Jaguar Performance CDI charge capacitor

Discussion in 'Performance Mods' started by jaguar, Aug 2, 2014.

  1. jaguar

    jaguar Well-Known Member

    I had mentioned before that a higher value of charge capacitor causes a longer spark time (the time during which the spark is happening) but would be counter-indicated for ported hi-rpm engines since a higher value needs more time to charge. Finally I got around to analyzing the difference between a 1uf and a 1.5uf capacitor, noting the peak voltage they are able to charge to at different RPM. Turns out that the larger cap allows less charge voltage at all RPM. very surprising. So although it gives a longer spark the spark strength would be diminished so I doubt there would be any advantage to it. Here is a graph of the results:
    Fabian likes this.

  2. jaguar

    jaguar Well-Known Member

    So at 5ooo rpm there is a loss of voltage of 21% which means the spark strength is lowered that amount.
    Due to the strange design of analog CDIs the change of capacitance also causes a change in the spark timing.
    So a CDI which allows you to choose between the two capacitances would confuse the results because of the timing change.
  3. Fabian

    Fabian Well-Known Member

    Interesting analysis.
  4. HeadSmess

    HeadSmess Well-Known Member

    bigger capacitor requires more current at a given voltage to charge the same amount in the same time. quite simple really.

    the magneto cant generate enough voltage which is dependent on the current required.

    the size of the cap has no bearing on spark duration. discharge it into the same resistance, ie, the coil primary, it just delivers more current. unfortunately the step up works on voltage rather than current.

    ohms law sneaks back into electronics at every turn, no matter how hard one tries to defeat it!

    farad... 1 amp at 1 volt for 1 second... or a coulomb or something...

    love wiki at times, it always backs me up :)

    One farad is defined as the capacitance of a capacitor across which, when charged with one coulomb of electricity, there is a potential difference of one volt.[1] Conversely, it is the capacitance which, when charged to a potential difference of one volt, carries a charge of one coulomb.[2] A coulomb is equal to the amount of charge (electrons) produced by a current of one ampere (A) flowing for one second. For example, the voltage across the two terminals of a 2 F capacitor will increase linearly by 1 V when a current of 2 A flows through it for 1 second.

    also, the point of triggering the scr is the opposite polarity to the charging current. so the capacitance cant affect it, it being separated by a blocking diode.. though i havent verified this with a silly scope, so i could just be talking shyte.

    reduce the resistance on the triggering side, voltage will increase less rapidly, due to increased current flow, trigger point will be retarded. increase resistance, and the opposite will occur.

    or use a CR delay circuit.

    increase current flow, and watch the gate on the SCR fry :)

    fiddle with alternate SCR that trigger at different voltage/current levels.

    or use an IGBT, that requires NO current flow to be triggered.
  5. jaguar

    jaguar Well-Known Member

    so very sorry HS but you are so wrong.
    The discharge time of a 50% bigger capacitor is 50% longer as long as the resistance stays the same (which it does in this case). So the time which the voltage/current thru the HVcoils primary is enough to be the root source for a healthy spark at the end of this cascade would also be longer although I'm not sure it would be 50% longer.
    [Fabian I told you he would be back]
  6. HeadSmess

    HeadSmess Well-Known Member

    very good. testing ya :) yep, basic RC circuit. so the rest is incorrect too, i guess? excellent :)

    but not as constantly as before ;)
  7. Fabian

    Fabian Well-Known Member

    and the forum is all the better for it, even if HeadSmess chooses his presence to be as inconsistent as a healthy dose of fibre free constipation aided with a teaspoon of cement - - - making economical use of his presence; though contributing greatly by pushing out (with fortitudinous effort) updates to his """dyno""" project, that can only be considered a magnum opus in the making.
    HeadSmess likes this.
  8. AssembleThis

    AssembleThis Member

    Charge Capacitor For CDI

    Hi jaguar!

    Interesting post. I'm going to be building a couple of cdi in the next couple of months if I get all the parts I ordered. Nothing as nice as the one you sell, was just curious so decided I'd do some experiments myself. I make my own printed circuit boards and have supplies ordered for that as well. I have a 2ufd cap 850volt, trying to find a 1ufd but haven't found 1 yet. You wouldn't know a supplier were I could get 1 would you? No timing changes just a basic unit. For the 2 engines I have if I'm successful.


  9. jaguar

    jaguar Well-Known Member

    you can find various ones at
    AssembleThis likes this.
  10. HeadSmess

    HeadSmess Well-Known Member

    try two .470uf instead. usually, in a case like this...whatevers cheaper :)
  11. ridinmike

    ridinmike New Member

    I hope I'm not stepping on your thread , but I would like to get one of "Jaguars" units. Where can I order one ?
  12. jaguar

    jaguar Well-Known Member

    Jaguar CDI distributors:
    Sick Bike Parts
    ridinmike likes this.
  13. ridinmike

    ridinmike New Member

    Thank you Jaguar and JNM, it amazes me how much a person can learn if they just keep their mouth shut and listen. I found the needed info further down on the same page and ordered one from JNM
    I haven't been so fortunate trying to buy an already modded engine, gonna have to do it myself I guess
  14. jaguar

    jaguar Well-Known Member

    click on my signature link to see all the mods I recommend.