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[GMCnet] Compression test [message #180455] Thu, 16 August 2012 00:16 Go to next message
ljdavick is currently offline  ljdavick   United States
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In his recent post “Help for engine” Jay Rabe noted his compression is about 150-165 psi. When I checked my compression a month ago I got 135-140. What should I expect from a 140,000 mile motor, or I suppose the real question is, what should it have been from the factory?

Larry Davick
A Mystery Machine
1976(ish) Palm Beach
Fremont, CA

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Larry Davick
A Mystery Machine
1976(ish) Palm Beach
Fremont, Ca
Howell EFI + EBL + Electronic Dizzy

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Re: [GMCnet] Compression test [message #180465 is a reply to message #180455] Thu, 16 August 2012 06:58 Go to previous messageGo to next message
Steven Ferguson is currently offline  Steven Ferguson   United States
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IIRC, all the cylinders should be within 10%. That shows even wear and no
sealing problems.

On Wed, Aug 15, 2012 at 10:16 PM, Larry Davick <ljdavick@comcast.net> wrote:

> In his recent post “Help for engine” Jay Rabe noted his compression is
> about 150-165 psi. When I checked my compression a month ago I got
> 135-140. What should I expect from a 140,000 mile motor, or I suppose the
> real question is, what should it have been from the factory?
>
> Larry Davick
> A Mystery Machine
> 1976(ish) Palm Beach
> Fremont, CA
>
> _______________________________________________
> GMCnet mailing list
> Unsubscribe or Change List Options:
> http://temp.gmcnet.org/cgi-bin/mailman/listinfo/gmclist
>



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Re: [GMCnet] Compression test [message #180504 is a reply to message #180455] Thu, 16 August 2012 11:29 Go to previous messageGo to next message
roy1 is currently offline  roy1   United States
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ljdavick wrote on Wed, 15 August 2012 22:16

In his recent post “Help for engine” Jay Rabe noted his compression is about 150-165 psi. When I checked my compression a month ago I got 135-140. What should I expect from a 140,000 mile motor, or I suppose the real question is, what should it have been from the factory?

Larry Davick
A Mystery Machine
1976(ish) Palm Beach
Fremont, CA

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He probably has a different cam or higher compression pistons then you have.

Roy Keen Minden,NV 76 X Glenbrook

[Updated on: Thu, 16 August 2012 11:30]

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Re: [GMCnet] Compression test [message #180507 is a reply to message #180455] Thu, 16 August 2012 11:39 Go to previous messageGo to next message
C Boyd is currently offline  C Boyd   United States
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Sir: compression tests can vary, throttle not open, wet test (squirt of oil) will vary from dry test. Carbon build up can raise compression and also contribute to spark knock. You might run some GM top engine cleaner thru the engine per directions and then run a dry test then a wet test and see if there is any difference. Higher wet is usually leaking rings, you have good compression as is so valves should be ok..




ljdavick wrote on Thu, 16 August 2012 01:16

In his recent post “Help for engine” Jay Rabe noted his compression is about 150-165 psi. When I checked my compression a month ago I got 135-140. What should I expect from a 140,000 mile motor, or I suppose the real question is, what should it have been from the factory?

Larry Davick
A Mystery Machine
1976(ish) Palm Beach
Fremont, CA

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C. Boyd
76 Crestmont
East Tennessee

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Re: [GMCnet] Compression test [message #180530 is a reply to message #180507] Thu, 16 August 2012 14:23 Go to previous messageGo to next message
xplorid is currently offline  xplorid   United States
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Hi all - what is the target compression for my '73? When I picked up this GMCII at Jim Bounds shop in June, he told me it had even compression, but at 91. That seems absurdly low given the numbers in this post, but I drove it 3000 miles up to Ohio then back here to Idaho. Hmmmm.

1974/94 GMCII by Explorer Manny 6.5 TD Al radiator 1 ton front 4 bags back

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Re: [GMCnet] Compression test [message #180587 is a reply to message #180530] Fri, 17 August 2012 00:11 Go to previous messageGo to next message
John Sharpe is currently offline  John Sharpe   United States
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Quote:

even compression, but at 91


Anything below 100 is bad. I doubt that the 91 is accurate. With that you probably would not have enough power to pull the 12000# over an overpass.

John Sharpe
Humble,TX
'78 Eleganza TBI
'89 Spectrum 2000 MPI V-10
'40 Ford Panel Delivery TPI
johnasharpe@gmail.com

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Re: [GMCnet] Compression test [message #180615 is a reply to message #180587] Fri, 17 August 2012 08:38 Go to previous messageGo to next message
xplorid is currently offline  xplorid   United States
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Exactly - and I seem to recall getting by an overpass or two, and a few hills in TN and KY on the way up to OH. We even have some hills here in ID. So I'll use this post and get some compression testing done!

1974/94 GMCII by Explorer Manny 6.5 TD Al radiator 1 ton front 4 bags back

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Re: [GMCnet] Compression test [message #180638 is a reply to message #180615] Fri, 17 August 2012 10:46 Go to previous messageGo to next message
A Hamilto is currently offline  A Hamilto   United States
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xplorid wrote on Fri, 17 August 2012 08:38

Exactly - and I seem to recall getting by an overpass or two, and a few hills in TN and KY on the way up to OH. We even have some hills here in ID. So I'll use this post and get some compression testing done!
The MM for '73 and '74s says OEM compression ratio was 8.5:1
That would be about 125 PSI, I think.
The MM also says:
Minimum compression recorded in any one cylinder should not be less than 70% of the highest cylinder. For example, if the highest pressure in any one cylinder is 1501bs., the lowest allowable pressure for any other cylinder would be 105 lbs. (150 X 70% = 105).

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Re: [GMCnet] Compression test [message #180644 is a reply to message #180638] Fri, 17 August 2012 12:34 Go to previous messageGo to next message
hal kading is currently offline  hal kading   United States
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That probably was a differential test using 100 psi as the reference. If so you only have 9% leakage - quite good.

"Hi all - what is the target compression for my '73? When I picked up this GMCII at Jim Bounds shop in June, he told me it had even compression, but at 91. That seems absurdly low given the numbers in this post, but I drove it 3000 miles up to Ohio then back here to Idaho. Hmmmm."

Hal Kading 78 Buskirk Las Cruces NM

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Re: [GMCnet] Compression test [message #180659 is a reply to message #180638] Fri, 17 August 2012 15:34 Go to previous message
ljdavick is currently offline  ljdavick   United States
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This thread makes me feel better about my compression readings.


Larry Davick
Fremont, California
A Mystery Machine
'76 (ish) Palm Beach

----- Original Message -----
From: "A." <markbb1@netzero.com>
To: gmclist@temp.gmcnet.org
Sent: Friday, August 17, 2012 8:46:26 AM
Subject: Re: [GMCnet] Compression test



xplorid wrote on Fri, 17 August 2012 08:38
> Exactly - and I seem to recall getting by an overpass or two, and a few hills in TN and KY on the way up to OH. We even have some hills here in ID. So I'll use this post and get some compression testing done!
The MM for '73 and '74s says OEM compression ratio was 8.5:1
That would be about 125 PSI, I think.
The MM also says:
Minimum compression recorded in any one cylinder should not be less than 70% of the highest cylinder. For example, if the highest pressure in any one cylinder is 1501bs., the lowest allowable pressure for any other cylinder would be 105 lbs. (150 X 70% = 105).
--
'73 23' CanyonLands
UA (Upper Alabama)
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Larry Davick
A Mystery Machine
1976(ish) Palm Beach
Fremont, Ca
Howell EFI + EBL + Electronic Dizzy

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