Leak down and Compression info
04-15-2006, 02:52 AM
#1
Race Car
Thread Starter
Leak down and Compression info
Guys:
In my searching for a car there has been alot of information on what the results mean.
For example: My comprssion was an average of 102 and my Leakdown was between 10 and 12 percent.
Comments about the compression being low and the leak being bad had me concerned. I spoke at length with the Porsche guy about these numbers and got some really good info. His feeling is that this engine is just fine considering it has 80k on it. No smoke on start up etc. helps the theory.
Then I was reading the articles on Pelican and they seem to confirm this diagnosis.
Here are some excerpts (including my compression converted to sea level figures)
My 10% leakdown looks to be ok considering the miles and the adjusted compression is 117 or 118% Not equal to a new engine but not bad either. And as stated in the article many anomolies can come into play with a test but one thing that is really nice to see is testing on the cyliders to be close in numbers across all 6 cylinders.
Leak down:
When you are running the test, it is a wise idea to make sure that the crank doesn’t turn at all. Have an assistant hold the crank steady or place a flywheel lock on the engine if it’s out of the car. Connect the leakage tester to the engine in the same manner that you would with the compression tester. Pump up the cylinder and let the leakage tester measure the amount of air lost. The gauge on the tester should give readings in percentage numbers. A newly rebuilt engine should have leak-down percentages of around 3-5%. An engine in good running condition should show 10% or less. Numbers around 20% indicate some wear of the engine, but are still adequate for good engine operation. Leakage numbers of around 30% indicate that there are problems brewing, and that a rebuild may be necessary. Needless to say a large leakage amount like 90% indicates that there is a hole in the combustion chamber, and the engine is probably not firing on this cylinder at all. Rotate the engine crankshaft clockwise 180° when you’re done, and check the next cylinder. Repeat the process for each of the six cylinders.
A newly rebuilt engine should have leak-down percentages of around 3-5%. An engine in good running condition should show 10% or less. Numbers around 20% indicate some wear of the engine, but are still adequate for good engine operation. Leakage numbers of around 30% indicate that there are problems brewing, and that a rebuild may be necessary.
Compression:
Altitude and temperature also affect the compression readings. Manufacturer’s specifications are almost always given at a specific altitude (14.7 psi at sea level), and 59° Fahrenheit. Both temperature and barometric pressure change as you go up in altitude, so you will need to correct your measurements if you wish to compare it with a factory specification. The following chart provides conversion factors for correctly compensating for changes in altitude:
Compression Test Altitude Compensation Factors
Altitude Factor
500 0.987
1500 0.960
2500 0.933
3500 0.907
4500 0.880
5500 0.853
6500 0.826
7500 0.800
A standard compression reading of about 150 psi at sea level in Los Angeles would measure significantly less in the surrounding mountains. For example, at an elevation of 6000 feet, the expected reading would be 150 psi X .8359 = 125 psi. The cylinders would be reading low if compared to sea level measurements, yet perfectly fine at this altitude.
The complete article can be found HERE
I am in no way an expert. I just thought I would share this great info with you guys.
In my searching for a car there has been alot of information on what the results mean.
For example: My comprssion was an average of 102 and my Leakdown was between 10 and 12 percent.
Comments about the compression being low and the leak being bad had me concerned. I spoke at length with the Porsche guy about these numbers and got some really good info. His feeling is that this engine is just fine considering it has 80k on it. No smoke on start up etc. helps the theory.
Then I was reading the articles on Pelican and they seem to confirm this diagnosis.
Here are some excerpts (including my compression converted to sea level figures)
My 10% leakdown looks to be ok considering the miles and the adjusted compression is 117 or 118% Not equal to a new engine but not bad either. And as stated in the article many anomolies can come into play with a test but one thing that is really nice to see is testing on the cyliders to be close in numbers across all 6 cylinders.
Leak down:
When you are running the test, it is a wise idea to make sure that the crank doesn’t turn at all. Have an assistant hold the crank steady or place a flywheel lock on the engine if it’s out of the car. Connect the leakage tester to the engine in the same manner that you would with the compression tester. Pump up the cylinder and let the leakage tester measure the amount of air lost. The gauge on the tester should give readings in percentage numbers. A newly rebuilt engine should have leak-down percentages of around 3-5%. An engine in good running condition should show 10% or less. Numbers around 20% indicate some wear of the engine, but are still adequate for good engine operation. Leakage numbers of around 30% indicate that there are problems brewing, and that a rebuild may be necessary. Needless to say a large leakage amount like 90% indicates that there is a hole in the combustion chamber, and the engine is probably not firing on this cylinder at all. Rotate the engine crankshaft clockwise 180° when you’re done, and check the next cylinder. Repeat the process for each of the six cylinders.
A newly rebuilt engine should have leak-down percentages of around 3-5%. An engine in good running condition should show 10% or less. Numbers around 20% indicate some wear of the engine, but are still adequate for good engine operation. Leakage numbers of around 30% indicate that there are problems brewing, and that a rebuild may be necessary.
Compression:
Altitude and temperature also affect the compression readings. Manufacturer’s specifications are almost always given at a specific altitude (14.7 psi at sea level), and 59° Fahrenheit. Both temperature and barometric pressure change as you go up in altitude, so you will need to correct your measurements if you wish to compare it with a factory specification. The following chart provides conversion factors for correctly compensating for changes in altitude:
Compression Test Altitude Compensation Factors
Altitude Factor
500 0.987
1500 0.960
2500 0.933
3500 0.907
4500 0.880
5500 0.853
6500 0.826
7500 0.800
A standard compression reading of about 150 psi at sea level in Los Angeles would measure significantly less in the surrounding mountains. For example, at an elevation of 6000 feet, the expected reading would be 150 psi X .8359 = 125 psi. The cylinders would be reading low if compared to sea level measurements, yet perfectly fine at this altitude.
The complete article can be found HERE
I am in no way an expert. I just thought I would share this great info with you guys.
