| Onan Compression [message #264875] |
Sun, 26 October 2014 20:52  |
dpottebaum
Messages: 100
Registered: January 2013
Location: Bondurant, Iowa
Karma: 0
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I performed a compression test on the Onan today. Looked though several manuals and surfed around on the forum, but can't find specs on that the compression should be. The right side was 95 lbs and the left side was just under 90. Not sure if that is within tolerance or not? Thanks in advance for confirmation of acceptable range.
Dave Pottebaum
Bondurant, Iowa
1978 Royale
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| Re: [GMCnet] Onan Compression [message #264955 is a reply to message #264886] |
Mon, 27 October 2014 22:16  |
dpottebaum
Messages: 100
Registered: January 2013
Location: Bondurant, Iowa
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I must be blind as a bat. I can't find the compression range anywhere in the Onan manuals in the links from bdub. There is a spot where one of them refers to "low compression", but they don't define what the psi is for low compression. Is 90 psi to 95 psi OK for a 6000 Onan? Thanks.
Dave Pottebaum
Bondurant, Iowa
1978 Royale
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| Re: [GMCnet] Onan Compression [message #264958 is a reply to message #264955] |
Mon, 27 October 2014 23:15 |
Otterwan
Messages: 946
Registered: July 2013
Location: Lynnwood (north of Seattl...
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dpottebaum wrote on Mon, 27 October 2014 20:16I must be blind as a bat. I can't find the compression range anywhere in the Onan manuals in the links from bdub. There is a spot where one of them refers to "low compression", but they don't define what the psi is for low compression. Is 90 psi to 95 psi OK for a 6000 Onan? Thanks.
Nope, not just you. I looked through both the Onan and GMC manuals, and didn't find any values for compression.
1977 Birchaven, Lynnwood WA - "We may not be able to stop all evil in the world, but I know that how we treat one another is entirely up to us."
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| Re: [GMCnet] Onan Compression [message #264985 is a reply to message #264984] |
Tue, 28 October 2014 10:22 |
James Hupy
Messages: 6806
Registered: May 2010
Karma: -62
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Dave, to be kinda technical about this, let this guide you. At cranking
speed with a fully charged battery, a "L" head engine like the Onan will
pump 75 to 80% efficiency. So if atmospheric is 14.7psi, and you multiply
that by .8 that will yield approx. 11.76 psi. Multiply that by 5.5, which
is the compression ratio of the Onan, and that will yield, in a perfect
world, 64.680 psi. So, if the engine is worn a bit, you could have an
acceptable range of 55 - 70 psi. cranking compression pressure. If you are
near that, all is well and good in Onanland. Other opinions WILL vary.
Jim Hupy
Salem, OR
78 GMC Royale 403
On Tue, Oct 28, 2014 at 8:07 AM, Matt Colie wrote:
> dpottebaum wrote on Sun, 26 October 2014 21:52
>> I performed a compression test on the Onan today. Looked though several
> manuals and surfed around on the forum, but can't find specs on that the
>> compression should be. The right side was 95 lbs and the left side was
> just under 90. Not sure if that is within tolerance or not? Thanks in
>> advance for confirmation of acceptable range.
>
> Dave,
>
> Engines of this type are low compression because:
> They didn't need any more to make the required output.
> The design made it hard to hold in much more.
> These are just two of many reasons that the flat head side valve engines
> are about gone.....
> By low compression, I mean the 6 or 7 to 1 region. By the time you
> include losses, you are about where you got it.
>
> A cranking compression test has so many variables that it is best used
> only to find damaged to one particular cylinder.
>
> So, clean it up and go....
>
> Matt
> --
> Matt & Mary Colie - Members GMCMI, GMCES
> '73 Glacier 23 - Still Loving the Applied Rear Brake Control Arms
> SE Michigan - Twixt A2 and Detroit
> _______________________________________________
> GMCnet mailing list
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| Re: Onan Compression [message #264986 is a reply to message #264875] |
Tue, 28 October 2014 10:52 |
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John Heslinga
Messages: 632
Registered: February 2011
Location: Edmonton, Alberta, Canada
Karma: 4
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Hi Dave.
Many small engines such as the Onan have cam grinds (and even mechanical devices) that will release some of the compresion through the exhaust valve at the slow cranking speeds encountered to allow for easier starting. (Sometimes referred to as "EZStart") trying to do a compression test in the normal engine direction at cranking speeds will produce low readings that are pretty meaningless. (Low) Generally small engines with easy start cam grinds need to be turned Reverse direction to do a proper compression test. (Mechanical types are not as easy) Your numbers seem consistent with what I've seen for this phenomenon. I suspect that is why you are also not finding compression data in the manuals. Your 90psi is probably OK.
John and Cathie Heslinga
1974 Canyonlands 260
455, Manny tranny and 1 ton, 3:70 LS,
Red Seal Journeyman, DTE, BEd. MEd.
Edmonton, Alberta
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| Re: Onan Compression [message #264992 is a reply to message #264986] |
Tue, 28 October 2014 11:46 |
Ken Burton
Messages: 10030
Registered: January 2004
Location: Hebron, Indiana
Karma: 10
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Also keep in mind that is an air cooled engine with a lot of extra clearance on the rings. Normally on that type of engine we test the compression hot with an differential compression tester. During the test the engine is not cranking and each piston held at TDC.
Ken Burton - N9KB
76 Palm Beach
Hebron, Indiana
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| Re: Onan Compression [message #265035 is a reply to message #264992] |
Tue, 28 October 2014 19:25 |
dpottebaum
Messages: 100
Registered: January 2013
Location: Bondurant, Iowa
Karma: 0
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I certainly appreciate all of your help guys. As Matt says...I'll clean it up and go. Thanks!
Dave Pottebaum
Bondurant, Iowa
1978 Royale
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