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Tech Question
Steve Whelan, High Wycombe, Australia, 1973 Dodge Challenger 440
HI Rick,
you must be sick of ECU/Ignition conversion
questions so I found out most of the info I was looking for from your
previous tech Q's as well as links from your website. But I can't find
anywhere that tells me the correct value of ballast resistor to use for the
Orange ECU upgrade from the stock black ECU. I learnt that the 5-pin black
ECU needs a dual ballast (5 ohm and 0.5 ohm) and that the new Orange 4 pin
ECU only requires a single ballast =- But what value?? Parts suppliers say a
1 ohm, but they supplied me with a 1.25. I have a collection here ranging
from (as measured on my ohmeter) 0.6 to 1.6 ohms. What is the correct value
to use and how critical is it to the operation? And another thing I can't
work out is why they don't use pin 3 on the ECU any more? I guess they must
have an internal resistor to drop the voltage for the unit? I am also doing
the upgrade on my other vehicle -- an Australian '73 Valiant Charger 340.
Thanks for any help for this simple question, and also thanks for an
informative and sharp publication.
Steve-
The dual ballast's 5 ohm side wasm't needed on 4-pin ECUs, which began in the late '70's, since the ECU's internal circuitry was completely modernized, rendering it unnecessary and superfluous.
The ballast's value is pretty much an inverse curve: as the resistance decreases, the spark out goes up ("hotter" spark), RPM range also goes up, and emissions and low-RPM misfire go down. Sounds like a no-brainer, right? Well, there's no free lunch, mate! As the resistance goes down, current draw (amperage) goes up, and so does power dissipation, which is what makes all those good things happen. But more dissipation = higher operating temps = reduced reliability. That's why the "Gold" box has an extra heat radiator on the switching transistor.
Now that the engineering lesson is over, a direct answer: around 1.0 ohm seems to be a good compromise. Anything less, for long-term street use, either toss spare parts in the glove box, or add an extra heat radiator (it's a TO-3 power transistor, see sources such as www.digi-key.com or Radio Shack, etc.)
Rick
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