Low Compression

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    Author Topic: Low Compression  (Read 6734 times)

    Offline Chryco Psycho

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    Re: Low Compression
    « Reply #30 on: May 02, 2014 - 10:47:31 PM »
    sounds to me like the heads & seats are going south on all cylinders just 1 is worse then the others , I doubt the compression will rise in any of the other cylinders with oil either , , time for head work
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    Offline Strawdawg

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    Re: Low Compression
    « Reply #31 on: May 02, 2014 - 10:53:44 PM »
    Quote from: Chryco Psycho on May 02, 2014 - 10:47:31 PM
    sounds to me like the heads & seats are going south on all cylinders just 1 is worse then the others , I doubt the compression will rise in any of the other cylinders with oil either , , time for head work

    yep...if the rings were that bad, the car would be smoking badly and showing a lot of blow by out the valve covers
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    Offline cudabob496

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    Re: Low Compression
    « Reply #32 on: May 03, 2014 - 01:34:15 AM »
    except for #8, looks like the average cyclinder pressure is about 140 psi, which may
    be perfectly normal for an older engine.  Assuming it started out new at 180 psi, even
    with perfect head work, may only get pressures to 160 psi, simply because of the
    older rings and cylinder wall wear and scratches, which cause a loss of cylinder pressure,
    and which Engine Restorer is suppose to help fix.
    « Last Edit: May 03, 2014 - 01:37:54 AM by cudabob496 »
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    72 Cuda, owned 25 years. 496, with ported Stage VI heads, .625 in solid roller, 254/258 at .050, 3500 stall, 3.91 rear. 850 Holley DP, Reverse manual valve body.

    1999 Trans Am, LS1, heads, cam, headers, stall, etc! Love to surprise the rice rockets with this one. They seem so confident, then it's "what the heck just happened?"

    2011 Kawasaki Z1000

    Offline cudabob496

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    Re: Low Compression
    « Reply #33 on: May 03, 2014 - 03:10:44 AM »
    checking plug color on #8 could also be a way to tell
    if loss of pressure is due to poor ring seal. Darker plug
    means burning oil past rings.
    « Last Edit: May 03, 2014 - 05:19:16 AM by cudabob496 »
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    72 Cuda, owned 25 years. 496, with ported Stage VI heads, .625 in solid roller, 254/258 at .050, 3500 stall, 3.91 rear. 850 Holley DP, Reverse manual valve body.

    1999 Trans Am, LS1, heads, cam, headers, stall, etc! Love to surprise the rice rockets with this one. They seem so confident, then it's "what the heck just happened?"

    2011 Kawasaki Z1000

    Offline cwestra

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    Re: Low Compression
    « Reply #34 on: May 03, 2014 - 09:25:48 AM »
    Here's a little more history on the engine.  I rebuilt it completely around 4 or 5 years ago with the following main things done.

    Bored .030" over.
    9.5:1 Keith Black cast pistons
    Lunati 60402 cam (262°/268° advertised duration, .475"/.494" lift at rocker, 112° LSA)
    Head work including hardened seats and valve replacement where needed.

    There has been less than 10,000 miles put on the car since the rebuild, most highway or around town but some aggressive driving mixed in.

    Compression check on engine in May 2010 was as follows;

    #1 -  150 psi          #2 -  135 psi
    #3 -  150 psi          #4 -  142 psi
    #5 -  150 psi          #6 -  142 psi
    #7 -  145 psi          #8 -  153 psi

    Except for cyl #2, all seem close.  Including #2 they are within around 10% of each other (just about).  Now I don't know what the  correct numbers should be with this engine combination (pistons & cam mainly), but I'm not sure if it would be up in the 180 psi range.  If I'm wrong, which certainly could be the case, then I'm open to feedback.  But they were reasonably consistent then and I felt with the 112° LSA on the cam that I might be losing some because of that.  Please, though, chime in with your thoughts.  If something doesn't look right I want to correct it.

    Also, I have attached a picture of the plugs when I pulled them out just before taking the latest compression readings.

    I'm not sure what I'll ultimately find the problem to be, but it seems too early after the rebuild to be experiencing this, regardless.  I know I have fought the 6 bbl. carb set up for quite a while and could have been running lean during that time.  Not sure if that could possibly explain burnt exhaust valves so soon though.
    « Last Edit: May 03, 2014 - 09:28:07 AM by cwestra »
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    Corey - in Northern Indiana

    Offline 72cudamaan

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    Re: Low Compression
    « Reply #35 on: May 03, 2014 - 09:54:16 AM »
    Running lean could definitely lead to burnt valves. It could also lead to melted pistons right at the ring lands, which
    could explain why no compression increase with oil. How is the rocker adjustment? Just a thought, though it sounds
    to me like taking the heads off and doing a thorough inspection is the best thing for you to do before any other
    damage can be created. By the way, is the lean condition fixed?
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    Andy  (phukker whither)

    Offline 72cudamaan

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    Re: Low Compression
    « Reply #36 on: May 03, 2014 - 09:55:39 AM »
    Northern Indiana isn't too far away. Maybe time for a tear down party?
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    Andy  (phukker whither)

    Offline cwestra

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    Re: Low Compression
    « Reply #37 on: May 03, 2014 - 10:07:08 AM »
    Quote from: 72cudamaan on May 03, 2014 - 09:54:16 AM
    Running lean could definitely lead to burnt valves. It could also lead to melted pistons right at the ring lands, which
    could explain why no compression increase with oil. How is the rocker adjustment? Just a thought, though it sounds
    to me like taking the heads off and doing a thorough inspection is the best thing for you to do before any other
    damage can be created. By the way, is the lean condition fixed?
    I checked the rocker adjustment on #8 and it was fine.  The spec says adjust to 0 clearance and add another 1.5 turns.  Rockers on #8 were closer to .75 turns.

    I did install an AFR meter last year and made some minor adjustments to improve the mixture situation, but I suppose that doesn't necessarily mean 1 or 2 cylinders aren't, for some reason, still running lean.  Attached are a couple of charts showing what I am getting.  At a cruise I am still running a little lean but thought that was normal.

    I will have a good look at pistons once heads are off.  Thanks
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    Corey - in Northern Indiana

    Offline cudabob496

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    Re: Low Compression
    « Reply #38 on: May 03, 2014 - 11:46:31 AM »
    were cylinders bored/honed with a torque plate? If not,
    can get distortion of cylinder wall diameter near top of cylinder.

    interesting, #8 was your highest reading in 2010.
    was pressure check with warm engine in 2010?
    « Last Edit: May 03, 2014 - 11:52:03 AM by cudabob496 »
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    72 Cuda, owned 25 years. 496, with ported Stage VI heads, .625 in solid roller, 254/258 at .050, 3500 stall, 3.91 rear. 850 Holley DP, Reverse manual valve body.

    1999 Trans Am, LS1, heads, cam, headers, stall, etc! Love to surprise the rice rockets with this one. They seem so confident, then it's "what the heck just happened?"

    2011 Kawasaki Z1000

    Offline cwestra

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    Re: Low Compression
    « Reply #39 on: May 03, 2014 - 01:51:59 PM »
    Quote from: 72cudamaan on May 03, 2014 - 09:55:39 AM
    Northern Indiana isn't too far away. Maybe time for a tear down party?
    Might be.
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    Corey - in Northern Indiana

    Offline cwestra

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    Re: Low Compression
    « Reply #40 on: May 03, 2014 - 05:24:10 PM »
    I just finished doing a leak test.  Below are my results.

    #1 -  94%          #2 -  89%
    #3 -  95%          #4 -  90%
    #5 -  95%          #6 -  87%
    #7 -  93%          #8 -  28%

    Number 8 I can clearly hear hissing from the tail pipe and not on any of the others.  On all, I can clearly hear hissing from the inside of the block (rings).  This is true even for the cylinders reading 95%.  My concern is with the cylinders showing 87% - 90%, as they appear to be on the border (right bank).  How much power am I losing with these borderline cylinders?  Is it worth putting rings in?  Also, why would I be experiencing issues only with one bank?
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    Corey - in Northern Indiana

    Offline Strawdawg

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    Re: Low Compression
    « Reply #41 on: May 03, 2014 - 05:37:29 PM »
    Looks like Neil hit it on the head.   In the greater scheme of things,  87% is not all that bad. Might be a little leak in the passenger side but I would guess valves.    Time to work the heads or swap another set on.

    Steve

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    Offline cwestra

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    Re: Low Compression
    « Reply #42 on: May 03, 2014 - 08:01:30 PM »
    Quote from: Strawdawg on May 03, 2014 - 05:37:29 PM
    Looks like Neil hit it on the head. 

    Steve
    As usual.
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    Corey - in Northern Indiana

    Offline cwestra

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    Re: Low Compression
    « Reply #43 on: May 03, 2014 - 10:38:34 PM »
    Here's a picture of the burned valve.  You can see the light from behind shinning through.  Not much but enough.  Also, I've include a picture of all the valves on the right side head.  Is the color of the exhaust valves indicating an extreme lean condition?

    The last picture is of the exhaust manifold.  For some reason one of the middle ports is dark sooty black and the other is almost a white color.  Has anyone seen anything like this before.  The other manifold is the same way.  May not be linked to my problem but just seems odd.
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    Corey - in Northern Indiana

    Offline Chryco Psycho

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    Re: Low Compression
    « Reply #44 on: May 03, 2014 - 10:45:59 PM »
    Well at least you have found the problem , too lean , probably , excessive heat one the exhaust valves , weird on the exhaust manifold though , most dual plane intakes feed both center ports from the same side of the carb , what intake are you using ?
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