Effects of high-duration camshafts and reliability of leak-down tests in gauging engine health

[Iron Mike]

I've heard and read various things regarding what is and isn't normal ranges for leak-down tests. Some sources say anything over 10% requires attention, others say over 30% is catastrophic and in need of rebuild, yet the tools used to perform such tests often show that 10-40% leak-down is considered 'low'. On the flipside, I have also heard that high duration camshafts and other variables can cause healthy engines to give readings of 30%+. In this particular instance also, a compression test showed nothing of concern and all cylinders were within 8psi of each other.

1. What else can cause leak-down test results to be 'high' in a healthy engine?

2. Can truly healthy engines produce high leak-down test results?

3. If high-duration camshafts can be the cause of high leak-down results, why do they cause this?

4. Are leak-down tests reliable measures of engine health for moderately worked engines? i.e. are the definitions of high and low results only relevant for standard cars?

Any definitive insight (rather than speculation) would be much appreciated.

Edited January 29, 2020 by Iron Mike

[GTSBoy]

High duration camshaft CANNOT cause high leakdown results. Leakdown tests are only done when both valves are closed. Ergo, cam duration is not an issue. cams with big duration will cause low compression test results, for the obvious reasons that they let the compression back out through the intake valve while the piston is closing.

Truly healthy engines can have higher than expected leakdown because forged pistons with higher clearances and wider ring gaps will simply leak more.

[KiwiRS4T]

1 hour ago, Iron Mike said:

I've heard and read various things regarding what is and isn't normal ranges for leak-down tests. Some sources say anything over 10% requires attention, others say over 30% is catastrophic and in need of rebuild, yet the tools used to perform such tests often show that 10-40% leak-down is considered 'low'. On the flipside, I have also heard that high duration camshafts and other variables can cause healthy engines to give readings of 30%+. In this particular instance also, a compression test showed nothing of concern and all cylinders were within 8psi of each other.

1. What else can cause leak-down test results to be 'high' in a healthy engine?

2. Can truly healthy engines produce high leak-down test results?

3. If high-duration camshafts can be the cause of high leak-down results, why do they cause this?

4. Are leak-down tests reliable measures of engine health for moderately worked engines? i.e. are the definitions of high and low results only relevant for standard cars?

Any definitive insight (rather than speculation) would be much appreciated.

A leak down test doesn't distinguish between modified and stock engines. There is no definitive answer its all relative. In my opinion anything more than 10% needs to be investigated which is not the same as saying you should panic and order a complete rebuild.

[Iron Mike]

52 minutes ago, GTSBoy said:

High duration camshaft CANNOT cause high leakdown results. 

I have heard from a reputable GTR workshop that it can though I did not get the chance to ask them why ?

[Dose Pipe Sutututu]

Unless you're dealing with a motor built for big boost and big horse power with very big ring gaps, then yes you'll get an undesirable leak down result.

Pretty much what @GTSBoy said but in simpler terms.

[Iron Mike]

5 minutes ago, Dose Pipe Sutututu said:

Unless you're dealing with a motor built for big boost and big horse power with very big ring gaps, then yes you'll get an undesirable leak down result.

Big ring gaps would cause low compression on a comp test but this particular engine is reading 162-170 psi across all cylinders.

[GTSBoy]

33 minutes ago, Iron Mike said:

Big ring gaps would cause low compression on a comp test but this particular engine is reading 162-170 psi across all cylinders.

Nope. Will cause lower psi than it might without the large gaps, but compression is a very fast process and the leak rate out through the gaps is "slow" compared to the compression pulses. Whereas, leakdown tests are steady state.

The sad fact is.....if the engine is truly leaking more than it should and you are "sure" that the bottom end is healthy, then the seats are bad.

[GTSBoy]

57 minutes ago, Iron Mike said:

I have heard from a reputable GTR workshop that it can though I did not get the chance to ask them why

Physics would like to argue.

[joshuaho96]

12 hours ago, Iron Mike said:

I have heard from a reputable GTR workshop that it can though I did not get the chance to ask them why ?

There's only two good explanations. Either they were thinking of compression tests when they said that or they're not actually as reputable as you say they are.

[Iron Mike]

Ok cheers. Also with the various ranges for acceptable leakage often being quoted at <10%, <20% , <30%, why then do all the leak down guages show it being "low" and in the 'green' under 40%? See pic attached for reference

[Ben C34]

Because there is no standard. Much like engine building each builder will have their own preference, with most agreeing within a certain range. 
it’s not clear cut. , and all the tests in the world on an assembled engine won’t tell you if it is about to Grenade itself due to a small mistake/issue.

 

[KiwiRS4T]

10 hours ago, Iron Mike said:

Ok cheers. Also with the various ranges for acceptable leakage often being quoted at <10%, <20% , <30%, why then do all the leak down guages show it being "low" and in the 'green' under 40%? See pic attached for reference

Because Snap On don't want you to blame their tools for telling you your engine is farked.  To say 40 - 60% leakage is "moderate"  is plain ridiculous. Have you actually carried out a leakdown test on your car? What were the numbers? If you have you will know that the test is carried out with the piston at TDC and both valves closed so its hard to see why you think the lift or duration of the cam should make any difference.

[Iron Mike]

12 minutes ago, KiwiRS4T said:

Because Snap On don't want you to blame their tools for telling you your engine is farked.  To say 40 - 60% leakage is "moderate"  is plain ridiculous.

Wouldn't it be the other way around? Ie. Wouldn't Snap on want that leakage bound to be conservative to avoid the scenario where an engine is actually f****d and the guage implies it's in good condition?

Regardless, it's not just Snap on. All the guages I've seen have the same bound for low leakage being 40%.

[GTSBoy]

Who cares what the gauge says?

[joshuaho96]

7 hours ago, Iron Mike said:

Wouldn't it be the other way around? Ie. Wouldn't Snap on want that leakage bound to be conservative to avoid the scenario where an engine is actually f****d and the guage implies it's in good condition?

Regardless, it's not just Snap on. All the guages I've seen have the same bound for low leakage being 40%.

Any more than 15-20% is cause for concern. If the engine is still being broken in the rings might not seal very well but afterwards it should be 10% or less when the engine is warmed up.