Question about leakdown tests

[Ghoste]

How much would consider the results to worsen in a warm engine over a hot one?  Not cold, but one that has cooled to the poitn where you can easily touch any part of the block with your bare hand and not feel any discomfort (maybe the temp of bathwater??)

[Tilar]

I wouldn't think there would be much if any at all between a warm and a hot engine, unless there was some crack that opened up when it was hot.

[elacruze]

How much would consider the results to worsen in a warm engine over a hot one?  Not cold, but one that has cooled to the poitn where you can easily touch any part of the block with your bare hand and not feel any discomfort (maybe the temp of bathwater??)

I would expect no change. If you see a significant change in leakdown between hot and cold, I'd look to valve seat sealing problems. Do you have aluminum heads? Solid lifters? What is your leakage now?

[FLG]

Might be some difference but as long at everything is concistant between cylinders your all good.

[c00nhunterjoe]

not neccesarily, even if they are all even at 15% leakdown, your motor is in bad shape.  most stock applications call for less then 10% but a performance build should be 5% at worst

[FLG]

not neccesarily, even if they are all even at 15% leakdown, your motor is in bad shape.  most stock applications call for less then 10% but a performance build should be 5% at worst

Joe, most deff...i was saying assuming the numbers are where there supposed to be your looking for consistency...though cold they might have a tad more leak down.

[c00nhunterjoe]

ah, got ya now 

[John_Kunkel]

Think about this, the control orifice in most leakdown testers is .040"...that's .00126 square inches. If you had an accumulated leak the size of .00126 square inches you'd have 100% leakdown but does the motor's power really suffer with a leak that small?

Then think about this, at 6000 rpm the complete combustion cycle occurs 50 times a second, nearly as fast as the electricity cycles in an AC light bulb. Fast huh? Fast enough that another combustion cycle occurs before the previous cycle's leakage has much effect?

I'd place more faith in a dynamic compression test that mimics actual running conditions rather than a static one.

(stepping off of soapbox)

[Ghoste]

You can't say things like that and then just get off the soapbox and walk away. 
You make an interesting point and it flies contrary to most diagnostic instructions where it is commonly held that the leakdown is the preferrred and more accurate of the two.
I think personally that it's an important tool for determining causes of issues that crop up in a compression test.  That is to say, one should be used with the other to narrow down a specific failure than alone to determine overall condition. (does that make sense?)

[John_Kunkel]

I see the only value of the leakdown test as being able to pinpoint the source of the leak i.e. a leaking valve/head gasket is more severe than a leaking ring.

My background is in aircraft maintenance where the leakdown test is referred to as "the A&P mechanic's full employment guarantee". Many cylinders are routinely removed from perfect running engines because of the unofficial 25% leakdown mandate. I've had cylinders that exceed 25% leakdown meet or exceed the dynamic compression of the perfect cylinder right next to it.

[c00nhunterjoe]

thats part of the reason i like to do a running compression test on suspect cylinders......not to steal the topic but what aircraft do you work on? i dont work on aircraft but i do have good knowledge on the agt 1500....(m1 abrahms)

[elacruze]

I see the only value of the leakdown test as being able to pinpoint the source of the leak i.e. a leaking valve/head gasket is more severe than a leaking ring.

My background is in aircraft maintenance where the leakdown test is referred to as "the A&P mechanic's full employment guarantee". Many cylinders are routinely removed from perfect running engines because of the unofficial 25% leakdown mandate. I've had cylinders that exceed 25% leakdown meet or exceed the dynamic compression of the perfect cylinder right next to it.

Um, yeah, when the ring lands are hammered like the grand canyon I'd expect that result. I had a 305 GMC pickup that ran like a clock once it was started, but the ring lands and cylinder TDC wear were so bad it wouldn't start without ether. It took a .040 bore to clean it up.
Static cylinder leakage usually correlates directly to dynomometer and track performance.
I'm not being argumentative, but what's the maximum RPM for piston aircraft engines? Generally, they are under much less stress and speed than automotive engines. An engine running perfectly and having good dynamic compression at 4000rpm is no guarantee it would remain so at 6000rpm.  
What is the measured wear on the pistons and cylinders removed from these 25% leak engines?

[John_Kunkel]

The basic reason aircraft prefer the leakdown over a dynamic test is the danger associated with the spinning propellor. The Air Force did use dynamic testing in addition to leakdown, I've seen the 18-cylinder compression testing kit and seen them installed (ever see a radial engine that looked like it was having a bad hair day?)

Air cooled piston aircraft engines have choked bores, IOW the top of the bore is smaller to account for the expansion that is greatest near the heat source which is the cylinder head. Most mechanics don't measure cylinders that are rejected for high leakdown, they just exchange them for overhauled/new. If a valve is the source of the leakage they usually repair that in house but not always.

One of the largest aircraft engine makers is Continental. Years ago they were receiving lots of complaints that their new/rebuilt large six-cylinder engines (520 cid) were failing leakdown shortly after installation, Continental attributed it to revised choke dimensions and tried to convince mechanics to ignore the low readings. When that failed they purposely sabotaged an engine so that it had 100% leakdown and compared the dyno numbers to the unmolested engine and published the results. The difference was no more than would normally be seen from one pull to another on the same engine. Of course, the crankcase pressure increased accordingly.

The fact that aircraft engines turn slower is irrevelant, the faster an engine turns the less effect leakdown will have because the time between combustion events is shorter.

[elacruze]

Interesting. Is a hot-engine leakdown specified? Does it come in line after it's up to operating temperature? Are the bores iron lined, or aluminum?
High RPM contributes to ring flutter, particularly if the groove clearance is too loose.

Since I've never witnessed an intentional choked bore in automotive or motorcycle engines, I'd suggest that we're comparing apples to oranges. It's easy enough to achieve 2-3% leakdown in these engines with a straight bore, and possible with care to achieve less than 1%.

The basic reason aircraft prefer the leakdown over a dynamic test is the danger associated with the spinning propellor. The Air Force did use dynamic testing in addition to leakdown, I've seen the 18-cylinder compression testing kit and seen them installed (ever see a radial engine that looked like it was having a bad hair day?)

Air cooled piston aircraft engines have choked bores, IOW the top of the bore is smaller to account for the expansion that is greatest near the heat source which is the cylinder head. Most mechanics don't measure cylinders that are rejected for high leakdown, they just exchange them for overhauled/new. If a valve is the source of the leakage they usually repair that in house but not always.

One of the largest aircraft engine makers is Continental. Years ago they were receiving lots of complaints that their new/rebuilt large six-cylinder engines (520 cid) were failing leakdown shortly after installation, Continental attributed it to revised choke dimensions and tried to convince mechanics to ignore the low readings. When that failed they purposely sabotaged an engine so that it had 100% leakdown and compared the dyno numbers to the unmolested engine and published the results. The difference was no more than would normally be seen from one pull to another on the same engine. Of course, the crankcase pressure increased accordingly.

The fact that aircraft engines turn slower is irrevelant, the faster an engine turns the less effect leakdown will have because the time between combustion events is shorter.

[John_Kunkel]

It's recommended that leakdown tests be performed on engines that are at operating temperature but that's pretty much impossible because cool down starts as soon as it's shutdown. The difference in aircraft/auto leakdown tests is that the recommended procedure for aircraft is to introduce air to the cylinder with the piston below TDC and then use the prop to lever the piston to TDC, this helps seat the rings against the bottom of their lands. Some guys (myself included) like to rock the piston back and forth TDC to get the highest reading possible, this sorta mimics the dynamic test.

The cylinders are aluminum with ferrous liners, the heads are aluminum and are threaded onto the cylinders.

[oldschool]

john are you a pilot too? i own and fly a bonanza. i too do annual compression leakdown test on my io 470.i also do them on my cars....