what those are for). Aircraft testers seem to denominated by the diameter of the piston, also.

From previous discussions about leakdown testers, I think the way they work - the two gauge ones - is that between the pressure you regulate for dial A, there is a small orifice leading to dial B. Because it is so small, negligible air flows/pressure is lost to dial A when dial B is connected to a cylinder. I analogize this to the tiny orifice in a CIS fuel distributor through the large diaphragm separating the top and bottom halves of the FD.

Or perhaps to a high value resistor which doesn't draw enough current from a circuit to lower its voltage? High impedance on volt meters in these days of good and inexpensive electronics (from one who grew up in the VTVM era)? Beyond some level of impedance, only for the most exacting scientific testing would more be better in practical terms.

My experience (all random) with air leaks is that if there is an opening, even if very small, near negligible pressure will show up in a soap bubble test. Maybe a rubber seal might work differently, as more pressure might force a seal to leak. But I don't see that kind of concern with engine leak downs. You aren't going to force valves off their seats, and my suspicion is that you won't move rings to where they don't seal as well.

Past discussions often involved the size of the orifice, and on how there was thought to be no standard for it.

With my first compressor, I couldn't get 100 psi on the inlet side, so used 80, or 50 to save the math, just like many others have. Now I can use 100, so I do.

My take on how Hazet has set up the one dial gauge is that its face is set to work properly only at a very specific inlet air pressure. It must still have a restriction between the inlet and the outlet. I suspect the adjustment knob is set so the one gauge is at 0, and its face is calibrated so 0 is for what their desired inlet side pressure is.

It seems to me that higher pressure just gives better analog dial resolution. Soon, I suspect, there will be electronic digital dials on the market - I just bought a digital tire gauge. Hard to know what its error range might be - anyone who has compared the several tire gauges we all collect has been a bit surprised to see how much they all vary. As mentioned, the gauges on my leakdown tester are quite probably not up to some technical institute standard of accuracy.

So I am not understanding the argument about why one and only one inlet pressure can work on a two gauge system, though it is critical on a single gauge system. Unless it has to do with internal orifice size. And it would seem that with a dual gauge system you could deal with that - after setting up the test, reset the inlet pressure so the inlet side is back to 100 or whatever you had chosen, and promptly read the outlet gauge.

I'd be leery about 20% leakage in an exhaust valve - if seat or valve is burned, it could get worse fairly quickly. Ring blowby you could live with for a while - maybe more frequent oil changes? Intake, I'd wonder why. But for a race or track motor, I'd not settle for 20% anywhere.

After blowing up a 2.7, I got a used replacement, with leakdowns in the 10-12% range per a mechanic with a big, fancy looking test rig. He said it should be OK, just not quite as fresh. I ran it a while, but persuaded myself that my lap times were suffering, and sold it - with full disclosure - to another racer. He said it worked fine for him until he built a 2.8 with carbs and cams and CR etc.

So Andrew, I think you will have some trouble convincing some of us (most of us aren't worried about being corrected - that's how you learn if you accept the fact that there is almost always someone out there who knows more than you do) that the the inlet pressure on a two dial tool will cause the percentage of leaking to vary if you operate it in the normal way.

[safe]

A healthy engine should be under 5%.
When I tested my 964 engine I had leak down at 1% (could not hear any leaks at all), on the same tester we did a friends 968 turbo, set up with larger ring gap. It had 15+ something, mostly leaking through the rings.

[otto_kretschmer]

Originally Posted by AndrewCologne

@hcoles

giving 135 PSI to a combustion chamber for a leak down test is far too much for a precise test result. By this most test results are between 2% and 5% which looks good on the first view.

Most quality leak down test Gauge/Units like from Hazet do internally limit the input pressure to approx. 4 Bar where by this a wider reading bandwith of the actuall pressure loss is possible.

Give em about 58 PSI and re check the leak down value.
As shown below everything down to 20% is acceptable as long as all cyclinders are almost matching

What is "acceptable" is arbitrary. The manufacturer of that tool, Hazet, has 0-20% in the green zone. I wouldn't be happy with 20% on a newly rebuilt and broken in engine.

If I got 20% on a daily driver with 300k miles on it, I may be OK with that for now but I would be planning on replacing/rebuilding the engine.

I built my own leak down tester with parts I bought from the local hardware store.



I've posted this picture before and I keep using it.

[AndrewCologne]

@ Otto
Read my explanation further above, ... it depends on the pressure which reaches the combustion chamber.
The Manufacturer Hazet of the tool above deals with an internal clamped pressure provided to the combustion chamber and thats why here "these" 20% are within the range.
Many people go with about approx 7 Bar at the combustion chamber when doing a leak down test there YES about 5% is ok and should not be exceeded.

BTW, YES, your gauge output is fully ok at approx 80 PSI. What is the sice of the ofrice you used on your DIY seutp? Nice btw!

Here's a reading I did a few weeks ago ... approx 2.5% loss when applying 4 Bar to the comb.chamber. Same very slight Noise on all six tests.

[dannobee]

Aircraft piston engines are measured using the two gauge leakdown tester, but the readings are listed as "compression tests," with the post orifice number listed first, then the inlet air. A reading would be advertised as, "Compression is 78/80." Even though we all know it's actually a leak down test.

[hcoles]

At the risk of throwing too big a wrench in the works.....
I was thinking... what we are trying to measure is an amount of air flow at a given pressure (80-100? psi) applied to a cylinder. The pressure applied to each cylinder should be the same. If that's the case then why isn't the downstream gauge set to read a constant pressure? What would happen is the gauges would read 105-100 instead of 100-96 or something like that. I know it's a small difference and probably not worth worrying about.

[winders]

No, you are measuring pressure difference. If the inlet pressure is 100 psi and the cylinder hold 95 psi, there is a 5 percent difference or leak down. I use a 2 gauge setup with one gauge showing the inlet pressure and the other showing the cylinder pressure.

[otto_kretschmer]

Originally Posted by AndrewCologne

@ Otto
Read my explanation further above, ... it depends on the pressure which reaches the combustion chamber.
The Manufacturer Hazet of the tool above deals with an internal clamped pressure provided to the combustion chamber and thats why here "these" 20% are within the range.
Many people go with about approx 7 Bar at the combustion chamber when doing a leak down test there YES about 5% is ok and should not be exceeded.

BTW, YES, your gauge output is fully ok at approx 80 PSI. What is the sice of the ofrice you used on your DIY seutp? Nice btw!

Here's a reading I did a few weeks ago ... approx 2.5% loss when applying 4 Bar to the comb.chamber. Same very slight Noise on all six tests.

ugh.. the FAA has some guidance of leak down tests

https://rgl.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/0/99c827db9baac81b86256b4500596c4e/$FILE/AC_43.13-1B_w-chg1.pdf

page 8-7

I built my tester 20 years ago and I think I just put a plug of epoxy in the pipe nipple that is in between the two gauges and let it cure and then drilled a hole with a .040" drill bit.

The FAA recommends a .040" orifice for engines with pistons 5" diameter or below and .060" for engines with pistons bigger than 5" diameter.

[otto_kretschmer]

Originally Posted by hcoles

At the risk of throwing too big a wrench in the works.....
I was thinking... what we are trying to measure is an amount of air flow at a given pressure (80-100? psi) applied to a cylinder. The pressure applied to each cylinder should be the same. If that's the case then why isn't the downstream gauge set to read a constant pressure? What would happen is the gauges would read 105-100 instead of 100-96 or something like that. I know it's a small difference and probably not worth worrying about.

Henry

Your engine is good and you should be jumping with joy at getting those leak down readings. Those are very good numbers. You will have to find something else to worry about.

Small variations in upstream line pressure from your compressor won't matter. I try to keep the test between 100 and 80 psi. My compressor maxes out at 100.

[Walt Fricke]

Andrew - first off, why is the pressure which reaches the combustion chamber anything other than the regulated pressure before the orifice, minus what leaks out? We both know that if the filter in a WUR is blocked so the WUR can't reduce the CP, the CP ends up the same as system pressure. And if the line from the WUR to its next stop is blocked, you get the same result. If you have zero combustion leaks, you will have no difference between the two gauges, without regard to what the regulated pressure is, won't you? And with zero leaks, the orifice diameter won't matter, will it? Of course it will, some, when connected to the spark plug hole.

Second, how do you respond to the argument that the Hazet, which has to make things work with one fixed gauge with a "backward" face so it reads directly in percent, has to use a very specific regulated input pressure for their system to work? Certainly, if you used more or less pressure than specified, you'd get different readings (though you couldn't really use more -the needle would hit its stop).

More to the point, Hazet says you need an air pressure source of 6-12 bar, so their setting pressure can't be over ~90 psi. If you somehow measured what the regulated pressure is when set is 80 something psi, I believe you.

My suspicion is that the gauge they use isn't custom made for them, though they have a custom face. Perhaps some gauges available in metric countries are specified at 5 bar?

Two gradation lines difference between cylinders they say are 4% loss, which they say is acceptable if that is the greatest variance between the high and low cylinders. You set the pressure until the dial pointer is on 0. Then they say green area is up to 23% on the gauge.

Like others, I am mystified at accepting a 23% leakage. If I had a 19% leakage in five exhaust valves, but 23% in one, would I do more than, say, chance driving home or to a shop for a rebuild?

If I had my car home, I'd perform a 50 psi and a 100 psi leakdown test, to see if that confirms that (within the differences in resolution) the % results were the same.

And am I wrong to analogize pressure to voltage, and flow to current? The Ohms law analogy does suggest that different orifice sizes will lead to different leak percentages, though, if the orifice is taken as the resistance. However, we are measuring a whole circuit - the leakage from the cylinder is part of the circuit - maybe that makes orifice differences, at least if small, not as R1, and the leakage as R2. Resistance electrically is additive. Here we have the area of a 0.04" diameter orifice , R2 is infinite, and R1 is irrelevant. If we assign a small enough decimal number to R1, and some rather lower number to R2, would that work?

It appears that the length of the restrictor, as well as its diameter, affects the pressure drop. That is a term in the equations doctors use in figuring blood flow drops, which are pretty much like Ohms Law with some special cases. But maybe the length, if relatively short, doesn't matter all that much? I have never seen an engineering analysis of the length, compared to the diameter, of a dent in, say, an oil line in terms of flow.

[safe]

If you have a tester with only 1 gauge, like the Hazet, it is dependent on the air source to be the "other" gauge and pressure regulator.
Its the orifice between the unrestricted air source and leaking chamber that do the magic in the test, it needs to be appropriately sized to the leakage of the piston rings. Its the only thing that can skew the test.
If you have a tester with a red/yellow/green zones and percentage instead of numbers you need the specific air pressure that the manufacturer of the gauge specifies. The Hazet pictured seems like a cheaper to manufacture and dumbed down tool.

[AndrewCologne]

The Hazet pictured seems like a cheaper to manufacture and dumbed down tool.

The Hazet Tool is used by many well known Porsche Workshops here which also offer engine overhauling etc. Hazet Tools aren't cheap manufactured at all, its the other way around as they provide most reliable hardware and are here in Germnay the first choice of mechanics.

If you have a tester with only 1 gauge, like the Hazet, it is dependent on the air source to be the "other" gauge and pressure regulator.

yes,IF the tester comes with no ofrice, but in case of the Hazet tool the source pressure should be within the specs above 5 Bar like mentioned above by Walt. Internally it works with reduced fixed 4 bar, thats why differnet source pressures will result in the same leak down values.
As you already know (as thats what you meant above), when just connecting a simple gauge shurely different incoming pressures will end up in diff. results.

If I had my car home, I'd perform a 50 psi and a 100 psi leakdown test, to see if that confirms that (within the differences in resolution) the % results were the same.

Thats cause internally the pressure brought to the combustion chamber will remain the same.

Like others, I am mystified at accepting a 23% leakage. If I had a 19% leakage in five exhaust valves, but 23% in one, would I do more than, say, chance driving home or to a shop for a rebuild?

I never said that I personally accept this. In case of Hazet and their specs, means their final reduced pressure brought to the combustion chamber, the green area on their gauge just tells the menachnic that the engine is ok. In our case where we want to archieve/keep maximum power at our engines, for shure I would be anything but not be happy with 20%. BUT ... I would also no accept 5-10% when using a tool providing much more pressure "to the combustion chamber" like real reaching 8 Bars or higher – Im not talking here only about the source pressure, the pressure "reaching the combustion chamber" here is relevant.

Second, how do you respond to the argument that the Hazet, which has to make things work with one fixed gauge with a "backward" face so it reads directly in percent, has to use a very specific regulated input pressure for their system to work? Certainly, if you used more or less pressure than specified, you'd get different readings (though you couldn't really use more -the needle would hit its stop).

With the Hazet tool you you set the pressure from the compressor connetion to more than 6 Bar (length of hose must be taken into account) to the Testing tool. the next point in the way of the air is the ofrice in the Hazet tool (and also in case of other tools) which clambs the pressure – thats why diff input pressures here on such tools wont change the result. Its all depening on the ofrice used and IF one is used.
FAA following devices are shurely a way to go, ... but beside other FAA compilant ones I also built myself one DIY version of a tester which does not come with an ofrice as by this I can provide the Full 8-10 Bar to a combustion chamber, wich not only once showed me that there is a "leak" between the Head and the cylinder. Covering that area with some dishsoap/water mixture showed me in that particual case that ateh sealing ring was flawed – which was prooved when the engine was taken apart.

It appears that the length of the restrictor, as well as its diameter, affects the pressure drop.

Shure

[safe]

Originally Posted by AndrewCologne

The Hazet Tool is used by many well known Porsche Workshops here which also offer engine overhauling etc. Hazet Tools aren't cheap manufactured at all, its the other way around as they provide most reliable hardware and are here in Germnay the first choice of mechanics.

yes,IF the tester comes with no ofrice, but in case of the Hazet tool the source pressure should be within the specs above 5 Bar like mentioned above by Walt. Internally it works with reduced fixed 4 bar, thats why differnet source pressures will result in the same leak down values.
As you already know (as thats what you meant above), when just connecting a simple gauge shurely different incoming pressures will end up in diff. results.


Thats cause internally the pressure brought to the combustion chamber will remain the same.

I never said that I personally accept this. In case of Hazet and their specs, means their final reduced pressure brought to the combustion chamber, the green area on their gauge just tells the menachnic that the engine is ok. In our case where we want to archieve/keep maximum power at our engines, for shure I would be anything but not be happy with 20%. BUT ... I would also no accept 5-10% when using a tool providing much more pressure "to the combustion chamber" like real reaching 8 Bars or higher – Im not talking here only about the source pressure, the pressure "reaching the combustion chamber" here is relevant.

With the Hazet tool you you set the pressure from the compressor connetion to more than 6 Bar (length of hose must be taken into account) to the Testing tool. the next point in the way of the air is the ofrice in the Hazet tool (and also in case of other tools) which clambs the pressure – thats why diff input pressures here on such tools wont change the result. Its all depening on the ofrice used and IF one is used.
FAA following devices are shurely a way to go, ... but beside other FAA compilant ones I also built myself one DIY version of a tester which does not come with an ofrice as by this I can provide the Full 8-10 Bar to a combustion chamber, wich not only once showed me that there is a "leak" between the Head and the cylinder. Covering that area with some dishsoap/water mixture showed me in that particual case that ateh sealing ring was flawed – which was prooved when the engine was taken apart.

Shure

I know Hazet isn't cheap, but it is still cheaper to produce a tester with one gauge than two.

All leakdown testers MUST have an orifice/restrictor between the air source and combustion chamber otherwise you can't measure a pressure drop.

[AndrewCologne]

Originally Posted by safe

I know Hazet isn't cheap, but it is still cheaper to produce a tester with one gauge than two.

ok, ... but it works and gives correct outputs – I dont see your point as you point out the argument cheaper. But ... no matter.

All leakdown testers MUST have an orifice/restrictor between the air source and combustion chamber otherwise you can't measure a pressure drop.

I never mentioned something different. But thats not needed for measuring the pressure drop itself, its needed to clamb down the incoming source pressure which will be forwarded to the combustion chamber to a specific pressure value – and that makes the incoming pressure independand as log as you keep the incoming pressure above a specific value depending on the device. As you btw statet further abvove as well.

Just do a test: Take a one gauge device with no ofrice and set the inpout pressure to exactly matching 4 Bar while the output is closed/sealed. If then connecting the output to the combustion chamber the leak in the combustion chamber will show the proportional pressure drop on the gauge. I did the test often and the result is exaclty matching to the ofrice containing Hazet device with 5-8 Bar applied to its input.
The logic is simple ... i.E. Walt compared it to the WUR pressure affecting approach.

[proporsche]

Originally Posted by winders

At 50 psi or 100 psi, 20% leak down is too much for an air-cooled 911. The last time I did a leak down on my race engine, which was just a few weeks ago, I has the same leak down percentage at 50 psi and 100 psi......

yup ...

[hcoles]

Originally Posted by otto_kretschmer

Henry

Your engine is good and you should be jumping with joy at getting those leak down readings. Those are very good numbers. You will have to find something else to worry about.

Small variations in upstream line pressure from your compressor won't matter. I try to keep the test between 100 and 80 psi. My compressor maxes out at 100.

I agree with you and I tend to worry too much about many things. Today for some reason I couldn't find the title. Wife is checking the safe deposit box at the bank today.