compression test and leakdown in disagreement?
10-14-2005, 03:52 AM
#1
Drifting
Thread Starter
compression test and leakdown in disagreement?
A few weeks back I posted about having low compression test numbers on my car (hot, dry test)
1: 130PSI
2: 130PSI
3: 145PSI
4: 135PSI
Tonight I did a leakdown test and got
4%
4%
3%
4%
all hissing through the dipstick hole, so rings. But these are good numbers arent they? Very confusing. Is my engine good or no?
As a side note the damper valve on my leakdown tester is 1/16" (0.0625") rather than the recommended 0.040"; would this make a real difference?
1: 130PSI
2: 130PSI
3: 145PSI
4: 135PSI
Tonight I did a leakdown test and got
4%
4%
3%
4%
all hissing through the dipstick hole, so rings. But these are good numbers arent they? Very confusing. Is my engine good or no?
As a side note the damper valve on my leakdown tester is 1/16" (0.0625") rather than the recommended 0.040"; would this make a real difference?
10-14-2005, 04:30 AM
#2
Race Car
Which car are you doing the test it on? Remember that atmosphere has an impact on the #'s I don't wanna go down that avenue. I just don't know enough.
My '83 after a refresh worked high 160's across the board cold.
The '83 was in the mid to high 170's hot.
The '87 cold is much lower. high 150's/low 160's even cold and high 170's +/- hot.
An S is going to run much higher. A turbo will run lower.
The #'s that you posted were very even but need to be looked at as they are low. Given your altitude I would work with them as all of the figures that Porsche discloses are at sea-level.
You are low on compression. I will try to find the post for the 9:5:1 ratio's that I have seen when I can find them. Higher altitude will change things. There are formulas to figure it out. Sorry I only made it to trig 2 and spelling 1. I have squished a bunch of other things out since then
Michael
My '83 after a refresh worked high 160's across the board cold.
The '83 was in the mid to high 170's hot.
The '87 cold is much lower. high 150's/low 160's even cold and high 170's +/- hot.
An S is going to run much higher. A turbo will run lower.
The #'s that you posted were very even but need to be looked at as they are low. Given your altitude I would work with them as all of the figures that Porsche discloses are at sea-level.
You are low on compression. I will try to find the post for the 9:5:1 ratio's that I have seen when I can find them. Higher altitude will change things. There are formulas to figure it out. Sorry I only made it to trig 2 and spelling 1. I have squished a bunch of other things out since then
Michael
10-14-2005, 12:25 PM
#3
Race Car
You're in Colorado, what's the elevation there?
If it's anything like Calgary, the numbers that you read on this board are useless for comparisons. I spent tons of time trying to figure out why my compression numbers were so low, yet my leakdown numbers were good on my 86, and as far as I can tell, the problem is the atmosphere, it's too darn thin up here.
My freshly rebuilt 951 motor just barely made 'good' compression numbers (~150psi on all 4), and the leakdown was essentially 0%, however I called it 1 and 2% to quantify the differences between the cylinders (however minor they may be). On my 86, I had between 115 and 120psi, and only 5% leakdown. This was a hot dry test, just like yours. It was still a darn quick car, and it ran very well.
The damper valve, if that is what I think it is, it shouldn't matter with such low leakdown numbers.
Remember, the CR on your motor is 10.something:1, in a perfect running engine, you should see a little over 10 atmospheres of pressure inside your cylinders (maybe 12, depending on heating of the air due to compression). I don't know what exactly atmospheric pressure is, but I think it's ~14.7psi at sea level, so it should be even lower at your altitude. So, assuming a perfect engine, with minimal heating you should see 12*14.7psi (176.4psi) if you were at sea level. At your altitude, take maybe 10% off of that? So 150psi would be perfect. With your numbers, I would say they are appropriate for your leakdown, and that compression-wise your motor is doing well.
On a side note, michael, your numbers are interesting, since the 87 should have higher compression pistons (10.something:1 that I mentioned earlier, as compared to the 9.6:1 CR pistons used in the pre-87 N/A's). Your numbers are also very high for your 83, so you must be at a low altitude, and the rings must have seated well after the rebuild.
If it's anything like Calgary, the numbers that you read on this board are useless for comparisons. I spent tons of time trying to figure out why my compression numbers were so low, yet my leakdown numbers were good on my 86, and as far as I can tell, the problem is the atmosphere, it's too darn thin up here.
My freshly rebuilt 951 motor just barely made 'good' compression numbers (~150psi on all 4), and the leakdown was essentially 0%, however I called it 1 and 2% to quantify the differences between the cylinders (however minor they may be). On my 86, I had between 115 and 120psi, and only 5% leakdown. This was a hot dry test, just like yours. It was still a darn quick car, and it ran very well.
The damper valve, if that is what I think it is, it shouldn't matter with such low leakdown numbers.
Remember, the CR on your motor is 10.something:1, in a perfect running engine, you should see a little over 10 atmospheres of pressure inside your cylinders (maybe 12, depending on heating of the air due to compression). I don't know what exactly atmospheric pressure is, but I think it's ~14.7psi at sea level, so it should be even lower at your altitude. So, assuming a perfect engine, with minimal heating you should see 12*14.7psi (176.4psi) if you were at sea level. At your altitude, take maybe 10% off of that? So 150psi would be perfect. With your numbers, I would say they are appropriate for your leakdown, and that compression-wise your motor is doing well.
On a side note, michael, your numbers are interesting, since the 87 should have higher compression pistons (10.something:1 that I mentioned earlier, as compared to the 9.6:1 CR pistons used in the pre-87 N/A's). Your numbers are also very high for your 83, so you must be at a low altitude, and the rings must have seated well after the rebuild.
10-14-2005, 12:49 PM
#4
Race Car
Zero10,
You are right. The '83 was 9.5:1 the '87 is 10.2:1 based on http://www.944central.com/lna/specs.asp. I guess that I/WE did a good job on the '83. I am just about at sea level as far as I know.
As for the #'s on the '87 I have been wanting to re-check things and just picked up some dealer only parts thismorning that I have been waiting on. I will post the #'s when I am done tinkering.
Michael
You are right. The '83 was 9.5:1 the '87 is 10.2:1 based on http://www.944central.com/lna/specs.asp. I guess that I/WE did a good job on the '83. I am just about at sea level as far as I know.
As for the #'s on the '87 I have been wanting to re-check things and just picked up some dealer only parts thismorning that I have been waiting on. I will post the #'s when I am done tinkering.
Michael
10-14-2005, 02:34 PM
#6
Race Car
Ahh, 12.3psi, that's even more than 10% lower than my calculations. Your engine is in pretty good condition. Are you having troubles with it, or were you just curious?
Michael, if you had the bores cleaned up (and they were within specs) and used new rings, those numbers should be expected. It's a good thing to see them though!
Michael, if you had the bores cleaned up (and they were within specs) and used new rings, those numbers should be expected. It's a good thing to see them though!
10-14-2005, 02:48 PM
#7
Race Car
OK, just re-checked the '87 w/115K as far as I know it has never been apart.
The #'s were:
- 135
- 140
- 140
- 135
Those were cold. I'll have to get it up to temp back in a few. DAMN those #'s are low.
Michael
The #'s were:
- 135
- 140
- 140
- 135
Those were cold. I'll have to get it up to temp back in a few. DAMN those #'s are low.
Michael
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10-17-2005, 10:49 PM
#9
Drifting
Thread Starter
OK, I got out my thermodynamics book to figure this out. Atmospheric pressure at 5000ft (my elevation) is 12.2psia. At sea level it is 14.7psia.
The factory wear limit is a (sea level) compression of 145psi.
The compression ratio and heating of the air due to compression stay roughly the same, so the ratio of compressed pressure to atmospheric pressure should be the same. In math words:
145/14.7=x/12.2
where x is the equivalent compression requiring a rebuild at this altitude. This works out to x=120psi.
Conclusion: I am fine. Damn altitude. My adjusted compression numbers are
#1 157psi
#2 157psi
#3 175psi
#4 163psi
granted there are more factors involved, but this is accurate enough.
The factory wear limit is a (sea level) compression of 145psi.
The compression ratio and heating of the air due to compression stay roughly the same, so the ratio of compressed pressure to atmospheric pressure should be the same. In math words:
145/14.7=x/12.2
where x is the equivalent compression requiring a rebuild at this altitude. This works out to x=120psi.
Conclusion: I am fine. Damn altitude. My adjusted compression numbers are
#1 157psi
#2 157psi
#3 175psi
#4 163psi
granted there are more factors involved, but this is accurate enough.
10-20-2005, 01:02 AM
#10
Rennlist Member
Originally Posted by patrat
Conclusion: I am fine. Damn altitude. My adjusted compression numbers are
#1 157psi
#2 157psi
#3 175psi
#4 163psi
granted there are more factors involved, but this is accurate enough.
#1 157psi
#2 157psi
#3 175psi
#4 163psi
granted there are more factors involved, but this is accurate enough.
Point being that compressed temperature can play a significant role in the compression pressure. I think this is why published compression pressure values are rarely published and it becomes useful to look at differences in pressure between cylinder pressures rather than just the pressure itself.
hd
10-20-2005, 05:28 PM
#11
Race Car
I agree with your math and your conclusion. Porschefig, I wouldn't worry, my 86 was down in that range and was doing great. Technically, all of it's cold numbers were sub-100psi, but I'm half a hair higher up than you altitude-wise 
10-20-2005, 08:36 PM
#12
Three Wheelin'
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Originally Posted by Zero10
I agree with your math and your conclusion. Porschefig, I wouldn't worry, my 86 was down in that range and was doing great. Technically, all of it's cold numbers were sub-100psi, but I'm half a hair higher up than you altitude-wise
10-21-2005, 01:20 PM
#14
Race Car
Wait, 20psi difference between cylinders?
That is pretty big. I thought you meant that you had done a few tests, and each time got results between 105 and 125psi (with the numbers in each set close to each other). As a general rule, if I see more than a 10% change between cylinders, I go looking for the reason.
In the case of my 86, most of my compression was lost though the valves not quite sealing, pretty easy fix, but I never got around to it.
Might want to do a leak-down test, but the numbers themselves (as a whole) aren't bad.
That is pretty big. I thought you meant that you had done a few tests, and each time got results between 105 and 125psi (with the numbers in each set close to each other). As a general rule, if I see more than a 10% change between cylinders, I go looking for the reason.
In the case of my 86, most of my compression was lost though the valves not quite sealing, pretty easy fix, but I never got around to it.
Might want to do a leak-down test, but the numbers themselves (as a whole) aren't bad.
