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CC Advertisers
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10-28-2006, 05:39 PM
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CC Member
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Join Date: Jun 2005
Location: nor cal,
ca
Cobra Make, Engine: ERA
Posts: 183
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Not Ranked
strange compression readings
my new 428(2k miles) has been idleing a bit strange for a while now.
i conducted a compression check and here are the readings.
cylinders 1-4 180psi
cylinders
5) 165 dry 175 wet
6) 175
7) 165
8) 165.
i only tested number 5 with a leak down. will do 6-7-8 with iol later. but i expect the same reading as #5.
why the lower reading on drivers side? what should new cylinders read?
my mech said he is going to r/r engine and look into it. will also change cam from 108 to 110. anyone have any input???
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10-28-2006, 07:08 PM
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CC Member
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Join Date: Dec 2001
Location: Louisville,
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Cobra Make, Engine: I'm Cobra-less!
Posts: 9,391
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Does it smoke? Possibly a bad valve seal? Head gasket?
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www.lykinsmotorsports.com
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10-28-2006, 07:10 PM
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Join Date: Jun 2005
Location: nor cal,
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Cobra Make, Engine: ERA
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no smoke from exhaust that i can detect. it does smoke out of the valve cover breathers though. no water consumption either. it didnt even smoke after i started it after leak testing number 5 cylinder.
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10-28-2006, 09:18 PM
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CC Member
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Join Date: Jun 2003
Cobra Make, Engine: ERA 427 w/496 Side Oiler, roller, dual quads
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That's less than 10% difference. If my memory is correct (questionable at times  ) Ford wouldn't repair it in the early '70s unless a cylinder was 25% low. Can't say what they'd accept now, but I suspect pretty close to the same.
Dan
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10-29-2006, 08:04 AM
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Senior Club Cobra Member
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Join Date: Jan 2001
Location: Santa Barbara, Ca.,
ca
Cobra Make, Engine: R.U.C.C. with a 427FE, toploader
Posts: 1,435
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Are you testing with the carb wide open 
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Mike Z
Nothing sucks more than that moment during an argument when you realize you’re wrong. 
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10-29-2006, 09:18 AM
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Join Date: Feb 2002
Location: Dadeville,
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Cobra Make, Engine: Sold my EM.
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As the higher compression ratios are all on the right side, is it possible the head on that side was machined down slightly more than the left creating smaller combustion chambers on that side? Or perhaps the head gaskets were not the same thicknesses?
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Tommy
Cheetah tribute completed 2021 (TommysCars.Weebly.com)
Previously owned EM Cobra
"Never attribute to malice that which is adequately explained by stupidity." - Hanlon's Razor
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10-29-2006, 11:08 AM
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CC Member
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Join Date: Jun 2005
Location: nor cal,
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the carb was wide open.... are these readings lower than should be? what should they be at? will this cause it to idle rough? or effect the performance?
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10-29-2006, 02:12 PM
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Member of the north
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Join Date: May 2003
Cobra Make, Engine: A Cobra
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Those don't look that bad to me. Could you share your test parameters?
The test instrument itself could have the margin of error you are seeing in those readings. What did you use to measure the compression?
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10-29-2006, 03:16 PM
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Join Date: May 2001
Location: California,
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Cobra Make, Engine: NAF 289 Slabside Early Comp Car with 289 Webers and all the goodies. Cancelling the efforts of several Priuses
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Tommy has hit upon a very good possibility. The combustion chamber volumes of one head are quite likely different than the other. It could have been caused by unmatched pair of heads, one cut more than the other or the valves being recessed a little more in one than the other. When the chamber volumes are equalized prior to assembly it eliminates this situation and equalizes compression readings in each cylinder, allowing each cylinder to make equal power.
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Rick
As you slide down the Banister of Life, may the splinters never be pointing the wrong way 
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10-29-2006, 04:00 PM
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Senior Club Cobra Member
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Your only 9% difference from the high to the low, not bad at all...........Most engine builders will allow for 10% between cylinders, so your o-k there...........
As for the difference from one side to the other, I think the others have nailed it down to the cylinder head...........
All in all, I would not touch a thing...........
David
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DAVID GAGNARD
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10-29-2006, 06:14 PM
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CC Member
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Join Date: Jan 2002
Location: Northern California,
CA
Cobra Make, Engine: -Sold- Contemporary 427S/C # CCX-3152 1966 427 Med Rise Side Oiler, 8v 3.54:1 Salisbury IRS, Koni's.. (Now I'm riding Harleys)
Posts: 2,567
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Those numbers are fine. If anything it's wierd 1-4 are all 180.
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michael
A man that is young in years, may be old in hours, if he have lost no time. But that happeneth rarely. Generally, youth is like the first cogitations, not so wise as the second. For there is a youth in thoughts, as well as in ages... Sir Francis Bacon (1561-1626)
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10-29-2006, 06:37 PM
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CC Member
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Join Date: Jun 2005
Location: nor cal,
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Cobra Make, Engine: ERA
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thanks again guys. i used a actron compression gauge from sears....:>
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10-29-2006, 11:10 PM
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CC Member
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Join Date: Jun 2005
Location: nor cal,
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Cobra Make, Engine: ERA
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michael, i rounded the passenger side off. they were all within 1/2 to 1psi from 180. same with drivers side.
ok, well i feel better. thanks. i thought they should all be the same or close to the same. thanks for the help guys as usual... ken
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