Engine compression test results
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Intermediate
Joined: Jan 2011
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Engine compression test results
I had a compression test done on my 09 Road King police (103).
It came out to 130 psi 5% leak down. The mechanic said it should be 180 psi and 10% leak down. I'll need to replace my piston rings this winter.
What does the 130 to 180 psi and the 5% to 10% leak down mean and what should it be.
It came out to 130 psi 5% leak down. The mechanic said it should be 180 psi and 10% leak down. I'll need to replace my piston rings this winter.
What does the 130 to 180 psi and the 5% to 10% leak down mean and what should it be.
Tourer
Joined: Sep 2014
Posts: 420
Likes: 5
From: Kingsport, TN
180 compression is in the normal range for a stock motor. 130 is way too low, indicating the compression stroke is not generating the right amount of pressure for optimized combustion. It is measured while the engine is being cranked over.
A leak down test measures how well the sealing of the rings and valves are holding the pressure in the combustion chamber once it is generated by the compression stroke. It is measured while it motor is at rest and is defined as the drop in pressure over time. It cannot be too low (a 5% pressure drop is good and better than 10%).
Couple of questions:
1) was the battery kept fully charged during the compression test? Was the throttle held wide open? Were the ACR's (automatic compression releases) disabled if so equipped?
2) does the motor have S&S "easy start" cams?
3) has the motor been rebuilt before and how many miles ago?
A leak down test measures how well the sealing of the rings and valves are holding the pressure in the combustion chamber once it is generated by the compression stroke. It is measured while it motor is at rest and is defined as the drop in pressure over time. It cannot be too low (a 5% pressure drop is good and better than 10%).
Couple of questions:
1) was the battery kept fully charged during the compression test? Was the throttle held wide open? Were the ACR's (automatic compression releases) disabled if so equipped?
2) does the motor have S&S "easy start" cams?
3) has the motor been rebuilt before and how many miles ago?
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Intermediate
Joined: Jan 2011
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I replied below the questions.
Couple of questions:
1) was the battery kept fully charged during the compression test? Was the throttle held wide open? Were the ACR's (automatic compression releases) disabled if so equipped?
> I don't know
2) does the motor have S&S "easy start" cams?
> no
3) has the motor been rebuilt before and how many miles ago?
> it was never rebuilt as far as I know. It have 22k miles on it.
1) was the battery kept fully charged during the compression test? Was the throttle held wide open? Were the ACR's (automatic compression releases) disabled if so equipped?
> I don't know
2) does the motor have S&S "easy start" cams?
> no
3) has the motor been rebuilt before and how many miles ago?
> it was never rebuilt as far as I know. It have 22k miles on it.
Tourer
Joined: Sep 2014
Posts: 420
Likes: 5
From: Kingsport, TN
I seriously doubt that you have worn rings at 22k miles. The compression test is suspect. Why did you have the testing done? If the bike rides fine (like it did when you got it), I would just ride it.
Cruiser
Joined: May 2014
Posts: 130
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From: CO
What Dynawg1 said. Only thing I'd add is I think you have the #'s mixed up concerning your last question. I usually do a leak down test with 100 psi. If your mechanic was using 100 psi & got 5%, that's good. I would think you should have netted higher compression #'s.
Before you commit to top end work, clarify your tests, or have them done at another reputable shop.
Before you commit to top end work, clarify your tests, or have them done at another reputable shop.
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Road Master
Joined: Apr 2012
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From: San Jose
Here is a recent tread where the OP had the same problem, 120# on compression test. He was unaware of disabling the ACR. Ran the test again after disabling ACR and he got 185#.
https://www.hdforums.com/forum/engin...n-testing.html
https://www.hdforums.com/forum/engin...n-testing.html
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