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Old 06-07-2007, 07:45 PM   #1
aenima4six2
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Default Compression Leak Down Test Results - Help!!

Ok, I had my Scubie dealer do a compression and leak down test on my car just to ensure everything was normal, and I must say I am very disappointed with the results. I did the tests because I got my car used and I wanted to see the condition of my engine. What confuses me is the tests seem to contradict themselves, that or my timing is off, or the tech did the test wrong. Please tell me what you think

2006 STi w/ 20600 Miles

Compression Test Results
#1 - 135 psi
#2 - 135 psi
#3 - 133 psi
#4 - 133 psi

Leak Down Test Results
#1 - 90/82 - 8 PSI Loss (Approx 8.8%)
#2 - 90/82 - 8 PSI Loss (Approx 8.8%)
#3 - 90/82 - 8 PSI Loss (Approx 8.8%)
#4 - 90/84 - 6 PSI Loss (Approx 6.6%)

Apparently, 2006 STi Spec is 142-171 PSI and the Lower Limit = 128
These results seem very odd as Leak Down results under 10% indicate the engine is in excellent condition, however these compression results would indicate that my engine is in bad shape

Please help.
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Old 06-07-2007, 08:36 PM   #2
RemlapaN
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My understanding of relative compression tests is the actual psi is not as important as the fact that all four cylinders are within 5% of each other (which yours are)

A lot of variables go in to the reading displayed on the gauge, including altitude, air temperature, engine temperature, engine wear, gauge accuracy, how many times the mechanic lets the motor crank over, throttle position, timing, cam advance, etc etc etc.

If one was 135psi and the rest were 145 it would be cause for concern, but with all four consistently being low, it's not the worse news, it just means you might need to run more boost than others to get the same HP, or the test was not done accurately.
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Old 06-07-2007, 08:46 PM   #3
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I don't see any conflict in the results and while those aren't the "wonderful" numbers you can only get when a motor is fresh, they aren't "bad" by a long shot. To me they simply say the motor is used. How many miles?
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Old 06-07-2007, 10:10 PM   #4
aenima4six2
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I have 20600 miles, thats why i was pretty concerned with the low PSI compression results.

Thank you both for all the input on the situation.
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Old 06-07-2007, 10:31 PM   #5
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those are good numbers, I work at a non-subaru dealership but I could probably borrow 3 other techs compression gauges and all of ours would read something different. one may read 130, one may read 135, one may read 140. could have been a colder engine temp, etc. too many variables. those seem damned close together, I would be happy.
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Old 06-07-2007, 10:35 PM   #6
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As long as the numbers are within each other, you're good to go. Up here in Denver, compression tests on STIs range from 112-120. I just did a compression test on mine earlier this week: 118, 118, 115, 113.
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Old 06-07-2007, 10:38 PM   #7
aenima4six2
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I love this forum, you don't know how much stress this relieves.

But really I was concerned because its far off from Subaru spec, but again from what i have heard, its the distance between each that really matters. Plus the leak down test had great results, my car does not seem to have any loss in power, and I was given the opportunity to driven a brand new 2007 STi and it felt only the slightest bit quicker than my own.

Thanks again for all the input!
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Old 06-07-2007, 11:07 PM   #8
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I had 135 across the board at 28k miles, before I put the gt35r on and broke some ring lands. Your results seem normal to me.
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Old 06-07-2007, 11:21 PM   #9
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How did you compute the % on the leakdown?

Quote:
Originally Posted by aenima4six2 View Post
Ok, I had my Scubie dealer do a compression and leak down test on my car just to ensure everything was normal, and I must say I am very disappointed with the results. I did the tests because I got my car used and I wanted to see the condition of my engine. What confuses me is the tests seem to contradict themselves, that or my timing is off, or the tech did the test wrong. Please tell me what you think

2006 STi w/ 20600 Miles

Compression Test Results
#1 - 135 psi
#2 - 135 psi
#3 - 133 psi
#4 - 133 psi

Leak Down Test Results
#1 - 90/82 - 8 PSI Loss (Approx 8.8%)
#2 - 90/82 - 8 PSI Loss (Approx 8.8%)
#3 - 90/82 - 8 PSI Loss (Approx 8.8%)
#4 - 90/84 - 6 PSI Loss (Approx 6.6%)

Apparently, 2006 STi Spec is 142-171 PSI and the Lower Limit = 128
These results seem very odd as Leak Down results under 10% indicate the engine is in excellent condition, however these compression results would indicate that my engine is in bad shape

Please help.
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Old 06-07-2007, 11:29 PM   #10
aenima4six2
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ex1: 1 -(82/90) or

ex2:you can use the given input PSI (90) and divide it by the amount of Leakage PSI printed on the report (8). This will yield the same 8.8%
90-82=8 , 8/90 = 8.8% Leak down

I should have stated
90 was the input PSI
8x was the PSI that held constant in the cylinder

For some reason the dealer printed the ratio as 90/82 instead of 82/90
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Old 06-07-2007, 11:33 PM   #11
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The compression test looks good to me...within a couple pounds across all four=great. Don't worry much about the psi reading, if the low readings worry you I have a compression gauge you could try...it would make you happy, always reads high, very consistent, but very generous.
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Old 06-07-2007, 11:49 PM   #12
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Quote:
Originally Posted by aenima4six2 View Post
I have 20600 miles, thats why i was pretty concerned with the low PSI compression results...
Nah, you're good to go. Keep on truckin'.
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Old 06-08-2007, 07:33 AM   #13
aenima4six2
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Gook looking out everyone, Just wanted to say thanks to everyone who contributed. Before this post I was seriously worried I had bad compression on the engine, and was under the impression either my valves or rings where pretty worn down.

I owe each of you a beer!
Cheers
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Old 06-08-2007, 09:40 AM   #14
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For compression test, it doesnt really matter the psi number, it is more important to have closer numbers together, right?

For leakdown, average number should be 10-20% leak and the % should also be closer with each other?
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Old 06-09-2007, 01:23 AM   #15
aenima4six2
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Quote:
Originally Posted by sc0013z View Post
For compression test, it doesnt really matter the psi number, it is more important to have closer numbers together, right?

For leakdown, average number should be 10-20% leak and the % should also be closer with each other?

From what here, thats exactly right!
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Old 06-09-2007, 12:18 PM   #16
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Quote:
Originally Posted by sc0013z View Post
For compression test, it doesnt really matter the psi number, it is more important to have closer numbers together, right?

For leakdown, average number should be 10-20% leak and the % should also be closer with each other?
The PSI number certainly does matter. What if they all read 90? That would be bad

Both factors have to be taken into consideration. You need a good compression as well as even numbers across the cylinders to have a good result. For leakdown it depends on the motor but I would say you should look for < 10%.
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Old 06-09-2007, 06:25 PM   #17
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20% leakdown is "replace your motor time"
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Old 06-09-2007, 06:47 PM   #18
aenima4six2
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but all in all, the majority of you would say my numbers above (and hence engine condition) is fine - and i should have no concern?
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Old 06-09-2007, 07:07 PM   #19
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Quote:
Originally Posted by aenima4six2 View Post
but all in all, the majority of you would say my numbers above (and hence engine condition) is fine - and i should have no concern?
You have only a little bit of variance in your compression number to the point that I'd consider it insignificant. A low compression reading could just be the result of the gauge used or whether the engine was warm or cold when the test was performed. Overall your numbers are not low and are of no concern. Your leakdown shows a little bit of wear but is fine as well. Your car is fine. Don't worry about it. When your compression drops below 120, has variance >7psi across the cylinders, or has leakdown >15% then your motor would be on its last legs.
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Old 06-10-2007, 01:39 AM   #20
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DOnt worry about the number on the Compression test. Gauges read different and the altitude really effects the reading. A test a sea level and a test at 6000 ft will read differently. The key is the balance. Your engine is well within the 7 psi + or - spec per cyl.

8.8% leak down again.. Its balanced. If you had a problem one cyl would be way more then the others and thats the red flag.

Also.. Remember that EJ motors could have A or B pistons. Its stamped on the top of the block. Mine is an A B A B Meaning I have two As and Two Bs.

Clark
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Old 06-10-2007, 10:30 AM   #21
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Quote:
Originally Posted by AZScoobie View Post

Also.. Remember that EJ motors could have A or B pistons. Its stamped on the top of the block. Mine is an A B A B Meaning I have two As and Two Bs.

Clark
? What's the difference?
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Old 06-10-2007, 01:09 PM   #22
aenima4six2
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Thats interesting, I wasn't aware that there were to different piston sets. I take it that the difference between A and B is the outside diameter of the piston to compensate for larger and smaller bore sizes between cylinders.
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Old 06-10-2007, 08:17 PM   #23
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Exactly right. The spec. clearance is .0004", with acceptable .0000"(!!!) - .0009". They use the A or B pistons to keep the jug clearance within the acceptable range...or so they lead us to beleive...
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Old 06-10-2007, 09:46 PM   #24
aenima4six2
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lol.. very interesting, if i ever have a rebuild ill have to look and see what i got. Ah yes, one last thing. From what i have been reading, any leak down results that are <10% i was led to believe was excellent, or the engine was in great condition. From the replies posted here, it would seem as this is not the case, and my 8.8% is so-so. Any clarifications on this?
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Old 06-11-2007, 12:55 AM   #25
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In the old N/A days the rule of thumb was: <5 was "new", <20 was "good", 20-30 was "tired", and >30 was "failure." The problem, I think, is that with an FI car, leakdown loss becomes more critical, and pretty much any loss above 20% will simply overwhelm the breather and turbo oil drain sub-systems with too much blow-by at high boost. I'm going off a hunch here, so I'll defer to others on this.
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