How much injector is too much for factory pcm?
12-13-2011, 11:02 PM
#21
So taking the ECU out of the equation, running peak/hold injector circuitry @ 14ohms limits the current to 1 amp. This is ideal for the "hold" current, however, you'll lack the higher "peak" current required to snap the injector open.
If we reverse the scenario and throw in a resistor that brings the current up to, let's say 4 amps, you now have the "peak" current to open the injector but you've also induced a higher "hold" current as well, and the ECU may eventually burn out.
As mentioned earlier current is controlled via a sensing resistor and transistor, and pretty much operates within the parameters of the injector circuitry (in our case 1amp). In order to run peak/hold injectors we actually need current that is controlled via switching to get that initial 4amps required to open the injector. I'm most certain that these ECU's don't have that capability, but this is essentially what an external peak/hold injector driver does.
So basically, what it all boils down to when running peak/hold injectors on a saturated system is that you introduce some latency in the initial stages of injector opening. Meaning the benefit of a shorter injector-opening response time is nil.
If we reverse the scenario and throw in a resistor that brings the current up to, let's say 4 amps, you now have the "peak" current to open the injector but you've also induced a higher "hold" current as well, and the ECU may eventually burn out.
As mentioned earlier current is controlled via a sensing resistor and transistor, and pretty much operates within the parameters of the injector circuitry (in our case 1amp). In order to run peak/hold injectors we actually need current that is controlled via switching to get that initial 4amps required to open the injector. I'm most certain that these ECU's don't have that capability, but this is essentially what an external peak/hold injector driver does.
So basically, what it all boils down to when running peak/hold injectors on a saturated system is that you introduce some latency in the initial stages of injector opening. Meaning the benefit of a shorter injector-opening response time is nil.
ie with low impedance injectors, one needs a driver box with the stock ecu
if one doesnt want to spend $500 for a driver box and harness, one can add resistors, from $8 to 50ish total cost depending on the resistors, and have the low z injectors work.
they will have a slower injector open time(megamanual reports an additional 1.0 ms) which means they act more like high impedance injectors, but otherwise, no other downside.
i think where we differ is I think the extra current will burn up the injector coils, you think it will burn up the ecu. moot point because we both agree you dont run low z injectors in a high z setup - something will burn up, and quickly.
btw, many manufacturers, used low impedance injectors and resistors right from the factory so they cant be all that bad
12-14-2011, 07:51 AM
#22
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71,
Yes I think we're on the same page, but great info none-the-less. Got the theory and the math covered.
Wicked,
I actually found a set of 72# injectors out of my old Supra (don't know how they stayed with me this long), and picked up a pair of Radio Shack 10ohm 10w resistors to give you a quick visual reference.
Stock Injector Resistance
Resistor should be spliced in on the positive (+) side
This is the resistance you're after. I used a 10ohm resistor and you can see there's pretty wide margin. No worries as long as you stay between 12-14 ohms, and verify your amperage draw (refer to 71's posts).
Also, check this site for your project box to mount your resistors in, if you'd like to go that route. Pick up some rubber grommets and thermal tape or paste. Pretty much go through it and get creative if you want. Most of this stuff is pretty inexpensive.
If you do use the project box be sure that all the resistors somehow come in contact with the box (separated by thermal tape or paste), which will act as a heat sink.
Hope this helps and good luck.
Ciao
Yes I think we're on the same page, but great info none-the-less. Got the theory and the math covered.
Wicked,
I actually found a set of 72# injectors out of my old Supra (don't know how they stayed with me this long), and picked up a pair of Radio Shack 10ohm 10w resistors to give you a quick visual reference.
Stock Injector Resistance
Resistor should be spliced in on the positive (+) side
This is the resistance you're after. I used a 10ohm resistor and you can see there's pretty wide margin. No worries as long as you stay between 12-14 ohms, and verify your amperage draw (refer to 71's posts).
Also, check this site for your project box to mount your resistors in, if you'd like to go that route. Pick up some rubber grommets and thermal tape or paste. Pretty much go through it and get creative if you want. Most of this stuff is pretty inexpensive.
If you do use the project box be sure that all the resistors somehow come in contact with the box (separated by thermal tape or paste), which will act as a heat sink.
Hope this helps and good luck.
Ciao
Last edited by bmech211; 12-14-2011 at 09:25 AM.
