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AMelbye
Joined: 17 Sep 2016
Posts: 3
Location: Oslo, Norway
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 Posted: Wed Sep 28, 2016 9:22 pm Post subject: what are good compression numbers for a 1980 euro spec 931? |
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Hi, I'm considering a 1980 eu spec 931.
It's on the other end of the country (Norway) so I got the seller to check out a few things. But I'm not sure what to make of the compression test results.
Here are the numbers:
Cyl1: 9.4bar / 136psi
Cyl2: 9.5bar / 138psi
Cyl3: 9.8bar / 142psi
Cyl4: 9.9bar / 144psi
He didn't say anything about a wet test, so I'm guessing these numbers are dry. about 5.5% difference between cyl1 and cyl4.
Are these numbers good or bad?
This is an euro spec model model year 1980 931. How high would compression be on a recent rebuild?
If this is bad. How bad is it? Engine out job? |
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morghen
Joined: 21 Jan 2005
Posts: 9095
Location: Romania
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| Posted: Wed Sep 28, 2016 9:26 pm Post subject: |
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Get a confirmation from other members but the numbers look good to me.
Both the value and the spread. As far as i remember difference should not be more than 10%
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Lau Christophersen
Joined: 17 May 2013
Posts: 123
Location: Denmark/Copenhagen
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| Posted: Wed Sep 28, 2016 10:35 pm Post subject: |
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| Nearly the same numbers i got on my 931(Danish - so also euro spec). and it was driving fine. |
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Fifty50Plus
Joined: 28 Feb 2008
Posts: 1422
Location: Washington DC area
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| Posted: Thu Sep 29, 2016 12:26 am Post subject: |
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Good numbers.
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Dutch924-racer
Joined: 23 Jul 2007
Posts: 1081
Location: The Netherlands
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| Posted: Thu Sep 29, 2016 9:51 pm Post subject: |
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was the test cold or with warm engine ?
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Carrera RSR
Joined: 08 Jan 2010
Posts: 2312
Location: Somerset, UK
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| Posted: Fri Sep 30, 2016 1:28 am Post subject: |
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Surprised to see #1 lowest and #4 highest. Usually #4 gets the raw deal and usually wears first
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morghen
Joined: 21 Jan 2005
Posts: 9095
Location: Romania
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| Posted: Fri Sep 30, 2016 4:19 am Post subject: |
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| Carrera RSR wrote: | | Surprised to see #1 lowest and #4 highest. Usually #4 gets the raw deal and usually wears first |
maybe they started counting from the other end 
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AMelbye
Joined: 17 Sep 2016
Posts: 3
Location: Oslo, Norway
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| Posted: Fri Sep 30, 2016 6:49 am Post subject: |
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Yeah, I'm sure he started counting from the other end.
How does it matter if the engine was cold? I'm guessing that if it was cold, warm engine numbers would probably be better?
What numbers would I get on a new rebuild? |
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Fasteddie313
Joined: 29 Sep 2013
Posts: 2595
Location: MI
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| Posted: Fri Sep 30, 2016 10:47 am Post subject: |
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| AMelbye wrote: | Yeah, I'm sure he started counting from the other end.
How does it matter if the engine was cold? I'm guessing that if it was cold, warm engine numbers would probably be better?
What numbers would I get on a new rebuild? |
Yes a warm engine would be better yet, higher numbers, because tolerances decrease with heat expansion..
If you squirt oil in the cylinders it will also increase the compression numbers..
On a brand new rebuild your compression test numbers may be worse if you test it before the engine has been broken in and the rings get seated well..
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AMelbye
Joined: 17 Sep 2016
Posts: 3
Location: Oslo, Norway
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| Posted: Fri Sep 30, 2016 7:47 pm Post subject: |
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| Fasteddie313 wrote: |
On a brand new rebuild your compression test numbers may be worse if you test it before the engine has been broken in and the rings get seated well.. |
Let me rephrase my question: What numbers (PSI or Bar) would you get on a recently rebuilt engine that has just been broken in, and is in top condition? |
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Cedric
Joined: 27 Aug 2004
Posts: 2805
Location: Sweden
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| Posted: Fri Sep 30, 2016 10:31 pm Post subject: |
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| Carrera RSR wrote: | | Surprised to see #1 lowest and #4 highest. Usually #4 gets the raw deal and usually wears first |
Maybe they started measuring at #1, so at the time they were at #4 the bores maybe started to get som oil film.
I have my compression numbers written down somewhere, will take a look
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Paul
Joined: 02 Nov 2002
Posts: 9491
Location: Southeast Wisconsin
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| Posted: Fri Sep 30, 2016 11:37 pm Post subject: |
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Those numbers are good, no reason to be concerned.
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peterld
Joined: 10 Dec 2006
Posts: 981
Location: Noosa Heads QLD Australia
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| Posted: Mon Oct 03, 2016 3:55 pm Post subject: |
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^ + 1. You would see similar numbers on a newly built engine.
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