Mercruiser 470 + Petronix II = coil failure (the amp issue)

[blkzoe]

For all the folks going thru coils on their Mercruiser 470/Petronix II ignition setup; I may have a possible resolution. SOP for Petronix installation has always been bypass the resistor wire ( about 2 ohms resistance). Depending on the internal resistance of the coil installed, bypassing the resistor wire could put you over the 4 amp limit resulting in a coil-b-q. Yes, I realize that there is a minimum voltage requirement for the Petronix to work (full 12v per Petronix). The question becomes; Where is the sweet spot where minimum voltage requirements and the maximum amp limit fall within spec?

From Petronix:


Four and six cylinder engines should not exceed 4 amps. Eight cylinder engines should not exceed 8 amps. If the total amperage in your system is higher than the amount recommended for your application, you should install a ballast resistor.
Example
Voltage 12
Resistance 1.5
12 / 1.5 = 8
Total amperage 8

OK, using this formula, one can calculate that a 4 cylinder engine should employ a minimum of 3 ohms resistance regardless of source.
Voltage 12
Resistance 3.0
12 / 3.0 = 4
Total amperage 4

The key is maintaining sufficient voltage to the Petronix unit while keeping the total system under 4 amps. One issue I've yet to address is what did Petronix mean with 12 volts? That is are they referring to a 12v key on or 12v running? Recall that your voltage while running at the coil can typically be somewhere in the 13.0-14.2v range. So does the amp calculation become revised to as follows requiring a minimum of 3.5 ohms resistance to meet the amp limit:

Voltage 14
Resistance 3.5
12 / 3.5 = 4
Total amperage 4

I'll be doing some testing / R&D today; determining key on vs running voltage at the coil. Then, will I have enough voltage to run when installing enough ballast to be under the 4 amp max for the 470.

Anybody else address this issue?

 

[Bt Doctur]

What they left out of the instruction was telling you to replace the coil with a internally resisted one and use full 12v for the electronic ign system. A lot have split the feed with full 12v to the ign red wire and a ballast resistor to go to the + side of the coil

 

[blkzoe]

What they left out of the instruction was telling you to replace the coil with a internally resisted one and use full 12v for the electronic ign system. A lot have split the feed with full 12v to the ign red wire and a ballast resistor to go to the + side of the coil

per figure #3?

 

[Bt Doctur]

correct on #3 , #2 is acceptable with a internally resisted coil

 

[stonyloam]

Petronix recommends using their Flamethrower II, 0.6 ohm coil for use with their Ignitor II module. Bypass the resistance wire and feed 12v to the coil and module. Like in fig.2 above. If you are going with the Ignitor II it is worthwhile to spend a little more and go with the Flamethrower II. Mine is set up that way and has worked flawlessly. One other thing that I did, as recommended by my "Ignition guy" is to use a relay to power the Petronix system. Basically I used relay (similar to a fuel pump relay) powered by the purple ignition wire on the choke. That delivers full voltage and plenty of current directly from the battery to the coil and module.

 

[stonyloam]

If you are going with the stock coil, simply leave the purple resistance wire in place, and splice the Ignitor II red wire into the purple IGNITION wire at the electric choke connector.

 

[blkzoe]

"Petronix recommends using their Flamethrower II, 0.6 ohm coil for use with their Ignitor II module."

I did and ended up with a crispy critter for a coil. The internet is full of folks who burnt a .6 ohm Flamethrower II with their Ignitor II module. Again, the issue is no more than 4 amps with a 4 cylinder engine. Internal resisted coils seems to max out at 3.5 ohms which will keep you under 4 amps @ 14v. Any less resistance requires additional ballast.

 

[stonyloam]

I believe the stock for your 470 is a 1.5 ohm coil and the resistance wire is about 1.8 ohms. So a standard coil should work with the resistance wire as long as you feed 12v to the Petronix module. If you use an internal resistance coil, you can not bypass the resistance when starting and the battery voltage is at a minimum. On your 470 the slave solenoid feeds the coil during starting, so you will want to keep the external resistor.