spyder348

I have searched many times for the resistors i need. I have heard so many diff. numbers and seeing if anyone has a recent update on what resistors to use with my injectors which are siemens 72's from racetronix. low impedence which i think re 2.3 2.4 ohms. I purchased 1 ohm resistors from somebody and they sent me 1.5 ohm resistors. I read a search that the car wont even run with 1.5's. Is this true are 1.5's not good or do i def need to get 1 ohm 25w?

Charles A. Toupin

Let me check mine. I have the same injectors as yours. And I have a few resistors left... But I don't remember what they are...

Charles

CarbonRevo

I have some Vishay Dale 1.25ohm resistors (I'd have to double check). I never used them. Car ran fine without them. Most people use the Vishay Dales.

toddk911

Don't need them

DDP

Uh, no.


If you are running the factory computer you should run resistors. The injectors you have are low impedance, this means you need to run resistors. Just because a car will run without resistors doesn't mean that it isn't a good idea to wire them in. The damage doesn't happen overnight, it can take time.

buddd

with siemen 72lbers the ohms are within factory range from what i've been told and because they are a larger injector (than say a 55 for example) the duty cycle is less which in turn means less heat through your computer.

johnd1267

Hey everyone,

I'm new on here and am replacing the motor in a 1984 944. Anyone in Central Mass that wants to lend a hand?

johnd1267

hey,

I'm new to this site. I'm in the middle of replacing the motor in my 84 944. I think i'm way over my head. Any tips or anyone in Central Mass want to help?

eniac

I had Siemens 86lb injectors that were running 100% duty cycle at 3.5bar fuel pressure. You should pick an injector size that will run 80-90% duty cycle. Picking a larer injector simply to lower the duty cycle below is incorrect. So duty cycle has nothing to do with needing resistors or not if everything is properly matched.

That said, Siemen injectors can run with or without resistors. I have done both with no problems either way. I suggest using them to be on the safe side. If you don't, it will push the DME to its upper amperage limit and could possibly burn out an old weak unit...although I have yet to ever see this happen in person.

Bri Bro

The 944 injectors are one-off on impedance. I do not remember ever seeing another one like it. The two Siemens injectors mentioned above are both 2.35 ohms. Factory injector are at 4.5 Ohms.

For track the resistors are good insurance, for street it shouldn't be a problem. Adding them will make ZERO impact on the performance of the injector, it just lessens to load on the DME driver transistor if you are running high duty cycles. And yes, the DME does get hot when running high duty cycles for extended periods of time.

Type of resistor to use:
https://rennlist.com/forums/7073603-post6.html

Source for injector resistor
http://search.digikey.com/scripts/Dk...ame=KAL251F-ND

mikeyoman

Running any Resistor WILL potentially affect the performance at the injector. You will not be providing the full 12v at the injector and because the resistor is limiting current available, to protect the Transistor Driver, then it follows that the current availalble at the injector may be limited.
Never found out what current Seimens or any other injector require to operate correctly but they are designed to operate at 12v. If you add resistors they will likely have approx 8v and thus less current and thefore may become sluggish to open or similar. This may not be apparent in use but it is still a factor.

I'm not advocating NOT using resistors if you feel your TR Driver is likely to blow with low impedance Injectors, but be aware that adding any resistor will limit the voltage at the injector.

good write up at Injector Dynamics website
http://www.injectordynamics.com/questionsanswered.html

Bri Bro

If you are saying ohm laws applies, you are correct, the current will be limited (that is what is needed) and a voltage will be dropped across the resistor E=IR.

But the DME uses a set and hold pulse for the injectors. The hold is an oscillating voltage well below 12VDC rating of the injectors. There is a min voltage for any solenoid (injector) will engage and another voltage that it will hold open and it is well below the operation voltage rating of the part. Many hundreds of after market injects have been run with resistor in 944s and I have never heard one problem to date.

In the picture, you can see the injector pulse from the DME. The DME grounds the signal line so it is active when low. The first low going pulse "sets" the injector to the open state. The oscillating voltage maintains the injector in the open state but notice it is 1/2 the average voltage. This is done to reduce the load on both the injector and the DME drive transistors. If you measure the ground pulse from the DME, you will find it also isn't really going to ground. The injector transistor in the DME is a Darlington type and it will drop 1-2 volts across it.

You need find the correct amount of power to allow the injector to both set then hold the injector which I doubt you will find in any injector spec. If I get a chance, I can test one and see what that range is. For now, I am sure we are well within it or lots of folks would be having problems.

You will also find that some vendor supply injector resistors with a much small power rating then the normally used 25 Watt. This can be done because the average power to the injector (and also to the resistor) is greatly reduced due to the DME supplying an oscillating hold circuit.

gt37vgt

it has a very small effect on the control of the injector and you will only ever notice it if you are realy streatching the limits of the injector or injector driver or perhaps if you are a well instrumented injector geek at Injector dynamics .
1.5and 2.3 seems to add up to pretty close to 4 to me put them in line with the injector then compare the resistance of the standard injector if it is within 1 ohm i would think your safe ..
the resitors are nesisary as they help limit the current inn the injector circut without them you are at risk of burning out an injector driver

Bri Bro

The math to support this.

https://rennlist.com/forums/4141369-post29.html

mikeyoman

Brian,
Yeah, I studied the DME circuit before and you're right, it is a self limiting P&H circuit.
I still don't see the requirement for extra limiting resistors in series with the injectors.
It seems to be a case of 'just add them anyway' .
The driver circuit monitors & limits the current supplied to the injectors, as per your illustration.
Still think the injectors would need a 12v initially to open, with a reduced voltage/current on hold.
Adding resistors in series with injectors will reduce the 16 amp current draw - surely the injectors need whatever current they are designed for to open correctly ?

I also intend to build some kind of test jig in the future, but no time at the moment.