Leak Down Versus Compression Test?
05-21-2003, 08:29 AM
#2
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Piers,
A compression test tells you just that, what each cylinders compression is. Period. A leak down, in talented hands, can tell you about the condition of the valves and rings. Our 964's seem to be hard to get a good leakdown readings on, perhaps, I'm surmising, due to the unique shape of the cylinders (they taper). So, a compression test and leakdown test provide different sets of information, and really complement each other (I'd expect a compression test that shows a cylinder with weak compression, compared to the others, to also have higher leakdown, and the leakdown can help focus on the problem). As a caution, if someone does a leakdown and says you need to rebuild your engine, get a second opinion at another shop.
A compression test tells you just that, what each cylinders compression is. Period. A leak down, in talented hands, can tell you about the condition of the valves and rings. Our 964's seem to be hard to get a good leakdown readings on, perhaps, I'm surmising, due to the unique shape of the cylinders (they taper). So, a compression test and leakdown test provide different sets of information, and really complement each other (I'd expect a compression test that shows a cylinder with weak compression, compared to the others, to also have higher leakdown, and the leakdown can help focus on the problem). As a caution, if someone does a leakdown and says you need to rebuild your engine, get a second opinion at another shop.
05-21-2003, 08:49 AM
#3
When I sold my SC, the out of town buyer asked that I take my car to a specific shop he selected for a PPI, which I did. The shop enjoys a good reputation among car owners of different marquees, Porsches, MB, Audi, BMW, etc. After the compression check showed 195 - 196 lbs across all six cylinders, the shop said that these numbers were so good that a leak down was not necessary. I kept my silence, but making a mental note not to have any serious engine work done on my new C2 at that shop! As Bill sez, you really need both a leak down and compression tests to get a better understanding what's happening inside. 
Chuck
93 C2
Chuck
93 C2
05-21-2003, 09:26 AM
#4
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A compression test can give you an idea of whether any leakage is coming from the valves or rings. After taking an initial reading, you pour oil into the cylinder and try again. If the reading has improved, then the fault is with the rings ? the oil sitting on top of the piston provides an improved seal.
Works great on most engines, but not horizontally opposed ones, for obvious reasons! D'oh!
Cheers
Phil
Works great on most engines, but not horizontally opposed ones, for obvious reasons! D'oh!
Cheers
Phil
05-21-2003, 10:11 AM
#5
Three Wheelin'
Thread Starter
Thaks guys!
The reason I ask is I just happened to be reading the results of the PPI on my recent 993RS purchase.
It included 'all cylinders less than 5%' which I presume was a leak down?
The specialist said this was a very good figure?
Piers
The reason I ask is I just happened to be reading the results of the PPI on my recent 993RS purchase.
It included 'all cylinders less than 5%' which I presume was a leak down?
The specialist said this was a very good figure?
Piers
05-21-2003, 10:39 AM
#6
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I understand that the actual process of performing a leakdown test differs quite a lot from the compression test.
With a compression test, you remove one plug per cylinder, hook up a pressure guage (to each cylinder in turn), crank it several times to read the maximum compression value. What you look for is a difference in values across each cylinder (a difference of greater than 10% usually).
With a leakdown test, you remove a single plug from each cylinder. Then for each cylinder, turn to engine to its Top Dead Center, connect up a pressure driven leakdown guage, load the cylinder with the required pressure and then note how long it takes for the pressure to drop below a set value.
That's my understanding anyway, feel free to correct me if I'm wrong.
Ade
With a compression test, you remove one plug per cylinder, hook up a pressure guage (to each cylinder in turn), crank it several times to read the maximum compression value. What you look for is a difference in values across each cylinder (a difference of greater than 10% usually).
With a leakdown test, you remove a single plug from each cylinder. Then for each cylinder, turn to engine to its Top Dead Center, connect up a pressure driven leakdown guage, load the cylinder with the required pressure and then note how long it takes for the pressure to drop below a set value.
That's my understanding anyway, feel free to correct me if I'm wrong.
Ade
05-21-2003, 11:23 AM
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Ade, Yep, that's pretty much a good description (although I never knew that London was formerly New Zealand )
Piers, 5% is very good, especially if all six cylinders are about the same. So what are you buying? I assume another 911 and not some French lean-too shed <img border="0" title="" alt="[Wink]" src="wink.gif" />
Phil
)Piers, 5% is very good, especially if all six cylinders are about the same. So what are you buying? I assume another 911 and not some French lean-too shed <img border="0" title="" alt="[Wink]" src="wink.gif" />
Phil
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05-21-2003, 11:30 AM
#8
Three Wheelin'
Thread Starter
</font><blockquote><font size="1" face="Verdana,Tahoma,Helvetica">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Helvetica">Originally posted by Phil Raby:
<strong>French lean-too shed <img border="0" title="" alt="[Wink]" src="wink.gif" />
Phil</strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Helvetica">LOL!
Or 'French Road Boat' as a friend describes them.
No, I've bought a 993RS. LHD, 29,000 miles, Yellow, tracked but tight! Take delivery in 3 weeks as wheels are being re-furbed and anodised.
Can't bl@#dy wait!
Piers
<strong>French lean-too shed <img border="0" title="" alt="[Wink]" src="wink.gif" />
Phil</strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Helvetica">LOL!
Or 'French Road Boat' as a friend describes them.
No, I've bought a 993RS. LHD, 29,000 miles, Yellow, tracked but tight! Take delivery in 3 weeks as wheels are being re-furbed and anodised.
Can't bl@#dy wait!
Piers
05-21-2003, 12:18 PM
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Piers
check out
<a href="http://www.pumaracing.co.uk" target="_blank">www.pumaracing.co.uk</a>
Articles there are very informative.
Oh, BTW, did I mention that I was jealous!!
check out
<a href="http://www.pumaracing.co.uk" target="_blank">www.pumaracing.co.uk</a>
Articles there are very informative.
Oh, BTW, did I mention that I was jealous!!
05-21-2003, 12:32 PM
#11
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</font><blockquote><font size="1" face="Verdana,Tahoma,Helvetica">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Helvetica"><strong>With a leakdown test, you remove a single plug from each cylinder. Then for each cylinder, turn to engine to its Top Dead Center, connect up a pressure driven leakdown guage, load the cylinder with the required pressure and then note how long it takes for the pressure to drop below a set value. </strong></font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Helvetica">That's just the first step. A good technician can listen to the intake for escaping air through the intake valve, exhaust for exhaust valve, etc. That's why it can be a diagnostic tool, in addition to providing a leak down measurement.
