Running a low ohm coil with an orange igntion box?

[Fran Blacker]

Was reading Mopar Action tech topics. E Booger said you needed to run a minimum 1.4 ohm coil with an orange box or it will be cooked. Running a MSD Blaster II 0.70 0hms heading for trouble?

 

[RJRENTON]

Fran,
I believe the concern is the current handling capacity of the switching transistor on the front of the box. When this transistor turns OFF (all the coil's primary current flows through this transistor), the collapsing field in the coil's primary winding generates the high secondary secondary voltage to the plugs. If the coil's primary resistance is reduced to 0.7 ohms, the transistor will have to handle roughly 50% more current and be subjected to a higher induced voltage caused by the coil's collapsing primary winding, a function of the coil's turns ratio. This presents both higher voltage and current conditions which will likely cause premature failure. If you feel you want or need more voltage to the plugs, look for a coil with the same primary winding resistance but has a greater secondary turns ratio, allowing it to generate a higher secondary plug voltage with the same primary current.
This is just my opinion.
Bob Renton

 

[Fran Blacker]

Fran,
I believe the concern is the current handling capacity of the switching transistor on the front of the box. When this transistor turns OFF (all the coil's primary current flows through this transistor), the collapsing field in the coil's primary winding generates the high secondary secondary voltage to the plugs. If the coil's primary resistance is reduced to 0.7 ohms, the transistor will have to handle roughly 50% more current and be subjected to a higher induced voltage caused by the coil's collapsing primary winding, a function of the coil's turns ratio. This presents both higher voltage and current conditions which will likely cause premature failure. If you feel you want or need more voltage to the plugs, look for a coil with the same primary winding resistance but has a greater secondary turns ratio, allowing it to generate a higher secondary plug voltage with the same primary current.
This is just my opinion.
Bob Renton

Can I assume since it still works its ok or could it damaged? Only has 6 to 8 hours of sporadic run time.

 

[Glenwood]

If you are using a ballast resistor, the current might be within the ecu's tolerance. Remember the br is cut out during starting so it can handle more. Carry a spare just in case.

 

[RJRENTON]

Gentlemen,
Which coil are we referring to? The original coil or the MSD coil? And is the origional ballast resistor still being used and what is the value of the resistor?
Electronic devices are not usually forgiving. And excessive current thru the switching transistor will result in more heat being generated thru the transistor's collector emitter junction. Heat has a cumulative effect which will result in failure. True, the ballast resistor (when in the circuit) limits the transistor and coil primary current but if components are substituted without thought or analysis is asking for trouble. Having a spare control box and ballast resistor and maybe a coil available on hand may be prudent.
Bob Renton

 

[Fran Blacker]

Blaster II. Resistor 1.41 to 1.55 ohms being used.

 

[6PKRTSE]

I used to have an orange box. Changed out to a Rev-n-ator box probably going on 8 to 10 years ago. MSD blaster II coil has been on for probably 25 years. Still using a ballist resistor. I have never had any known ignition issues with either box.

 

[RJRENTON]

Fran,
What is resistance of the Blaster 2's coil primary? Its reading is taken across the 2 threaded connections; one will be marked (+), the other will be marked (-). To measure the high voltage connection, measure between the (-) and the high voltage connection. The reading will be significantly higher....maybe 2000 - 5000 ohms but it depends on the manufacturer. The higher the resistance usually indicates the voltage produced (volts per turn ratio).
Given the resistance and voltage, compute the current using Ohms law: I (current) = E (voltage) ÷ R (resistance). The other unknown is the switching transistor's specs. If you know the number of the transistor it could be looked up thru GOOGLE, referencing the transistor's Ic maxumum (collector current) to see if your computed value exceeds that value. The specs also shows max temp capability. Hope this helps your quest.
Bob Renton