Points gap for model engines?

Discussion in 'General Engine Discussion' started by awake, Feb 12, 2020.

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  1. Feb 12, 2020 #1

    awake

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    Another design question:

    For an older car with points et al., I believe the gap is usually set to .016" - .020". Is this the right range for a model engine?
     
  2. Feb 12, 2020 #2

    stevehuckss396

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    It is if you are using the same system as an older car. If you are using points and one of the new cdi systems then you should shoot for .005 or so. The idea is to try to eliminate the points bouncing when they are closing.
     
  3. Feb 12, 2020 #3

    Peter Twissell

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    In general, spark ignition systems do not scale. The requirement of the system is to produce a spark hot enough to ignite the fuel/air mixture in the immediate vicinity of the plug gap. Whether that happens in a cylinder of 0.5cc or 500cc is irrelevant.
    Thus , for a given type of system (e.g. points/coil) the gaps are the same, the voltages are the same and the power consumption is the same. The components may be made smaller, but only at the expense of convenience and durability. It is possible to get away with less insulation and more heavily loaded coils when the engine will bever be run in the rain, or under the bonnet of a car in Dubai at 50°C ambient.
     
  4. Feb 12, 2020 #4

    awake

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    Thanks, Steve and Peter. I should have been more specific - yes, planning on an old-fashioned points-condensor-coil ignition.
     
  5. Feb 12, 2020 #5

    stevehuckss396

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    Ok then set it up like it is in an old fashioned car.

    Dont forget to put in a ballast resistor.
     
  6. Feb 12, 2020 #6

    awake

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    Right, thanks. There is supposed to be one built into the coil I bought, but I have that in the back of my mind as something to check out.
     
  7. Feb 12, 2020 #7

    Brian Rupnow

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    Set the points for 0.020" gap. You don't need a ballast resistor. All the ballast resistor did was cut the 12 volts down to 8, and supposedly the points would last longer. You do need a condenser.
     
  8. Feb 13, 2020 #8

    BillC

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    Point gap is also called the 'dwell angle' or as you may visualize; the period of time or dwell time between when the points make contact and break contact. This also effects ignition timing since a close or narrow point gap has very little dell and vice-versa. There is a period of time when the points close that the primary voltage saturates the coil primary windings in the coil. At that point there is no longer a high voltage induced in the secondary windings by the magnetic field of the primary - it is saturated - no change. When the points open the magnetic field collapses which again induces a voltage in the secondary but also in the primary so without a condenser, the high induced voltage will burn or arc the points. The condenser absorbs or charges that primary voltage so helps to prevent arcing the points on opening. The condenser is sized by the time it takes for the coil to saturate in microseconds. Coil ignitions' limiting factor is that time or the saturation time especially in low voltage systems - so now obsolete. Points are the greatest limiting factor with bounce, effective contact and arcing.
    Sorry for the long winded response but one thing leads to another... I've been building engines and making junkers run for a long time so I have been into this up to my eyeballs as pure enjoyment. (might say easily entertained!)
     
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  9. Feb 13, 2020 #9

    awake

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    Thanks, Bill. I am slowly wrapping my head around all of this. As a newbie I hesitate to ask, but ... my understanding has been that dwell and points gap are not quite the same thing, though the latter does affect the former. I understood the dwell angle to be the angle of rotation while the points are closed - and that angle is primarily going to be set by the way the ignition cam is cut. For example, if the cam is cut to resemble a typical intake valve, with one protruding lobe, the dwell angle is going to be something like 200° or more. But if it is cut like a flat tire, the dwell angle will be much shorter, maybe 60°. Of course, I am speaking of a single cylinder setup - on my old slant-6 Dodge Dart, the cam looked more like a rounded-over hex, as best I can recall. But in any case, changing the point gap will make only a small difference, maybe a degree or two - ???

    I realize I may have totally butchered the whole concept!
     
  10. Feb 13, 2020 #10

    awake

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    Thanks, Brian. I read somewhere (maybe on this forum?) that the biggest issue with not having a resistor is heating up the coil too much - ? In any case, the one I bought is *supposed* to have a resistor built in, so we shall see how it does.

    I'm getting closer to first spark - I modeled the ignition cam and the points mounting bracket and 3d printed them to check - all looks good, so now I need to make them in metal. Meanwhile, I still need springs, rocker arm, carburetor, and gas tank. Springs first, so that I can begin to check compression ... or lack thereof!
     
  11. Feb 13, 2020 #11

    Peter Twissell

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    Points gap and dwell angle (properly dwell period) are two different things. Incorrect points gap will also effect dwell period, but they have different functions.
    The points gap is the space between the contacts necessary to prevent arcing when the points are open. The ignition cam timing will be set at a particular gap, and the gap must be maintained in order to preserve the timing.
    Dwell period is the time for which the points are closed before they open to trigger the spark. The dwell period (typically a few milliseconds) should be the time required for current in the coil primary to reach its peak value. If the points are closed for any longer than the ideal dwell period, the spark will not be any stronger but more electrical power will be consumed and the coil will be heated up.
    Electronic ignitions control dwell in time domain. mechanically actuated points approximate the dwell period to an angle, so the actual dwell period is longer than necessary at low rpm.
     
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  12. Feb 13, 2020 #12

    BillC

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    Well, considering the six cylinder point cam: the 'corners' of the hex are the open areas of the points. If the points are adjusted so that they open only slightly at the cam lobes, the dwell angle will be great: each lobe will act on the points during a very small angle or rotation of the cam. The remainder of each segment of the cam will be the dwell angle. Say each lobe acts during a 5 degree angle, the remaining segments or 330 degrees/6 is the dwell angle: 55 degrees. If the points gap is closer the cam acts on them during a greater period: say 15 degrees, the dwell angle is then 45 degrees. Timing is effected by half of the difference between these two settings or 5 degrees. Depending on where the timing is set this can have an effect on the engines' ability to start, accelerate and basic efficiency.
    General Motors distributors had a small sliding door in the distributor cap which exposed an allen socket head point adjusting screw which could be rotated to change the dwell angle (points gap) while the engine was running. Adjusting the dwell angle would greatly effect the running engine by changing timing and coil efficiency. Might add: dwell is as Peter says for efficient coil activity/efficiency but basic timing is not set by changing dwell or point gap but by rotating the points around the points cam - changing actual basic spark plug firing event of the engine.
     
    Last edited: Feb 13, 2020
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  13. Feb 13, 2020 #13

    awake

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    Thanks, Bill and Peter - I think I understand. Which may be famous last words ... :)

    I have designed the ignition system so that I can rotate the mounting of the points around the axis of the crank shaft (on which the ignition cam is fixed) to adjust the timing. As currently designed, I have 70° of rotation to work with - I'm thinking of placing the clamp screw such that I will be able to adjust the timing from as much as 45° BTDC to 25° ATDC. Hopefully that is way more range than I will actually need ... :)
     
  14. Feb 13, 2020 #14

    BillC

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    That would be considered a 'wasted spark' system since it fires the plug on every stroke rather than just on the compression stroke - unless that is, you are building a two stroker. If not; yes, that is far more range than you really need.
     
  15. Feb 13, 2020 #15

    awake

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    Yes, wasted spark - since this engine does not have a true camshaft, it is easier to put the ignition on the crank.
     
  16. Feb 14, 2020 #16

    bluejets

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    Unless of course it is an Atkinson Cycle engine. :)

    Which coil are you using?

    Just to clarify, the ballast resistor is used when an 8v coil is used on a 12v system.
    The idea is to bypass the resistor on startup and therefore supplying 12v to the 8v primary to give better start.

    I use points on the Atkinson I built.
    They are from the now obsolete 4 cylinder Simca car and best part is they are very small as they used 2 sets in the distributor.
    For the condensor I used around 0.22uF 400V pcb capacitor.
    As I mounted it in a switch box with all the other wiring etc. it doesn't need any metal casing for heat isolation.
    Still electrically across the points but the 6" to 8" distance from the points doesn't seem to affect the spark properties and it runs quite well.
    Coil was from a small motorcycle and at the time unknown as to whether it was CDI, kettering or magneto.
    I suspect the latter as it works well with a 7.2v lipo and have known others who tried cdi coils on 12v and instantly cooked them.
    Currently looking at some videos from doc0455 on you tube where he simply adds high power resistance to the primary of CDI coils for use on a buzz setup on 6V.
    Simple if it works as the little GY6 coils are readily available on Ebay etc. for maybe AUS$6.00.

    Also found this indepth detail on ignition systems.(pdf below)

     

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    Last edited: Feb 14, 2020
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  17. Feb 14, 2020 #17

    awake

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  18. Feb 15, 2020 #18

    bluejets

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    Seems fine.
    Most try to miniaturise the ignition components as much as possible however, ignition coils do not always allow this.
    Hall effect switch is one which works well in place of bulky points but requires a small amount of extra circuitry.
    I have such a design if you feel the need or just for your stock of records.
    I imagine you are not bothered by the large physical size or intend to place the coil out of sight.
    Depending on which spark plug you use, you may be able to lower the primary voltage a little.
    Sometimes it can be too much spark and it jumps everywhere except where it is required.
    Same thing happens in some CDI systems.

    Additionally I have somewhere the type of "miniature" HT plug wire.
    Made by Belden if my memory is correct.
    See if I can find the details.
     
    Last edited: Feb 15, 2020
  19. Feb 15, 2020 #19

    awake

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    Yes, this is a full size coil - I am planning to borrow a page from Brian Rupnow and build an ignition box (battery and coil - maybe also the condensor??) that I can take from one engine to the next. Of course, at the moment, I don't even have one engine ... :)

    In theory the points I'm using are much larger than the scale of the engine, but when I 3d printed the mounting system and tried it, it doesn't look look significantly out-of-scale. Oddly, what does look enormous is the condensor - hence the possibility of trying the condensor in the ignition box, even though Iknow that is not the optimal placement.

    Bluejets, thanks for the offer. Even though I decided to start out with MDI (I think that is the right way to describe the old-fashioned points-condensor-coil ignition?), I always welcome more information! And would be very interested in the miniature high tension plug wire as well.
     
  20. Feb 16, 2020 #20

    bluejets

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