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Northstar Engines and System Technical Discussion Discussion, Piston detonation evidence - 1998 Northstar in Cadillac Engine Technical Discussion; The marks are from corrosion. A piston from an engine in detonation looks like a mouse has been gnawing on ...
  1. #16
    dkozloski's Avatar
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    Re: Piston detonation evidence - 1998 Northstar

    The marks are from corrosion. A piston from an engine in detonation looks like a mouse has been gnawing on the top edges. The pits are of no concern. I'd be more worried about upsetting the static balance by removing even more metal. Leave the piston top alone.
    Don't mess with Binky Bear!


  2. #17
    daveyhouse is offline Cadillac Owners Fanatic
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    Re: Piston detonation evidence - 1998 Northstar

    I didn't even think about balance. I already have one vehicle with a terrible vibration at certain rpm's, I'm sure he don't won't his motor doing the same.

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    Re: Piston detonation evidence - 1998 Northstar

    Koz is absolutely correct - and the British even have a term for piston damage due to detonation: "The mice have been at it !"

  4. #19
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    Re: Piston detonation evidence - 1998 Northstar

    That is signs of detonation, and with the carbon buildup you described also a sign of oil ingestion from the intake air charge which causes detonation in itself as oil does not burn clean like gasoline and one more thing, if you leave it rough like that you will have "hot spots" as it affects both the burn pattern and the quench area.

    Do not resue that piston w/out dressing the top...but even taking it down far enough to eliminate them will increase the CC's of the stroke/chamber slightly.

  5. #20
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    If it is in the budget, id replace them all

  6. #21
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    Re: Piston detonation evidence - 1998 Northstar

    I would have to say they are not within his budget.

    He has been slowly buying parts a little at time depending on his paycheck. He is still saving to buy re-manufactured heads yet. He is only making $9.00 hr with less than 40 hrs a week. He is driving my full size van which is only getting about 8 mpg in town if he is lucky, so feeding that beast is putting a dent in his wallet also.

    I think I will lightly wet-sand the pitted areas. After all, the pistons are not perfectly smooth because they also have numbers etched into the top along with the directional arrow. Unless somebody has a couple used pistons they want sell

  7. #22
    Submariner409's Avatar
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    Re: Piston detonation evidence - 1998 Northstar

    daveyhouse, Use the piston(s) as-is. The slight erosion is inconsequential and IS NOT due to "oil ingestion" from the PCV system. The slight erosion WILL NOT cause "hot spots" or preignition. Several thousandths of an inch of carbon buildup is a perfectly normal consequence of burning a hydrocarbon fuel - gasoline - along with trace amounts of lube oil from cylinder wall lubrication. In 1,000 miles of driving the rebuilt engine the slight erosion will disappear - the carbon bugaboo.

    Hot spots caused by that slight erosion ??? Too bad the fairly sharp edges in the fly-cut valve head reliefs all were left out of this revelation.

    "Increase the combustion chamber cc volume" - It takes quite a bit of metal removal - visible metal removal - to change combustion chamber volume - in this instance, a slight polish job will remove practically NO piston metal.

  8. #23
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    Re: Piston detonation evidence - 1998 Northstar

    Quote Originally Posted by Submariner409 View Post
    daveyhouse, Use the piston(s) as-is. The slight erosion is inconsequential and IS NOT due to "oil ingestion" from the PCV system. The slight erosion WILL NOT cause "hot spots" or preignition. Several thousandths of an inch of carbon buildup is a perfectly normal consequence of burning a hydrocarbon fuel - gasoline - along with trace amounts of lube oil from cylinder wall lubrication. In 1,000 miles of driving the rebuilt engine the slight erosion will disappear - the carbon bugaboo.

    Hot spots caused by that slight erosion ??? Too bad the fairly sharp edges in the fly-cut valve head reliefs all were left out of this revelation.

    "Increase the combustion chamber cc volume" - It takes quite a bit of metal removal - visible metal removal - to change combustion chamber volume - in this instance, a slight polish job will remove practically NO piston metal.
    The respect I have had for your usual correct posts is disapearing....what world of engine building do you live on? You are correct on a "polish job" not removing enogh...but that piston would need to be resurfaced (turned on a lathe) to get down enough to eliminate them. Those erosin marks are no smooth valve releifs, but jagged errosin and it will only get worse with time.

    How many engines have you built in your lifetime? I do them steadily and have for over 38 years....and the invitation to vist the facility in person to see is still open. Misguiding someone on something like this helps no one.

  9. #24
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    Re: Piston detonation evidence - 1998 Northstar

    daveyhouse, Use this link to a Federal Mogul PDF for some idea of what detonation damage looks like and the origins of the phrase used by me and dkoz (Post #16 - and the man is an airframe and powerplant mechanic). A few more minutes spent with Google will turn up more on detonation and preignition. Don't take my word for it - do your own study.

    http://www.mlcmotorfactors.co.uk/Tro...s/PistonTT.pdf

  10. #25
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    Re: Piston detonation evidence - 1998 Northstar

    Point made....2 engines and that says enough.

    More good links:
    http://www.enginebuildermag.com/Arti...d_pistons.aspx

    http://www.contactmagazine.com/Issue...ineBasics.html

    And this I like the best...read the causes of detonation:
    PISTON DAMAGE DUE TO COMBUSTION DISTURBANCES


    Normal combustion of the air-fuel mixture takes place in the cylinder with a precisely predetermined sequence of events. It is initiated slightly before top dead center by a spark at the sparking plug. The flame spreads out from the plug in a circle and travels through the combustion chamber with steadily increasing speed from 5 - 30 m/sec. The combustion chamber pressure thus rises steeply, reaching its maximum level briefly after top dead center. The rate of pressure rise per degree crank angle should not exceed 3 - 5 bar to ensure long life of the power unit components. However, normal combustion can be disturbed by the most varied effects, of which the following three, which are quite different from each other, are the most important: 1. Pre-ignition: causes thermal overload of the piston.
    2. Detonation (knocking): causes erosion-like material removal and mechanical overload of the piston.
    3. Fuel flooding: causes wear with consequential oil consumption and piston scuffing.
    1. PRE-IGNITION
    In the case of pre-ignition, combustion is initiated before the spark by a glowing area in the combustion chamber. Possible sources of pre-ignition are the valves (usually the exhaust valve), the sparking plug, the gasket, deposits on these parts, and the surfaces surrounding the combustion chamber. The pre-ignited flame them operates with uncontrolled timing on the components causing a sharp rise in piston crown temperature. Continued pre-ignition causes the piston material to reach melting point after only a few seconds. In engines with substantially hemi-spherical combustion chambers this results in holes through the piston crown, which usually occur on an extension of the sparking plug axis.

    On engines with larger squish areas, scorching and melting of the top land occurs at the most highly loaded zone adjacent to the squish surfaces. Melting often continues down to the oil ring and onwards towards the engine interior.

    CAUSES OF PRE-IGNITION
    -- Sparking plug with incorrect heat range.

    -- Damaged, leaking valves or too little valve clearance.

    -- Glowing deposits on the piston crown, cylinder head, valves or sparking plugs.

    -- Soft carbon formed in the combustion chambers of high performance engines with extended city driving.

    -- Unsuitable fuel.

    -- Diesel fuel in petrol.

    -- Oil in the combustion chamber due to leakage past the piston rings or valve guides. -- High engine or high inlet temperature.

    Pre-ignition can also be caused by the severe heating of individual combustion chamber components due to detonation, which always leads to high surface temperatures.

    2. DETONATION
    In the case of detonation, combustion is initiated by the sparking plug spark. The flame front generates pressure waves as it spreads out from the sparking plug, thereby cauisng critical reactions in the unburned mixture. This results in self-ignition simultaneously in many areas of the remaining mixture, giving a 10 - 15 fold increase in the flame speed. The rate of pressure rise per degree crank angle and the peak pressure then become substantially increased. High frequency presure oscillations then occur during the firing stroke.

    In addition the surfaces surrounding the combustion chamber are overheated. When combustion chamber walls are burned clean of deposits, it is a sure sign that detonation has occured.

    Light, interrupted detonation can be withstood by most engines over a long period without damage. More severe, maintained detonation results in erosion-like removal of piston material from the top land and piston crown. The cylinder head and head gasket can also be damaged in a similar way. Components in the combustion chamber (e.g. the sparking plug) can then be so severely heated up that pre-ignition and the associated severe thermal overload and melting of the piston occur.

    Heavy, sustained detonation results in land and skirt fractures within short periods. These are not usually accompanied by melting or scuffing.

    CAUSES OF DETONATION
    -- Use of a fuel with insufficient detonation resistance (too low octane).

    -- Diesel fuel in petrol.

    -- Oil in the combustion chamber due to leakage past the piston rings or valve guides diminishes the detonation resistance of the fuel.

    -- All engine related factors which cause high temperatures at the end of combustion also lead to detonation. These are:

    -- High compression ratio, possibly due to deposits on the piston crown and cylinder head or to excessive grinding of the cylinder block and head surfaces.

    -- Ignition timing too advanced.

    -- Inlet temperature too high due to insufficient underbonnett ventilation or high exhaust back pressure. Failure to switch the inlet air flap to summer operation or a defective automatic inlet temperature control system also result in a significant increase in the inlet air temperature.

    FUEL FLOODING
    Fuel flooding can be caused by a too rich mixture, loss of compression pressure or combustion irregularities which lead to incomplete combustion. The lubrication of piston, piston ring and cylinder bearing surfaces becomes increasingly ineffective. Mixed friction with wear, oil consumption and scuffing are the consequences.

    ----------

    The pictures he posted show this:

    Detonation causes three types of failure:
    1.Mechanical damage (broken ring lands)
    2.Abrasion (pitting of the piston crown)
    3.Overheating (scuffed piston skirts due to excess heat input or high coolant temperatures)


    I do this for a living, I am NOT a backyard do it yourselfer (even though there are tons that actuall do have enough knowledge to recognize this).

    This is what I do for a living and my qualifications speak for themselves.


  11. #26
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    Re: Piston detonation evidence - 1998 Northstar

    I would like to thank everybody for their input and help on this. That was quite a bit of reading in those links.

    I have decided to wet-sand the affected pistons to dress them up a little.

    I imagine the previous owner probably ran the motor quite hot. Again this motor came out of a 96 Eldorado with unknown mileage. We just got the crank back today from the machine shop, we had it polished.

    Now I guess I should start a build thread. I am no master mechanic and could use all the tips this great site and members provides.

  12. #27
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    Re: Piston detonation evidence - 1998 Northstar

    couple hours later



    Couple more hours with black fingers



    How about buffing the piston tops when I am done, any benefit?

  13. #28
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    Re: Piston detonation evidence - 1998 Northstar

    What grit sandpaper did you use?

    I'd wet sand it to 800/1000 and be done with it.

    320 then 400, then 600, then 800/1000. Wash the piston thoroughly and that's it.

    Looks like you've removed most of the issues, you won't get it all but nothing is perfect. .

    Good luck.

  14. #29
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    Re: Piston detonation evidence - 1998 Northstar

    Not necessary to buff the tops, but again, before the heads go on it looks trick and would make a nice file picture..............

    Just for grins and kicks, here are some of the possibilities for a Northstar - BUT many are NOT workable in the FWD configuration - the modifier - CHRFAB - does some gorgeous engines for sand rails and kit cars.................. this article merely explains some of the Northstar technical points.

    http://www.hotrod.com/pitstop/hrdp_1...n/viewall.html

    SC2150 asked how many engines I have built. I have no idea - a lot - been doing engines of various types, makes, models since my first MG OHV 1250cc overhaul in 1954. Worked part-time as an engine mechanic at 3 foreign car shops in DC during high school. Did a flathead V-12 Lincoln Zephyr in the late 50's, along with a bunch of B-Block flatheads. Even throughout my Navy career I still built engines as a hobby - and Navy bases seem to have great automobile hobby shops where sailors can work and store stuff while at sea. In '83 I started on the Ford 302 series in the Mustang "GT" Fox body cars. Among other engines, my marine business builds Olds 455 engines from slow-speed workhorse to tweaky screamer (although - you twist a 455 over 5500 for a while, it comes apart). Anyway, I enjoy the challenges and the friendships - as SC2150 knows, as an engine builder you become acquainted with every gearhead and machinist in a wide radius of your work. Right now, I'm in the middle of doing a '65 Pontiac 389 for a guy in the Kent Island Cruisers - he's restoring a '65 LeMans sedan as a sleeper. I will probably never touch a later Cadillac engine than 2003 - no desire to start over, and arthritis is slowly eroding my ability to do fine assembly work. One thing I can take pride in is that over the years none - not one - of my engine rebuilds has been a "come back".

    EDIT at 1255: 1968, Naples, Italy. Apartment parking lot. My '65 Chevelle Malibu SS 327/350 developed a bad head gasket - coolant leaking into #2 cylinder. Did a decarbonising and top overhaul in the open with essentially NO proper tools or parts. The top of the piston looked exactly like dkoz' "mouse bites" analogy. Rolled it to TDC, packed the edge of the crown with grease and rotary wire brushed the edges of the hundred or so pits smooth. Cranked the piston down an inch and cleaned the bore. I blew that engine at the strip in Willimantic, CT in 1972.
    98eldo32v and 98eldo32v like this.

  15. #30
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    Re: Piston detonation evidence - 1998 Northstar

    I was using 1200.
    The only other grit I could find in my hoarding mess was 150 LOL, I think I have a file smoother than that.
    Wish I had lower grits, that would save me a lot of work and finger swapping.

    ----------

    Sub you just made me drool on my keyboard with link.

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