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  1. #1
    Tech Guru Resident Guru
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    About Racing CDIs

    This is particularly important to Yamaha twin owners.

    In the early years of high performance 2 strokes, virtually every engine had an ignition that is “straight firing”. That means that the ignition timing was exactly the same at all engine rpms. It didn’t take engineers long to realize that high rpm engines “liked” more retarded timing, as that helped stave off detonation. Sadly, that retarded timing value resulted in very poor mid-range power. In the mid-range, the engine preferred a more advanced timing value. With that, they developed CDI boxes that offered an “advance-retard” curve. These CDI boxes have a retarded timing value for easy starting, but the CDI box advanced the timing progressively a few degrees until 5000-6000 rpm (to get stronger mid-range power). After 6000rpm, the timing value then retards progressively as rpms increase …. It was the best of both worlds, and this technology is in use on every stock two-stroke PWC.
    When race shops started developing PWC race engines, they employed cylinder port timings and pipe dimensions that offered great high rpm power … but these same mods seriously harmed mid-range power. In an effort to recover some of that lost mid-range, they developed CDI boxes that had a much more advanced timing value at 6000rpm … a lot more advanced. On these racing CDI boxes the timing value at peak rpm was exactly the same as the stock CDI, so safe peak rpm power was still possible. The down side of these racing CDI boxes is that they induce a very high risk of detonation (and resulting piston scoring) if the engine is cruised steadily at 5000-6000 rpm.
    Since race boats all run 100% high-octane race gas, and they are typically operated in a “all on or all off” fashion, the excessively advanced mid-range timing is not a reliability problem. However, if one of these racing CDIs is used on a recreationally modified “91 octane pump gas” machine, the risk of detonation during mid-range cruising is “extremely high”. This creates the odd scenario where a 91-octane machine can be ridden very aggressively at high rpms with no problem (because timing at high rpm is stock). But that very same setup is at high risk of detonation and piston scoring when cruised at mid-range. Most riders assume that there is only a piston scoring risk when the engine is at high rpm …. And that is true with a stock curve CDI box. However the racing CDI boxes have their highest detonation risk areas at mid-range rpms. The takeaway from this is, racing CDIs are great for high-rpm closed-course race boats running on race gas, but they are a very dangerous (reliability) choice for anyone running a recreationally modified 91-octane pump gas setup.
    Unless otherwise stated, responses to all forum posts are based on my personal experiences working within the PWC industry, and developing high performance PWCs since 1987.

    www.groupk.com

  2. #2

    Re: About Racing CDIs

    Thanks Harry. Not to throw any manufacturers under the bus, but which ones do think are harmful for the average rec- rider?

  3. #3
    I dream skis
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    Re: About Racing CDIs

    Thank you for the write up Harry. Just out of curiosity, could a happy
    medium be found with a 91 octane rec modified ski by retarding the
    static timing maybe 2 degrees. (EX: a rec modified 701 running 180 psi
    on 91 octane with a MSD enhancer ). This would be a engine that would
    typically never see anything over 7,500 rpm under load.
    Last edited by ACP; 03-21-2018 at 06:56 AM.
    2001 Superjet; R&D, Blowsion, UMI, Riva, Wetwolf, Ocean Pro.

    1987 650sx: Mariner, UMI, Neo Designes, R&D, Jetsports, Renthal, Hydro-Turf, Skat-Trak, Reworked stock pipe, 40mm carb, Wetwolf F/S cone.

    1986 JS300; lots of mods, fast for a 300...I think

    1987 X2: Rips pretty good and has some 80's flare.

  4. #4
    PWCToday.com Is My Home Away From Home Waxhead's Avatar
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    Re: About Racing CDIs

    That a great article and its true in that situation in a runabout. Not many people cruise along at 6000 rpm on a stand up most people are braaappp braaapp on the throttle, The other point to remember as well is the dynamic compression is quite low on a two stroke until it gets on the pipe and then is spikes quickly. That's when you need to pull the timing out quickly.
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  5. #5
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    Re: About Racing CDIs

    Wax,
    Sadly, that would only be doing 2 wrongs to make a right .... going with a stock curve cdi is the best solution.

    Regards, Harry Klemm


    Quote Originally Posted by Waxhead View Post
    That a great article and its true in that situation in a runabout. Not many people cruise along at 6000 rpm on a stand up most people are braaappp braaapp on the throttle, The other point to remember as well is the dynamic compression is quite low on a two stroke until it gets on the pipe and then is spikes quickly. That's when you need to pull the timing out quickly.
    Unless otherwise stated, responses to all forum posts are based on my personal experiences working within the PWC industry, and developing high performance PWCs since 1987.

    www.groupk.com

  6. #6
    PWCToday.com Is My Home Away From Home Waxhead's Avatar
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    Re: About Racing CDIs

    Quote Originally Posted by Group K View Post
    Wax,
    Sadly, that would only be doing 2 wrongs to make a right .... going with a stock curve cdi is the best solution.

    Regards, Harry Klemm
    I don't see it as two wrongs Harry. I see it as maximizing efficiency for a given cylinder pressure. When the cylinder pressure is low you can run large advance to make more torque down low. Maybe not in a runabout with two large guys on it towing their equally sized wife's in a rubber tube on last years fuel, but in high performance applications there is plenty to be gained. Even the stock gp1200r ignition curve it quite advanced in this rpm range your'e talking about
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    Last edited by Waxhead; 03-21-2018 at 05:16 PM.
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  7. #7
    Tech Guru Resident Guru
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    Re: About Racing CDIs

    Whoops, Wax, my apologies ... my response was meant for the ACP post ... sorry for the confusion. Harry

    Quote Originally Posted by Waxhead View Post
    I don't see it as two wrongs Harry. I see it as maximizing efficiency for a given cylinder pressure. When the cylinder pressure is low you can run large advance to make more torque down low. Maybe not in a runabout with two large guys on it towing their equally sized wife's in a rubber tube on last years fuel, but in high performance applications there is plenty to be gained. Even the stock gp1200r ignition curve it quite advanced in this rpm range your'e talking about
    Unless otherwise stated, responses to all forum posts are based on my personal experiences working within the PWC industry, and developing high performance PWCs since 1987.

    www.groupk.com

  8. #8
    PWCToday.com Is My Home Away From Home Waxhead's Avatar
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    Re: About Racing CDIs

    Quote Originally Posted by Group K View Post
    Whoops, Wax, my apologies ... my response was meant for the ACP post ... sorry for the confusion. Harry
    All good Harry, Keep up the good work
    Www.waxracingproducts.com
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  9. #9
    Resident Guru JC-SuperJet's Avatar
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    Re: About Racing CDIs

    Good Info Harry.

    Thanks!

  10. #10
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    Re: About Racing CDIs

    Hey Harry what about advance plates on kawasaki 750's and 800's, what is the purpose of those. I also remember back in the late 80's we would twist the stator on our 650 limiteds to full advance ... were we crazy for doing that? Thanks for the great write ups

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