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Ressister for injections


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Not with a JAM ecu but i've fitted ballast resistors to lots of cars that I've installed aftermarket ecu's into. Ohms law means you can work it all out.



First you need to work out the existing current the injector drive is capable of. The stock ecu setup is 1x inj drive supplies 2x injectors, so:



Standard injectors are 14.8 ohm from memory so using say 14v / 14.8ohms = 0.95 amps x2 per injector drive = 1.89 amps (with some rounding)


4agze injectors at 2.9 ohm so 14v / 2.9ohms = 4.83amps x 2 per injector drive = 9.66 amps (with some rounding) aka way too much



Ballast resistors need to be wired into the +ve feed side of the injector wiring and you need one per injector so:



To be exact the calculations are:


Original resistance per injector less 4agze resistance per injector will give you the required size of the ballast resistors ie 14.8 - 2.9 = 11.9ohms



But you don't need to be too exact - as a rule of thumb for aftermarket ecus I use 4.7ohm 50watt ballast resistors which work out fine.


Calcs for using 4.7ohm injectors are:


4agze injectors at 2.9ohm + 4.7ohm = 7.6ohm so 14v / 7.6ohm = 1.84 amps x2 injectors = 3.68amps which should be fine on the stock injector drive



Oh by the way, don't use small 1w/5w resistors that you can buy at a electrical component store - they'll burn out almost instantly. its better to use either 25w or ideally 50w ballasts. If you mount them on a piece of ali and mount them in the engine bay they wont get hot from load. You can see them in this photo:



engine-bay-02.jpg



Confused? :p


Edited by Stu
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Hi Stu

Thank you for the quick reply.

So the Way i'm Reading your calculations it's Best if i Got 4x 50 watt 12 ohm resistors.

14/14,9 *2 = 1,879 amps which is as close to stock as possible

(2,9+12)

Is this correcly understood.?

Any other for or Against for choosing smaller ohm resistors ? :)

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Yeah 12ohm would mean that the ecu is seeing the same current draw as the original injectors.



HOWEVER - I'm not sure on how well the JAM ecu will do adjusting for the extra fuel supplied right across the map. Bear in mind that say at idle the ze injectors will be pumping in nearly 24% more fuel. The narrow band oxygen sensor will tell the ecu its running rich but I don't have any idea on how far the JAM ecu will be able to scale that back to normal idle AFR targets. Its likely to run like a dog/foul plugs (and potentially run hot with that much fuel) until the map is adjusted back. Let alone at boost - Im not sure if the narrow band sensor does anything at all in the positive pressure zone. Just something to consider.


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  • 2 weeks later...

Sorry for butting in this thread as I'm curious too about injectors and as I'm in the middle of rebuilding my engine I want to check my injectors are all working, I'm just presuming for stock injectors when testing them with an ohms meter is it all 4 should read about 12ohms is that correct? Thanking you in advance

Pat

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I didn't end up with the ze injectors on the jam ECU due to the connectors being different than the stock one.
was told they would fit, but they definitely dont. so injector project is on hold for now. but thank you very much for your expertise & answer

//Mathias

Edited by Mathias10
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