Pinging - is my cam the problem?

Discussion in 'Engine Topic' started by Marty Burks, Jul 22, 2021 at 1:37 AM.

  1. Marty Burks

    Marty Burks New Member

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    Here's the story - I built a 383 for my 1972 C10 pickup. The plan was for a torquey engine with a stable idle for this heavy truck. Basic specs are as follows;

    AFR aluminum heads with 195CC intake runners/64cc chambers
    KB pistons with 18cc dish (I make this combination to be about 9.7/1 Comp ratio)
    Howards cam @.050 lift;
    In duration 215
    Ex duration 225
    In lift .455
    Ex lift .465
    Lobe separation 112 degrees
    New HEI distributor from DUI - has a nice smooth 24 degrees of mechanical adv.

    I can't get it to not ping under acceleration, even fairly light acceleration, as well as lugging the starter when hot. I'm down to 4 degrees BTDC on static timing and it still pings but starts reasonably well. Any slower and it runs too rough and the power falls way off. I've also tried heavier advance weight springs to slow down the advance curve but with little effect. I'm running with no vacuum advance also. I've built a few small blocks in the past but not for 20 years or so, and this is the first 383 for me. I've worked on all the ignition timing tricks I know and have searched thread after thread to look for some hints as far as what I may be missing here. Most of the recommendations I see are for something around 12 BTDC static. No way I can get close to that with this engine.

    I'm wondering if I need a more aggressive cam for this compression ration? Any help from the experts will be greatly appreciated.
     
  2. Lowend

    Lowend Administrator. .a car, a man, a maraca. Staff Member Lifetime Gold Member

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    That cam shouldn’t be causing any issues with that setup.
    What are you running for spark plug gap?
    Have you tested for vacuum leaks?
     
  3. xten

    xten Veteran Member

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    Octane?
     
  4. COPO

    COPO Veteran Member Lifetime Gold Member

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    Did you degree the cam or just go straight up? I have 10.2:1 cr, initial between 14-16* and can hit 6,000 rpm shifts all day long without a ping using Shell 91 Premium with 0% ethanol. I remember when I had 11.0:1 cr in the early 80’s and had to turn down my timing to 2 or 4* BTDC for it to not ping when it was fully warmed up. If your using regular gas which I think is 89 octane then that’s a problem and maybe your cr calculations are off and you have more than you say. Your cam specs are not the problem.
     
  5. 70lt1z28

    70lt1z28 Veteran Member Gold Member

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    What do the plugs look like? Quadrajet? What jets? You may be running real lean.
     
  6. G72Zed

    G72Zed Veteran Member

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    First off, welcome to the best second gen Camaro site on the net.

    It's a known fact one needs 90cc @ TDC to hit 9.7:1 CR with a 383 ci. So, adding up your 64cc chamber, a typical HG like the FelPro 1003 @ 9.1cc and your KB 18cc pistons, and let's assume the 18cc includes the deck height and crevice volumes, this comes in at 91.1cc and that = 9.6:1. Some of those AFR chambers actually may pour more than 64cc, and if your piston is below deck, and the low(er) top ring placement of the KB pistons, you may be less than 9.6:1 cr. Did you cc the heads, pistons and check the final deck height relationship?

    I will take it the engine's overall tune is up to spec as far as plug heat ranges and fuel delivery and curves/afr's in the idle/transfer and mains are within spec as well.

    I will assume it's a hyd cam? as COPO mentioned above, was the cam degreed in, and not talking the "dot to dot" degreering either. I have seen cams with advance built in, and a degree or 2 more with stacking tolerances coming into play, I have seen early intake closing and moderate CR not play nice together.

    Look at it this way, you have 383 cubes pulling on a intake runner that is known for very high port velocity for the given CSA, factor in high vacuum signal at part throttle with an early closing intake trapping the charge, along with a heavy vehicle with tall rear tires and a highway gear and you are into detonation hell. You have a great combo, and quality parts (AFR/DUI ignition)

    Question, have you done a cranking pressure test? engine up to temp, plugs out, carb open, note the first pump in psi, then 4-6 pumps after, not max psi and report back. If those KB pistons are of the hypereutectic material, you do not want to rattle this piston, you will be blowing upper ring lands out the tail pipes soon. And if your tight on the rings end gap, it's not a matter if, it's when.

    It's all in the combo and how the parts interact with each other, either complimentary or fighting with each other.

    FWIW as an example of what can be done, my 350+.060 sbc runs 49cc Iron heads with 10.8:1 CR, runs 91 none Ethanol, or 94 with 9% Ethanol without issues, with 24* initial, 36* by 2,350rpm and vacuum advance with another 9* there's no pinging or detonation, but, intake duration @ .050 is 249. Close attention to the chambers, piston tops and deck height measurements are the key in running high CR on pump gas. I have done work to "soften" a CNC chamber to address some issues, unless you have the Comp AFR with the finer program.

    The other issue that can play into excessive "pinging" is vehicle weight and overall gearing. My Camaro's 2nd gear is a 2.10, combined with a 3.73 rear gear, it's like a TH350 with a 5.15 rear gear, so not much loading.

    Lot's to chew on, but that's why you are here, hope this helps.
     
  7. Marty Burks

    Marty Burks New Member

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    Lots to chew on for sure. I cc'd one chamber to come up with the 64cc number and selected the pistons with that in mind, wanting to stay under 10:1. Thin metal felpro head gaskets and I remember about .010 under deck height, I didn't cc the pistons. Edelbrock performer dual plane intake and Edelbrock 1406 carb. Cam is Hyd flat tappet. I use the local 92 octane ~10% ethanol. I've done a little bit of checking for vacuum leaks but haven't found any and believe the intake side is sealed up pretty well. The plugs look pretty nice to me, just a little bit of tan after 1000 miles or so. I did not degree the cam, I used a roller chain set that had key locations for advance and retard as well as a "std" position, I used the std keyway and timing marks. 3.73 rear end and TH 350. I haven't done a cranking pressure test but will when I get a chance. Thanks everybody for all the replies.
     
  8. 1320feet

    1320feet Veteran Member

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    Did you check the timing mark on damper with TDC at the piston before installing the heads? If not, you can check with a piston stop thu spark plug hole.
     
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  9. 1980RS

    1980RS Veteran Member

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    Your cam is not the problem. Three things to look at when it's pinging, timing (and total), too hot of a spark plug and oil in the upper combustion chamber. I guess poor quality fuel also so 4 things to look at.
     
  10. Marty Burks

    Marty Burks New Member

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    I did check TDC and set the pointer before putting the heads on. I believe it's pretty close as I finally started to adjust the timing "by ear" and when I eventually set it so slow that it idled and ran very poorly, my light showed that to be in the 2-3 BTDC range. My best results have been to set static at 4 BTDC and then dial in 2 more degrees with the adjustable vacuum pot I have on the dist. so that it will idle reasonably well. With vacuum disconnected, I get about 28 degrees total at about 3000 RPM. This is a little higher RPM than the original setting from DUI since I put one slightly heavier advance weight spring in. It's not using any oil that I can measure with the stick, no smoke is visible. I get fuel from a busy truck stop/gas station on the interstate so I assume it's good. I've got a 'cold' set of plugs that I haven't tried yet. I'm not optimistic about the plugs since it doesn't have to be hot for the problem to occur, it will ping with a hint of extra throttle before it reaches thermostat opening. Thank you for the suggestions.
     

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