Porsche Leakdown Test Opinion
02-07-2008, 01:25 PM
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Porsche Leakdown Test Opinion
Let me preface this by saying that I am relaying information and have very little personal technical hands-on knowledge. I think this profile fits about 99% of the Rennlist community 
The head tech at Rusnak, Mike Schatz, is a friend of mine and very well respected. Rusnak management declined to post on a web board as they see no upside. Kimberly, the service advisor gave me permission to recount what they told me and supplied me with an excerpt from the manual about leakdowns.
I recently sold a car and a leakdown was done at Porsche Beverly Hills as part of the PPI.
Then I was asked to look at a car locally at Rusnak Westlake Village for a Rennlister. The values were in the 8-10% range. In each instance, the dealership said the car was good and the buyer, based on information acquired from Rennlist, thought the car might have a problem.
Mike and Mark, the two techs from Rusnak Westlake and Jack Murphy, the head tech from Porsche Beverly Hills consulted on this issue. Between them they have 100 years of Porsche-only experience.
They told me that they view those numbers as perfectly acceptable. They knew of no official Porsche documentation giving % value good/bad cut off ranges. They also said that they could not diagnose any SAI problems from a leakown test, and cars with as little as 25K miles could pop up with the problem regardless of the leakdown number. Also, since the leakdown test is performed with the engine at TDC (with the valves closed) it can give no insight into the valve-guide wear. They said that these cars typically have some level of valve-guide sealing issues.
They ONLY use leakdown tests to diagnose gross mechanical issues.
The head tech at Rusnak, Mike Schatz, is a friend of mine and very well respected. Rusnak management declined to post on a web board as they see no upside. Kimberly, the service advisor gave me permission to recount what they told me and supplied me with an excerpt from the manual about leakdowns.
I recently sold a car and a leakdown was done at Porsche Beverly Hills as part of the PPI.
Then I was asked to look at a car locally at Rusnak Westlake Village for a Rennlister. The values were in the 8-10% range. In each instance, the dealership said the car was good and the buyer, based on information acquired from Rennlist, thought the car might have a problem.
Mike and Mark, the two techs from Rusnak Westlake and Jack Murphy, the head tech from Porsche Beverly Hills consulted on this issue. Between them they have 100 years of Porsche-only experience.
They told me that they view those numbers as perfectly acceptable. They knew of no official Porsche documentation giving % value good/bad cut off ranges. They also said that they could not diagnose any SAI problems from a leakown test, and cars with as little as 25K miles could pop up with the problem regardless of the leakdown number. Also, since the leakdown test is performed with the engine at TDC (with the valves closed) it can give no insight into the valve-guide wear. They said that these cars typically have some level of valve-guide sealing issues.
They ONLY use leakdown tests to diagnose gross mechanical issues.
Last edited by ScottMellor; 02-07-2008 at 01:41 PM.
02-07-2008, 02:02 PM
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Interesting, thanks for posting.
02-07-2008, 02:35 PM
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Scott, the valve guide issues that are cars have as they age CAN be measured if you remove the turbochargers to get to the rockers and valve springs.. While air pressure is applied to the cylinder you can take a screw driver and apply pressure to the top of the valve stem. If you can move the stem, you will see a direct impact on the leakdown reading. People just don't realize that you are applying 100 lbs of air pressure in a cylinder. Scott on your car you only had ONE cylinder tested at 8% wasn't it?? They didn't even get to the usual high failing cylinders. I would suspect your numbers would be in the 8 to 12% loss reading which points to having a valve grind done.
At 8% or even 10% you hear the air coming out of the exhaust, because usually it is the exhaust valve. If a customer or owner starts to balk.. Then put the money as a wager and take off the valve cover to show how a 10% leakdown increases as you walk the valve in the worn valve guide. The 10% number grows to over 25% as the valve is walked off of it's seat. BTW the proceedure done on a brand new cylinder head will never see these issues. You have to hit the valve open with a mallet to drop the air out of the cylinder on a leakdown test.
A burnt exhaust valve will typically give you a 70 to 90% leakdown. The valve head will look like you wife cut a pie and removed a slice. A incident of a missed shift usually have the valves kissing the pistons, and the leakdown numbers will be 40 to 60% loss on that cylinder.
Like I have said on previous threads, if your engine is sitting at 8% then you seriously need to start saving your pennies, because your engine will need a topend overhaul sooner vs later.
At 8% or even 10% you hear the air coming out of the exhaust, because usually it is the exhaust valve. If a customer or owner starts to balk.. Then put the money as a wager and take off the valve cover to show how a 10% leakdown increases as you walk the valve in the worn valve guide. The 10% number grows to over 25% as the valve is walked off of it's seat. BTW the proceedure done on a brand new cylinder head will never see these issues. You have to hit the valve open with a mallet to drop the air out of the cylinder on a leakdown test.
A burnt exhaust valve will typically give you a 70 to 90% leakdown. The valve head will look like you wife cut a pie and removed a slice. A incident of a missed shift usually have the valves kissing the pistons, and the leakdown numbers will be 40 to 60% loss on that cylinder.
Like I have said on previous threads, if your engine is sitting at 8% then you seriously need to start saving your pennies, because your engine will need a topend overhaul sooner vs later.
02-07-2008, 02:37 PM
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My 2 ¢
A year ago I had a leakdown test done on my engine. The numbers were 14, 10, 26, 8, 12 , and 12. Ignoring the 26% cylinder the rest averaged 11%. Averaging all 6 cylinders the percentage was 14%. When the engine was torn down loose valve guides and worn seats were found on all 6 cylinders and on the 26% cylinder a piece of swarf was found trapped between the head and the barrel with clear signs of compression loss. Thank you Porsche for the factory installed extra.
Before the rebuild the car made 400 bhp on a rolling road with other 993 turbos making similar numbers. Just one problem - my car had the factory 430 kit so looked to be about 30 bhp down. And on a vmax event the car topped out at 167 mph. I haven't had the car back to a rolling road since then but the vmax speed on the same runway went from 167 to 184 and the car's 60-130 mph time dropped over a second. Off boost response is improved as well.
What am I trying to say? I guess I'm trying to say that an average of 14% leakdown is a measurable power loss and it doesn't need to be twice that to be noticeable. And once the valve guides are appreciably worn the longer they're left that way the more the moving valve heads will beat up the valve seats.
Would I have noticed the loss had I not had some pseudo-objective ways of measuring it? Perhaps not. The car was still very quick. It's just a matter of whether or not you want 100% (or as near as possible) of the car's rated hp or will settle for 90%.
Agreed; the SAI system isn't "exposed" when a cylinder's at TDC with the valves closed because the SAI system injects air into the exhaust port, not the combustion chamber.
Not sure I agree - valve guide wear leads to valve seat wear which results in loss of compression and is visible during a leakdown test.
Before the rebuild the car made 400 bhp on a rolling road with other 993 turbos making similar numbers. Just one problem - my car had the factory 430 kit so looked to be about 30 bhp down. And on a vmax event the car topped out at 167 mph. I haven't had the car back to a rolling road since then but the vmax speed on the same runway went from 167 to 184 and the car's 60-130 mph time dropped over a second. Off boost response is improved as well.
What am I trying to say? I guess I'm trying to say that an average of 14% leakdown is a measurable power loss and it doesn't need to be twice that to be noticeable. And once the valve guides are appreciably worn the longer they're left that way the more the moving valve heads will beat up the valve seats.
Would I have noticed the loss had I not had some pseudo-objective ways of measuring it? Perhaps not. The car was still very quick. It's just a matter of whether or not you want 100% (or as near as possible) of the car's rated hp or will settle for 90%.
Not sure I agree - valve guide wear leads to valve seat wear which results in loss of compression and is visible during a leakdown test.
02-07-2008, 02:46 PM
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It's too bad the 3 techs decline to post. As I said I have no hands on knowledge or experience.
Kevin: Yes, oddly the buyer of my car only did one cylinder. I have no dea why that was.
Phelix: I do see on the tech sheet that they want to see less than 5% difference across the cylinders.
Kevin: Yes, oddly the buyer of my car only did one cylinder. I have no dea why that was.
Phelix: I do see on the tech sheet that they want to see less than 5% difference across the cylinders.
Last edited by ScottMellor; 02-07-2008 at 03:12 PM.
02-07-2008, 03:14 PM
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Scott, the valve guide issues that are cars have as they age CAN be measured if you remove the turbochargers to get to the rockers and valve springs.. While air pressure is applied to the cylinder you can take a screw driver and apply pressure to the top of the valve stem. If you can move the stem, you will see a direct impact on the leakdown reading. People just don't realize that you are applying 100 lbs of air pressure in a cylinder. Scott on your car you only had ONE cylinder tested at 8% wasn't it?? They didn't even get to the usual high failing cylinders. I would suspect your numbers would be in the 8 to 12% loss reading which points to having a valve grind done.
At 8% or even 10% you hear the air coming out of the exhaust, because usually it is the exhaust valve. If a customer or owner starts to balk.. Then put the money as a wager and take off the valve cover to show how a 10% leakdown increases as you walk the valve in the worn valve guide. The 10% number grows to over 25% as the valve is walked off of it's seat. BTW the proceedure done on a brand new cylinder head will never see these issues. You have to hit the valve open with a mallet to drop the air out of the cylinder on a leakdown test.
A burnt exhaust valve will typically give you a 70 to 90% leakdown. The valve head will look like you wife cut a pie and removed a slice. A incident of a missed shift usually have the valves kissing the pistons, and the leakdown numbers will be 40 to 60% loss on that cylinder.
Like I have said on previous threads, if your engine is sitting at 8% then you seriously need to start saving your pennies, because your engine will need a topend overhaul sooner vs later.
At 8% or even 10% you hear the air coming out of the exhaust, because usually it is the exhaust valve. If a customer or owner starts to balk.. Then put the money as a wager and take off the valve cover to show how a 10% leakdown increases as you walk the valve in the worn valve guide. The 10% number grows to over 25% as the valve is walked off of it's seat. BTW the proceedure done on a brand new cylinder head will never see these issues. You have to hit the valve open with a mallet to drop the air out of the cylinder on a leakdown test.
A burnt exhaust valve will typically give you a 70 to 90% leakdown. The valve head will look like you wife cut a pie and removed a slice. A incident of a missed shift usually have the valves kissing the pistons, and the leakdown numbers will be 40 to 60% loss on that cylinder.
Like I have said on previous threads, if your engine is sitting at 8% then you seriously need to start saving your pennies, because your engine will need a topend overhaul sooner vs later.
What would you wager a 20% leakdown (exh. valve) on one cylinder is caused by? Slight defect in seat/valve?
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02-07-2008, 03:24 PM
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Let us re-read the essence of the post....
I am no tecchie either, but the above is also the exact opinion of a couple of experienced Porsche engine builders I have discussed this issue with not too long ago, across both sides of the ocean, and who actually have really built dozens, or rather certainly over a hundred Porsche engines.
Is it better to have 4%? Certainly!!
Phelix, the issue with your engine's power drop was specifically that 26% cylinder, not the average of 11% of all the others. You cannot disregard that you had a serious compression problem with one of the cylinders, so this does not really relate to the question raised by Scott IMO
Is it better to have 4%? Certainly!!
Phelix, the issue with your engine's power drop was specifically that 26% cylinder, not the average of 11% of all the others. You cannot disregard that you had a serious compression problem with one of the cylinders, so this does not really relate to the question raised by Scott IMO
02-07-2008, 03:27 PM
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Scott, I'm sure that Phelix didn't see my post prior to posting his..
To wrap it up, the engine will be down on power.. But more importantly is the added heat that is created. Detonation is increased and cylinder head temps increase.. I will post here shortly the damage that is done to a cylinder head. What answer do you want to hear?
Then if you had 3 techs on your sideline, why didn't they insure you engine with a warranty for the sale of your car?? You didn't have to accept the offer and sell your car. There talk is cheap when you are trying to sell a car. The numbers are the numbers.. I bet you a plane ticket that if I fly down to LA, I will find more than one cylinder greater than 8% on your car that you sold. I cannot tell you how many people on this VERY SAME forum have purchased cars from people and have listened to tech say the very same thing. Only to find out that when they get the car home, they can't PASS emissions due to the incomplete combustion. Yes, I have a bug up my ***. I am the guy that gets the Sunday call with a new buyer re-evaluating his life when his new mechanic has just given him his $15K estimate to rebuild his topend (and he has already been to a previous shop with the same results) And all I have to offer after the fact is support and a reassurance that you will have a Great running engine once it is fixed and running. Re-read Phelix post.. He had a cylinder that had debris (metal) located between the cylinder head and cylinder liner.. Exhaust gases were passing between this raised area and his leakdown showed 26% This does have a iimpact on the power output of the engine. If you have incomplete combustion you have heat that is being generated. This heat wrecks other components.
To wrap it up, the engine will be down on power.. But more importantly is the added heat that is created. Detonation is increased and cylinder head temps increase.. I will post here shortly the damage that is done to a cylinder head. What answer do you want to hear?
Then if you had 3 techs on your sideline, why didn't they insure you engine with a warranty for the sale of your car?? You didn't have to accept the offer and sell your car. There talk is cheap when you are trying to sell a car. The numbers are the numbers.. I bet you a plane ticket that if I fly down to LA, I will find more than one cylinder greater than 8% on your car that you sold. I cannot tell you how many people on this VERY SAME forum have purchased cars from people and have listened to tech say the very same thing. Only to find out that when they get the car home, they can't PASS emissions due to the incomplete combustion. Yes, I have a bug up my ***. I am the guy that gets the Sunday call with a new buyer re-evaluating his life when his new mechanic has just given him his $15K estimate to rebuild his topend (and he has already been to a previous shop with the same results) And all I have to offer after the fact is support and a reassurance that you will have a Great running engine once it is fixed and running. Re-read Phelix post.. He had a cylinder that had debris (metal) located between the cylinder head and cylinder liner.. Exhaust gases were passing between this raised area and his leakdown showed 26% This does have a iimpact on the power output of the engine. If you have incomplete combustion you have heat that is being generated. This heat wrecks other components.
02-07-2008, 03:40 PM
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Jean, then what are your suggestions for a pre-purchase guideline for leakdown?
It has been my opinion for what it is worth, that if the BUYER has a guideline of a leakdown figures from 0 to 6% he has nothing to worry about. Leakdowns from 8 to 12% point to valve guide wear. Greater than 12% points to valve to seat issues.. When the the exhaust valve cannot function by wicking out the heat due to the natural process of the sodium filled stem, the valve can fail. The excessive heat causes the valve head to split. Once this happens it is a short duration that the valve head breaks off and usually find a location in the top of the piston.
It gets really ugly when a valve plants itself into a piston. The progressive damage usually will destroy a cylinder head/piston and rod. If you are lucky you excape without crankshaft damage.
Why drive your engine to this exteme. The leakdown test is a barometer of health of your engine. There are plenty of "great" examples of cars that have leakdown test between 0 and 6% out there. Find them and buy them.. A engine that has 8% and greater I would pass on or discount the purchase price for a topend rebuild.
It's a bitch when you get your 70K car home and not be able to pass emissions.
It has been my opinion for what it is worth, that if the BUYER has a guideline of a leakdown figures from 0 to 6% he has nothing to worry about. Leakdowns from 8 to 12% point to valve guide wear. Greater than 12% points to valve to seat issues.. When the the exhaust valve cannot function by wicking out the heat due to the natural process of the sodium filled stem, the valve can fail. The excessive heat causes the valve head to split. Once this happens it is a short duration that the valve head breaks off and usually find a location in the top of the piston.
It gets really ugly when a valve plants itself into a piston. The progressive damage usually will destroy a cylinder head/piston and rod. If you are lucky you excape without crankshaft damage.
Why drive your engine to this exteme. The leakdown test is a barometer of health of your engine. There are plenty of "great" examples of cars that have leakdown test between 0 and 6% out there. Find them and buy them.. A engine that has 8% and greater I would pass on or discount the purchase price for a topend rebuild.
It's a bitch when you get your 70K car home and not be able to pass emissions.
02-07-2008, 04:01 PM
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What I agreed with ( or at least meant to, in case I didn't) was that a leakdown test showing between 8-10% across cylinders is perfectly acceptable. I never said it is great, nor that one does not have to worry about having to open his engine at some point down the line, after all, who does not. If this is not a potential risk a buyer of a 10 year old car is willing to take, then he should buy a more recent car.
I do think, based on asking people in the know as well, FWIW, that I can comfortably buy an engine with 10% leak down across cylinders. However, If I saw an unusual leak in one of the cylinders, I would run.
I recently saw a difference in compression in only one of my cylinders, so I performed a leak down test, and found a 4% higher leak down number in that same cylinder while the others were stable. I opened the next day the engine. I had a slight head leak and broken chain tensioner.
As to advising someone to buy an engine with a 6% leakdown instead of 8%, that makes perfect sense obviously. I must say that I would never pay $70k for a 993TT that has 10% leak down, but I did not think this was the original question in the thread. I have no idea what is behind it as well.
I do think, based on asking people in the know as well, FWIW, that I can comfortably buy an engine with 10% leak down across cylinders. However, If I saw an unusual leak in one of the cylinders, I would run.
I recently saw a difference in compression in only one of my cylinders, so I performed a leak down test, and found a 4% higher leak down number in that same cylinder while the others were stable. I opened the next day the engine. I had a slight head leak and broken chain tensioner.
As to advising someone to buy an engine with a 6% leakdown instead of 8%, that makes perfect sense obviously. I must say that I would never pay $70k for a 993TT that has 10% leak down, but I did not think this was the original question in the thread. I have no idea what is behind it as well.
02-07-2008, 04:01 PM
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Kevin, I no longer have a "dog in this hunt". A lot of people have been inquiring about what these numbers mean. I recently obtained some information on the topic garnered from what would appear to be a fairly qualified source and posted it for general information. Sorry about the insect in your **** cavity.