heliflyer

Hi Guys,
Following on from my oil and smoke issues (see thread Provent disaster),
I have done a leakdown test and would appreciate some advice on the results.
I wasn't able to find a commercial leakdown tester locally so made my own from a compression tester and tire inflator (remove the valve from the end of the compression tester adapter).

I then connected this to a 1.5hp air compressor delivering air at 100psi and tested each cylinder on a warm engine with each cylinder at TDC, and compared the pressure on the tire inflator to the output pressure of the compressor.

First off - none of the cylinders would actually hold a pressure - is that normal? I know that leakage is expected but how much?

Cylinders 1-4 all showed 90psi on the inflator guage, with all air leaking into the sump and no leaks past either valve or into the coolant system.
Cylinders 5-8 were all slightly higher, averaging at 95 psi, again with all air leaking into the sump and no other leaks.

In your opinion does this seem ok, or does it indicate new rings needed.

Thanks

Jadz928

I'm sure you would get more sophisticated answers, but 5-10% doesn't seem high to me.

What is missing from your results is how much time it took to leak down.

blown 87

What size orfice did you use?
I see no way you are going to get any kind of data from that set up that we can compare to.
A leak down tester puts a metered amount of air into the cylinder, which I am not seeing here, or I am missing something.

heliflyer

The opening at the end of the adapter is approx 3mm diameter (a standard tire valve insert fits in it).

When you say a metered amount of air, are you talking volume or flow rate?
This set up does not insert a set volume, but the flow rate is maintained at 100psi, output from the compressor is 7cfm.

As soon as air is introduced into the cylinders it begins to blow past the rings into the sump.

blown 87

3mm is way to big, I think the size is .040, but it is a set size and it is a metered amount that you need to get any kind of comparable numbers.
You can not just throw air into a cylinder and get meaningful numbers.

I found some wiki info and IIRC it sounds right.
http://en.wikipedia.org/wiki/Leak-down_tester

On aircraft we use 80 PSI as the control number on a two gauge set up, I use a single gauge % tester here at the shop.
I do each cylinder at TDC on compression with a warm engine that is locked at TDC.

I hope this helps.

Straight shop air in a set up like you have can be used to find gross leaks though.

tveltman

Greg,
I'm not so sure orifice size actually matters. You are measuring a pressure differential between two chambers. I think the orifice is just for mass balance, since the input pressure is constant, as regulated by the air compressor. You should just be measuring how leaky the one cylinder is. I think many people make their own gauges, and I've never read anything about how they specifically sized the hole in the middle. I could definitely be wrong, but I don't think it matters

heliflyer

Thanks for the link Greg - I'll add a restrictor of the suggested size and redo the tests.

blown 87

It does matter, that is why they have specific sized orfices for different engines, if you are letting a larger amount of air into the cylinder you will show less pressure loss on the second gauge.
And many people make leakdown gauges and get wrong readings, trust me on this.

blown 87

You also need a regulator at the input of your gauge, not at the compressor, 25 feet of hose will change the pressure.
It is critical for each cylinder to be at TDC on compression BTW.

tveltman

I stand corrected. I just bought the HF gauge...

jcorenman

Orifice size does not matter if all you want to do is compare each cylinder to others in the same engine, or to other engines that were measured with the same gauge with the same orifice. All you are doing is measuring leakage rate by looking at the pressure drop across a restriction, if you want real numbers then you need a known resistance.

The problem is that if the rings are all badly worn but equally, then comparing the cylinders on the same engine won't tell you anything about the amount of wear. You need a restriction such as the 0.040" x 0.25" long orifice in the wiki article to make the actual number meaningful. Or a known-fresh engine to use as a reference point.

Jay, I think your tester is fine (assuming your 100psi shop air is regulated), just add a 0.040" x 0.25" long restriction before the gauge.

heliflyer

Thanks Jim - shop air is regulated so will add the restriction and see what the difference is.

smiffypr

If you pull the trigger without the adaptor screwed into a spark-plug hole, what does the pressure gauge drop to? That pressure would be 100% leak. So if that gave 40psi, a test result of 94psi (6 off 100) would be 10% (of 100-40).
I made a similar set-up to yours, but connected the hose directly, i.e. without the quick release coupling, and using the above "maths", the results were similar to a proper snap-on leak-down tester. I think 10-15% leak-down is normal for a 928.

Personally I think a leak-down test is useful for seeing (hearing) where the leaks are, more than getting a good/bad figure.

Also, when turning the engine to the test position, you must only turn it clockwise, and you must stop before TDC so that the compression rings are hard against the bottom of the ring grooves.

You will need one person to hold the still against the air pressure and another to operate the air trigger.

Smiffy

heliflyer

Thanks for the info Smiffy - your guess of 40psi is just about spot on. As you suggested, engine was only rotated clockwise, and crankshaft was held just before TDC on each cylinder while it was tested.
Looks like my averages are between 10-15% across the cylinders.

As the engine runs very well and pulls strongly, I think at this stage I'm just going to pull the engine to resolve my leaking cam carriers issue and then try running the car for a while using something like Ametech oil - http://cgi.ebay.co.uk/ws/eBayISAPI.d...=STRK:MEWAX:IT and see if there is any improvement in the slight smoke issues.

Dave928S

+1 on what Smiffy said ... but I've achieved meaningful figures using your method and a stopwatch, after doing the following.

Test #1 ..... compression test of all cylinders noting first compression stroke increase ... then final highest reading. All my cylinders showed about 75psi on the first compression stroke, then rose to the final reading with the next and a fraction more with the third ... at normal cranking speed. I had normal compression readings on all but one ... which was 50psi down. I also checked with a borescope to rule out gross bore damage.

Test #2 ...... using a syringe I squirted the same small amount of oil into each cylinder one at a time and then did a compression test again. All normal cylinders rose 10 - 15 psi as a result of normal improvement of ring sealing .... with the bad one rising 60 psi to give the same reading as the others.

Conclusion: Had to be a ring problem as oil improved compression ... valves or head gasket wouldn't have responded to oil.

Test #3 ..... Same as your test .... taking it to 100psi then timing how long it took to drop to 10psi. Tests on all good cylinders gave a time of about 6 seconds ... with the bad cylinder giving 1.3 seconds.

Test #4 ... Holding 100psi to each cylinder, as a double check, I then checked for pressurisation of the cooling system and crankcase, and air leakage past valves to the intake and exhaust systems. There was no air getting to the exhaust from each cylinder at TDC, none to the intake system and no pressurisation of the cooling system ... but,with the oil filler cap off, the air coming from the crankcase from the bad cylinder was massive (enough to spin the strainer).

Conclusion: Had to be rings ... after stripping the motor I found it was a broken top compression ring ... with no other problem with valves or gaskets.

So ..... I found that results can be meaningful using a simple stopwatch leakdown test. Every motor is different, so I don't see that a comparison of leakdown on a motor compared to a standard orifice size gives you any more information than using a stop watch will ... as long as you have a good cylinder to compare to ... which is highly likely. One broken ring gave that huge difference so I'm sure anything else would have been just as dramatic.

Edit: just saw your last post. Sounds encouraging if they're all the same. I'd be interested if you have the engine still at the stage where you can take a reading to see what time you get to leak down from 100psi to 10psi.