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Off topic gm wasted spark ignition coils

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  • Off topic gm wasted spark ignition coils

    Some gm cars have 2 spark plugs firing at the same time from i coil. One plug is on the compression stroke and the other is on the exhaust stroke. The plug firing on the exhaust stroke does nothing ,hence it is wasted. My question is, why would the electricity choose to jump gap on the plug that is under compression. I allways thought that the higher the pressure ,the harder it is for a spark to jump. I seems that the electric would take the eazy way out and jump the plug that is on the exhaust stroke..Edwin Dirnbeck

  • #2
    This might help you picture what's happening.

    Location: Saskatoon, Saskatchewan, Canada


    • #3
      ARCANE, This explains it all .Let me say just one thing,YOU ABIDE. Once again ,I am amazed at the knowledge and the willingness to share on this forum..Thanks again Edwin Dirnbeck


      • #4
        It's still a wasted spark, meaning - the one on the exhaust stroke sparks for no reason other then wearing out the spark plugs of the engine at DOUBLE the rate,,,

        give me individual coil packs any day,,,,,,, the only reason why they used this system is to save expense... Subie's or in-line four bangers would also be stranded if you lost a coil cuz trying to get home on two cylinders just don't cut it, but 3 can work in a pinch and does - the computer picks up the malfunction and cuts fuel to that cylinder so you don't waste gas and also dilute your engine oil and wear that cylinder and you can limp home and replace your single coil --- as it should be,,,,,,,,, hell - Id rather run a distributor over wasted spark... it was a transitional era, it did simplify things to a degree but by far not a perfect system ...


        • #5
          The energy from the spark doesnt wear out the sparkplug, which you will replace anyways every 100k miles.

          Individual coil packs are twice the expense as wasted spark, carry a spare if you want better reliability.

          If there is an intermediate circuit card between the ecu and the coils, carry a spare one. Gasoline turboed VW's from around 2005 have them; oem is hundreds of dollars. The 20$ units on ebay work fine..

          And/or carry a spare distributor cap, rotor, ig coil, sparkplug, connecting rod, oil pump, oil pan, bondo (15 min cure time (to patch the hole in the cast iron block when the connecting rod punches a hole through it). Rod bearings are cheap and lightweight, carry spares for sure. Haha

          oh and carry spare wheel bearings, cv axle, ECU, and a spare driver for when one of them dies of hypothermia while you are changing the oil pan.


          • #6
            It was common on smaller Japanese twin motorcycle engines with 360 degree cranks. One double ended coil and only one set of ignition points. In theory the polarity difference was supposed to give uneven plug wear, but it wasn't noticable in the real world.


            • #7
              Originally posted by johansen View Post
              The energy from the spark doesnt wear out the sparkplug, which you will replace anyways every 100k miles.

              You stand corrected --- every single time that a spark plug fires it blows off a chunk of itself, in fact and in theory without this effect we would all be sitting on the side of the road going absolutely nowhere (exceptin for diesels)
              so a convention spark plug that normally lasts only 30,000 miles has a life expectancy of only 15,000 miles with the wasted spark system ----- and in fact the wasted spark system was one of the huge reasons as to why things like platinum and iridium started to get used to take care of the extreme wear factor....

              This from the people of NGK;

              Incidentally --- NGK has been my go to plug for over 50 years, from back to owning my first mini-bike - running a motorcycle service department - migrating to foreign cars to exclusively Japanese I bet I have a 97% replacement rate with NGK and over all those years ------- 1 bad one at the motorcycle service department,

              compare that to champions which I grudgingly had to use in my BMW one time when doing a tune up ------- 2 out of 4 shot craps right out of the pack.....


              • #8
                I will state this, because it was not mentioned in plain English.
                The 2 spark plugs and the coil are in a series circuit.
                The coil is not "grounded" to the chassis like a conventional coil.
                The sparks develops through one plug, on through the coil, and
                on through the other mating plug. A series circuit.

                My 4.6 Crown Vic has wasted spark coil packs.
                To account from wear on the plugs from sparking twice as many
                times as a conventional ignition, they come with "double platinum"
                spark plugs. Platinum tips on the center electrode and the ground
                electrode. So me not exactly being cheap, just being experimental,
                but my car has 190,000 on it now, and I developed a misfire in OD.
                So I pulled the plugs (my anti-seize lubricant was still on them) and
                the gaps were like 1/8". No wonder it was miss firing. So instead
                of buying new plugs, I just re-gapped them back to .045" and put
                them back in. The ground electrodes were bent over so much that
                they looked like question marks ! ! ! ? ? ? Ha !
                Runs good now. The platinum tips were long gone. No doubt went
                out the exhaust pipe a long time ago. Maybe I can get 300k on the
                original set of plugs. Will keep you posted.



                • #9
                  Now with plug mounted coils you don't even have the cables
                  Helder Ferreira
                  Setubal, Portugal


                  • #10
                    To begin with, any ignition system (even the point type ones) will generate more than enough spark to jump a gap. A slightly larger gap will cause the ignition system to produce a stronger spark to jump it. This can be done on small engines (or any engine) that get temperamental in starting & works to get better results. To further illustrate this, you will see that for decades manufacturers have purposely used resistance spark plugs & wires to reduce radio interference and the ignition systems have no problem with providing more than enough spark to run the engine.
                    The fact that spark plugs currently last 50,000 to 100,000 miles is proof that wear is not a problem. In the “old” days, spark plugs were commonly replaced every year or two but many ran for years without replacement. Granted they weren’t as well constructed as the ones we have available today.
                    A waste spark is therefore inconsequential from a wear stand point & there is more than enough spark energy to power both plugs at once on these systems. Coil failures can happen but in my experience this in relatively rare.


                    • #11
                      The Kohler engine (K582) in the garden tractor has but one ignition coil.
                      On cold winter startups, one cylinder is always sluggish to fire. But it always comes around soon enough.

                      Polarity .....


                      • #12
                        Originally posted by Robg View Post
                        A waste spark is therefore inconsequential from a wear stand point.
                        Doing "double duty" is never "inconsequential" - it always results in replacing plugs at half their rated interval, they may have got it covered with better spark plugs but keep in mind these plugs are not "free" some engines require plugs that cost up to 16 bucks or more apiece...


                        • #13
                          Originally posted by A.K. Boomer View Post

                          Doing "double duty" is never "inconsequential" - it always results in replacing plugs at half their rated interval, they may have got it covered with better spark plugs but keep in mind these plugs are not "free" some engines require plugs that cost up to 16 bucks or more apiece...
                          True enough.

                          The ignition must produce a certain energy to reliably light the charge. With two gaps, the energy needs to be about double. Probably a little less than double, but.....

                          Any ignition system is an EDM machine.... Same thing happens, a little wear from each spark. You can make the parts wear slower by choosing materials, but you cannot make it zero. So the "unused" spark wears that plug, whether you like that or not. Slowly, but surely. And, at about maybe 1.5x the rate it would be worn without the extra unused sparks.

                          Keep eye on ball.
                          Hashim Khan

                          Everything not impossible is compulsory


                          • #14
                            Oh, and I promise you, after 190k, the platinum tips are eroded and gone.
                            The ground electrodes are eroded with a concave crater and the center
                            electrodes are eroded almost level with the porcelain.


                            Edited to be more polite. Sorry.
                            Last edited by Doozer; 07-23-2021, 02:48 PM.


                            • #15
                              Sorry to be stupid, but what is the purpose of these twin spark plug systems? What do they do that is better than a coil/ single spark plug system, especially when many of you have said that spark plug life in a twin system is about half of what it would be for a single plug system?
                              'It may not always be the best policy to do what is best technically, but those responsible for policy can never form a right judgement without knowledge of what is right technically' - 'Dutch' Kindelberger