|
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??? |
Does it smoke? Possibly a bad valve seal? Head gasket?
|
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.
|
That's less than 10% difference. If my memory is correct (questionable at times :LOL: ) 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 |
Are you testing with the carb wide open:confused:
|
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?
|
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?
|
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? |
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.
|
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 |
Those numbers are fine. If anything it's wierd 1-4 are all 180.
|
thanks again guys. i used a actron compression gauge from sears....:>
|
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 |
| All times are GMT -7. The time now is 04:21 AM. |
Powered by vBulletin® Version 3.8.0
Copyright ©2000 - 2025, Jelsoft Enterprises Ltd.
Search Engine Friendly URLs by vBSEO 3.6.0
The representations expressed are the representations and opinions of the clubcobra.com forum members and do not necessarily reflect the opinions and viewpoints of the site owners, moderators, Shelby American, any other replica manufacturer, Ford Motor Company. This website has been planned and developed by clubcobra.com and its forum members and should not be construed as being endorsed by Ford Motor Company, or Shelby American or any other manufacturer unless expressly noted by that entity. "Cobra" and the Cobra logo are registered trademarks for Ford Motor Co., Inc. clubcobra.com forum members agree not to post any copyrighted material unless the copyrighted material is owned by you. Although we do not and cannot review the messages posted and are not responsible for the content of any of these messages, we reserve the right to delete any message for any reason whatsoever. You remain solely responsible for the content of your messages, and you agree to indemnify and hold us harmless with respect to any claim based upon transmission of your message(s). Thank you for visiting clubcobra.com. For full policy documentation refer to the following link: