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I have a 85ESL69E with a bad amp unit. The manual says the amp puts out 300V to the coil and 25kV to the plugs. 25k/300=~84. If I use an MSD 6A outputting 470V that means the plugs will see ~39.4kV. Does anyone know if the '69 cap and rotor can take this kind of voltage increase? The cap is $205, the amp $305 and rotor is NA. Shure would hate to blow out parts I cant get anymore.
Thanks

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You are quoting theory.

Actually the ignition system only generates what voltage is needed to jump the small spark plug gap.

At idle, that may only be around 5000 volts and increases on acceleration and WOT as the compression pressures increase. Unless there is an open circuit somewhere, the voltages do not get as high as you stated.

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Thanks seahorse,
I'm the 1st to admit i'm in over my head here, but had to start somewhere. How does the ignition system know where in the RPM range the motor is running? It doesnt see compression, but that has an effect on amount of voltage it takes to jump a given gap. Thanks for any info you can share here.

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You have to understand that compression pressures (not the ratio) build up as the throttle is opened, and that increases the amount of voltage needed to jump the sparkplug gap.

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So would I be safe to "assume" that the arc will jump when the conditions are right based on plug gap/atmosphere regardless of the amount of charge the CD box is capable of producing? Then the only concern of burning out distrubtor cap/rotor would be in an already faulty item?

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"Shure would hate to blow out parts I cant get anymore."

I`m pretty sure you can get both mechanical and electric parts for that old Johnson from:

Garry`s Marine repair
www.garysmarine.com
727-863-9277

He has more NOS parts for old OMC outboards than I`ve ever seen. Bought parts for my `61 Evinrude 75hp V4 from him. He is north of Tampa Fl.