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Nick Mann
14-06-2009, 10:57 AM
I have searched on this site and on google, but I'm struggling to find any good answers.

If I wanted to use low impedance injectors I would need a resistor pack or ballast to make sure I don't have too much current flowing through the ecu. (That's how I understand it anyway!)

I believe that these resistor packs can be home-made using parts from Maplins, but what parts would I need?

Also, I have heard that you lose some control over duty cycles by using this method - is this a significant issue?

Madhav
14-06-2009, 11:10 AM
From what I understood while searching, it has been done, but from the money you save you end up spending 10 fold in terms of time stuffing around.

IMO you are better off getting high impedance that slot straight from siemens or RC from the states. 300 pounds is all you would have to spend.

Nick Mann
14-06-2009, 11:23 AM
I know it has been done on a VR4. I have some low impedance injectors laying around, so if it is £50 and a few hours extra to sort those out, then it saves me a lot.

If I have to spend £300 upwards just on injectors then the project will be on hold for a lot longer.

Davezj
14-06-2009, 11:42 AM
in simplest possible terms you add a resitor in line (series) with each injector to make the total resistance the same as the high impedance original.

explanation:-

lets say low impedance is 5 ohms and high impedance is 20 ohms. i know they are not exact but it will make the numbers look better. and you can transfer the exact numbers in when yo do the cal for youself.

V=IxR

R=V/I

I=V/R

Assume 12V to injectors.

The standard set up is this,
I=12/20 = 0.6A

proposed setup

I=12/5 = 2.4A

to drop that to 0.6 A you would need to add a 15 ohm resistor in line (series) with each injector. Making the total reistance seen by the injector driver in the ECU 20 ohms.

however you will need to pick a resistor with the correct current rating and the closest prefered vale to the one you calculate you need. you will not find a 13.72 ohm resistor. you may get a 13 ohm or a 16 ohm.
If you can't find the correct one then you will have to make one, by putting a number of resistors together in a paralell arangement to tune the resistance to exactly what you want.

post the numbers up and will do the calcs for you if you like.

not sure the resistor pack will cause a drop in performance. but you may find the activation current of the low impedance injectors is not reached, so that could be an issue. you would need to know the electrical spec of the low impedance injectors to be sure.

however when these sort of circuits are designed the don't pick a FET only capable of driving 0.6 A if they need to drive 0.6A they are usually well over speced to do the job. but like everything else in the ECU there is no manufactures part number o my injector driver FET so who knows.

sorry for the woffle.

Mark 4
14-06-2009, 11:43 AM
Nick, I have a set of resistors as specified by NJ, thing is that I don't need them as I bought a set of Sard injectors and they're high imp.

Gowf
14-06-2009, 01:30 PM
Nick, as you know i have done this and have had issues. The issues i have had have come from using inferior resistors from maplin that just couldnt cope with the power. So instead of using 10w 5.6 Ohm resistors, i am now using 50W. Get them from RS and they have so far, given me no problems whatsoever.

With regards to the speed of response of the injectors, it may be slightly slower, but certainly not slow enough to cause any issues with duty cycle.

Just stick one of these resistors in the ground line of each injector and you will be absolutely fine.

foxdie
14-06-2009, 02:20 PM
Question, the injectors run at 12V right? Why not use a transistor to trigger the injectors?

My electronics knowledge is very rusty but could someone elaborate on whether this circuit could help?

uploaded/51922/1244985597.gif

Or maybe this page may be of some help? http://www.ggimages.com/rx7/pwm.html

Nutter_John
14-06-2009, 02:22 PM
what about the skew rate of the transister ???

Davezj
14-06-2009, 03:01 PM
i was think of doing a second set of injector, in stead of going bigger. but that's another story.

to drive the second set of injectors i was going to use an opto isolator to drie the FET tagged on to the existing injector signal line, so you don't load/change the original injector signal, when you manipulate it to scale it for a gradual increase with resect to RPM/TPS. but if you are not gongfor a second set of injector there is no need to opto couple it.

i think there is an issue with using the injector signal to drive a higher current transistor directly, as according to the research and reading i have done the injector signal does not drop to 0V when it is in it's none active state. I think it is 0.6V when none active, which would switch the transistor on perminently.

If someone know any different please correct me, i have not measured it our injectors but just read up on what others have done.

orionn2o
14-06-2009, 04:04 PM
Yeah but Dave you could lose that 0.6v easy with a diode in the circuit so you could pull the whole injector signal line down.

With regards to resistor pack, it's not an exact science and never will be, even using alike high impedance injectors will have different impedances therefore proving that the system isn't that susceptable to differences. Adding an approximate resistance will generally be sufficient. Techincally you need a resistance and a coil to create an inductance. Afteral we are talking impedance, not resistance.

Finally, you can ignore skew rates and time delays in injector pulsing as you can map this out as it'll be a constant change over the rev range.

Mark 4
14-06-2009, 07:40 PM
The resistors that I have are 50w.

orionn2o
15-06-2009, 08:27 AM
Out of interest Mark, which resistors do you have??

Eurospec
15-06-2009, 09:08 AM
Make sure they are power resistors though- normal ones havent got the balls and will melt. You need the big puppies with the ceramic heat sincs on them.

Gowf knows the values you are supposed to use- i cant remember!

Cheers,

Ben.

orionn2o
15-06-2009, 09:11 AM
Gowf used 5.6 Ohms 50W.

RS 107-4131

Actually has anyone measured the resistance of the standard injectors? It would be interesting to know what they are..

Nick Mann
15-06-2009, 09:35 AM
Can you do a resistance check on them? Some things change in resistance significantly when under load I believe.

Thanks Matt for the part number! Mark - are those the ones you have?

orionn2o
15-06-2009, 09:39 AM
You can do a resistance check as DC resistance doesn't change with load (well apart from temperature variants). You can't measure reactance as this changes with frequency.

Resistance + Reactance @ 90 degree phase = impedance.

The resistive component will however give a very rough indication of the impedance.

You could more accurately measure the impedance using a known impedance source, a sine wave generator and a multimeter. Then just make yourself up a potential divider and measure the outputs. The only thing you would need to know is the frequency range in which you want to measure.

Mark 4
15-06-2009, 10:06 AM
Yes Nick, I also have a small aluminium enclosure that I was gonn fit them into.