Interpreation of leakdown test results

[logician]

Hi,

So how exactly does one evaluate leakdown test results? I recently did a leakdown test with an instrument that I purchased from Pelican Parts (CLT-2PB). I got results between 5 and 9% for all cylinders with the engine warmed up and no oil injected into cylinders. Those results would seem to be pretty good. However, there are two parameters specific to the instrument that seem like they need be considered to evaluate these readings:

1) the orifice size of the instrument, and
2) the calibration pressure of the instrument.

Orifice size is important because the instrument basically measures leakage of your cylinder relative to the orifice size. If you get a leakdown measurement of 10% with one instrument, and then measure again with a different instrument having a different size orifice with half the area, then that other instrument would report 20%. Also, a larger engine will naturally have a larger amount of leakage for equivilent health. So the percentage number by itself is not very useful. My instrument did not indicate the orifice size, so I have no idea what the percentage actually means. Even if I knew the orifice size, I don't know what a good reading is given that orifice size. Is there a standard orifice size used in these instruments?

The "calibration pressure" on my tester was 36psi. This is the pressure that is indicated in the calibration gauge when the measuring gauge reads 0% and the flow is blocked. I had expected this pressure to be higher, something closer to 85psi from the research that I had done. For example, the following Longacre gauge uses a calibration pressure of 100psi:

> http://www.longacreracing.com/instructions/inst.asp?INSTID=25

I would think this pressure might be important to overcome some sticktion type effects. Too low a pressure might not overcome those effects, and so would give an optimistic reading. How important is the calibration pressure, and why would Longacre choose 100psi compared to the PP tester that uses 36psi?

Can anyone with some knowledge of Leakdown testing shed any light on these questions about leakdown testers and how their results should be interpreted?

-Juan

[jmohn]

I'm no expert, but I have a slight understanding, so I'll try an explanation. Since the test is to determine the "percentage" of air leaking from the cylinder, theoretically, it will leak the same percentage regardless of engine size, orifice size or pressure. If a cylinder is leaking 9% at 36 psi, it will also leak 9% at 75 psi. The "amount" of air leaking at 75 psi will be greater, but it's still 9% (it will be 9% of whatever test pressure your using). In the "real world" a large engine will have more surfaces that can leak, the orifice size must be at least large enough to flow more air than the cylinder can possibly leak and the test pressure must be large enough to get an accurate measure of loss but low enough (or the source large enough) that it can be maintained for the test (30-60 psi would seem a reasonable pressure for this test, your not measuring compression just leak-down). Once again the simplest expanation is to remember you're trying to test the "percentage" of air leaking, not the "amount" of air leaking. As far as evaluating the results, you need to know what the manufacturer considers normal. IIRC, for my 3.0 liter, anything less than 10% is considered "good" and anything less than 5% is considered almost "like new".

Jerry M
'78 SC

[RoninLB]

I use 100psi to the gauge.. and leak down numbers alone is not a good engine test.. if anything it should be used with a compression test............Ron

[dad911]

My Leakdown gauges also use 90-100 lbs as a starting pressure. A leakdown test measures the how well the combustion chamber is 'sealed' at TDC. If you have a cylinder that is low, you would then listen to where the air is 'hissing' out of the engine. If it comes out the breather, there is a ring/cylinder wall issue. If it hisses out the intake or exhaust, it is the corresponding intake or exhaust valve not sealing.

[logician]

Hi,

> Since the test is to determine the "percentage" of air leaking
> from the cylinder, theoretically, it will leak the same percentage
> regardless of engine size, orifice size or pressure.

I think orifice size does make a difference. Consider the following example: 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 area.

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

Thus if you want to compare measurements from different instruments, you need to consider their relative orifice sizes. Also, the orifice size really is needed to interpret whether the percentages you measure indicate a healthy or sick engine.

-Juan

[john70t]

Doesn't the cylinder pressures get behind the compression rings to help seal them to the wall? 36lbs might not be enough.

[RoninLB]

Originally posted by john70t

36lbs might not be enough.

sounds good to me.................Ron

[Formerly Steve Wilkinson]

You can also use a leakdown test to determine _what_ is leaking. Listen at the exhaust for hissing that would indicate exhaust-valve leakage, listen at a crankcase source--breather, etc.--for noise that would indicate leakage past rings, carbs for intake valves. Admittedly, I've never done that on a 911, but that's what you do with flat, air-cooled airplane engines, which have totally unconstricted exhaust system and a convenient oil-fill tube for access to the wet-sump crankcase. would assume you could do at least some of that on a 911 engine.

Stephan

[logician]

Hi,

So I've been doing some research on leakdown testers and ended up posting to another thread in the Engine Rebuild forum. There seems to be a lot of confusion about exactly how leakdown testers work, and what that leakdown percentage measures. What is it a percentage of? Does that percentage get larger for larger engines?

So I've come to the following conclusions:

1) Leakdown % is an absolute measure of leakage. It is not relative to any aspect of the engine.

2) The leakdown % tells you the size of the leakage compared to a standard .04" reference orifice in the leakdown instrument. So 10% leakdown says that the total leakage area in the engine is approximately one tenth the size of this .04" reference orifice.

3) It is important to use a 100psi calibration pressure, as that seems to be the standard.

So forget all those other intepretations of leakdown percentage as somehow a percentage of gas escaping (percent relative to what?). Considering the percent as measuring the absolute size of the leakage area is much more useful and direct. It is easy to see why a large engine will have a larger leakage percent than a smaller engine at equivilent health, and why acceptable leakdown percentages are specific to particular engine designs.

To see why I've come to these conclusions, look at the following thread in the Engine Rebuild forum:

> leak down test

-Juan