Ok I think I might regret this but I want injectors soon so here goes nothing.

I have spent a while searching but there is much conflicting advice.

Our cars should have injectors in the range 3.5 to 5 Ohms.

At least one tuner sells 2.8 ohm injectors that fit straight in.

Some people claim this fried their DME - however there are also people with fried /hot running DME's and standard injectors so this may or may not be true.

I am not fully clear on the relationship between impedance and resistance but resistance is measured in OHMs and to overcome a higher resistance you usually need to d more work so it seems to be with lower resistance the DME should have an easier time of it.

An injector supplier states that we need high impedance resistors 12ohm but the general consesus seems to be there may be duty cycle issues with these.

2.8 is a lot closer to 3.5 than 12 is to 5 so it would seem the 2.8s would be the better match.

Ballast resistors can be added to increase the resistance of the low impedance resistors but ignoring the difficulty I have understanding how higher resistance is somehow easier to overcome for the DME I believe this involves messing with the wiring loom at some point and this could introduce unreliability.

So in my quest to understand

1) how does a lower impedance cause the DME to work harder (and if you can explain it simply, how are impedance and resistance related?)

2) How does a 12ohm injector work better than a 2.8 ohm on a system designed to work in 3.5 to 5 range.

3) Is there anyone who makes a 55lb injector that fits our car that falls within the 3.5 to 5 ohm range.

Thanks, please try to keep the chat / rumours / guesses to a minimum in this thread, thanks
Tony

 

Okay, some basic electrical theory here. Resistance and impedance are the same thing. Basic governing equation for electrical flow is V = I * R, where V is voltage, I is current flow in amps and R is resistance in ohms. Since you have a fixed voltage to the injectors (say 12 volts), current flow will be inversely proportional to resistance. Using a 4 ohm injector, I = V/R = 12 volts/4 ohms = 3 amps. Using a 2.8 ohm injector, I = 2 volts/2.8 ohms = 4.3 amps. In other words, current provided by your DME increases by 43%.

In the electrical world, power is measured in watts, not horsepower. The equation for power is W = I*V. In other words, power increases directly with current flow. In the above example, for a 4 ohm injector, W = 3 amps * 12 volts = 36 watts. Using a 2.8 ohm ohm injector, the power consumed is W = 4.3 amps * 12 volts = 51 watts, or 43% higher. Therefore, a 2.8 ohm injector makes the DME work harder (provide more electrical current) than a 4 ohm injector, generating more heat. Conversely, a 12 ohm injector using less current and less power, and will not damage the DME.

Try this at home: measure the resistance of a 100 watt and a 40 watt light bulb. A 100 watt light bulb has a lower resistance. That allows more current flow, which is why it generates more light (and heat.) The above calculations won't work, however, as the resistance of the filament is much lower at room temperature than at operating temperature.

NOTE: The injectors in our cars are wired in parallel sets of 2 and 2. Parallel circuit theory is a bit more complicated, so the above simple example IS NOT directly applicable to our cars. I have kept it simple to get the point across. Danno has provided the parallel circuit calculations and recommendations for ballast resistor impedance and current rating on at least a couple of occasions.

 

Hmm, all of the stock 951 injectors I've measured were 5.1-5.6 ohms. There was a bug in the earlier manual that says 3.5ohms, but they've been updated to say 5.5 ohms.

I'll defer to the EE guys who do this for a living: https://rennlist.com/forums/944-turbo-and-turbo-s-forum/65934-dme-and-relays-getting-hot.html

 

Actually resistance in parallel is quite simple. The equation is R1xR2/R1+R2. It is evem more simple when the 2 resistance values are the same. It just 1/2 of the one resistance. If the stock injectors are 5.5 ohms then 2 in parallel would be 2.75 ohms. Using 2.8 ohm injectors in parallel would net a resistance of 1.4 ohms.
Hope this helps.

 

The circuit is a little more complicated. One output transistor fires two output wires in parallel, which in turn each fire two injectors in parallel. In other words, one transistor, two parallel ballast resistors, four parallel injectors.

Resistance through four stock 5.5 ohm (Danno's value) injectors = 5.5/4 = 1.38 ohms. Resistance through four 2.8 ohm injectors = 2.8/4 = 0.7 ohms. (Note that the resistance is now alomost exactly half, requiring the DME to output twice the current.) Difference = 1.38 - 0.7 = 0.68 ohms. Two ballast resistors wire in parallel each require 0.68 ohms * 2 = 1.35 ohm resistance. Any value between 1 ohm and 1.5 ohm would probably be close enough. It would be more important to ensure that the power rating of the resistors is high enough; Danno's linked thread recommends resistors rated at 25 watts.

The resistors are wired into the DME wiring loom, wires 14 and 15 from the DME 35-pin connector. Given the heat that the ballast resistors can potentially generate, the Guru Racing solution of mounting them on a metal heat sink seems like a good idea. See the picture under "injectors." HTH

Edit: Link fixed.

 

Waterguy, Danno's link thread does not work.
One important issue that has not been addressed in this thread is duty cycle. I think the reason some people are able to run aftermarket injectors without ballast resistors is that they are running a lower duty cycle.
To make things simple let's say the stock injectors are running 90% duty cycle with full power being 36 watts. Then net power consumption would be only 32.4 watts.
Now lets say you use an after market injector that because of the increase in size is only running at 60 % duty cycle. Even though at full power the injector would draw 51 watts of power because they are only on 60% of the time the net wattage would be 31 watts.
I think where people are running into problems is when they are making enough horsepower to run lower impedance higher flow rate injectors at the same duty cycle as stock injectors do on a lower HP car.

 

Thanks very much excellent info and I think I am getting it (And a reminder I should really brush up on my basic electrical theory trouble is, I don't use it and it gets forgotten).

Now this heat that comes from the balast resistors doesn't that take energy (current?) to create as well?

With the standard injectors (mine measured 5ohms) where does the heat go?

Tony

 

"Waterguy, Danno's link thread does not work."

It's the same link I posted earlier.

"Now lets say you use an after market injector that because of the increase in size is only running at 60 % duty cycle. Even though at full power the injector would draw 51 watts of power because they are only on 60% of the time the net wattage would be 31 watts.
I think where people are running into problems is when they are making enough horsepower to run lower impedance higher flow rate injectors at the same duty cycle as stock injectors do on a lower HP car."

But people never, ever get larger injectors because they want to run lower duty-cycles. It's usually when they hit 100% on the stock injectors, and then they've bumped fuel-pressure up to some ungodly 50psi or so. THEN they get larger injectors, but only one size up to 55#, which they then run right up to 90-100% (around 350-370rwhp).

It's also not so much the total current draw over time that's the issue (integrate amps vs time = total coulombs), it's the peak current draw when the injectors first opens up. The DME signal is not a square-wave. It's a peak & hold signal that literally blasts the injector open, then it's cut back to only 20-25% of that current to hold the injectors open for the remaining duration. It's this initial current that we have to be careful about. As Beab951 and Lorenfb (both EE guys doing this for a living) posted in the link, the average current draw is right up to the limit of the driver transistor's rating, not allowing for a 50% overhead. And the initlal peak blast is definitely over the limit.

Injectors are current-driven devices, not voltage. You just need the minium current of 2-3 amps to open them, then hold with 0.75-1.0 amp.

"Now this heat that comes from the balast resistors doesn't that take energy (current?) to create as well?"

That heat coming off the ballast resistors would otherwise be coming from the driver-transistor in the DME.

"With the standard injectors (mine measured 5ohms) where does the heat go?"

The heat is generated in the solenoid coils. It usually dissipates into the injector and is carried away by the fuel and surrounding air. The injectors can usually take a lot more heat due to their surface area than the DME transistor...

Also, this whole discussion is really about durability, not functionality. There's absolutely no performance difference because injectors are binary devices, they either open or closed. So with or without ballast-resistors, the injectors will be injecting the exact same amount of fuel (because the DME is output the same duty-cycle signal). It's just than when you're sending through 5-amps through each injector when only 2-3 is needed, the extra current generates extra heat in the injectors and DME and will lower their lifespan. It's not a question of whether it'll work or not, it works the same either way. It's just a matter of how long they'll work and last. David Salama had a typical experience with his injector wiring when upgrading to large injectors...

 

Excellent thanks again,
I have seen the light

Tony

 

So to sum up, if I (or anyone else) buys siemens 2.8ohm injectors we should install two 1 or 1.5ohm 25watt resistors.

(my current injectors are 5ohm so 5/4 =1.25ohm
siemens low impedence 2.8/4=0.7ohm
diff =0.55ohm
2 injectors in parrallel 1.1 ohm resistor)

1 ohm should be OK would 1.5 be better?

My current injectors are 5ohm and I note Danno hasn't seen them this low. I will measure a couple of others and the new injectors and report back here.
I do have a calibrated fluke87 multimeter so at least I should get that right

Then install one resitor in series with wire on DME pin 14
and one resistor in series with wire on DME pin 15.

Install resistors onto a decent heatsink with heat transfer paste.

Tony

Faith In Rennlist Restored

 

Weird thing is, i contacted Tony G about injectors vs resistors vs chips etc......

I was informed that he purchased a set of 72lbs injectors from Lindsey racing (Siemens Brand) and literally "dropped them in", fine tuned with the power perfect and that's it.

No chip adjustment, no resistors etc........weird.

 

Yes Lindsey do sell them to 'drop in' and they will work but for how long will the DME last? - Guess it depends how good your transistor is.
Tony

 

Normally resistance is the real value of impedance, which is a complex number and a function of frequency (rise-time), so essentially resistance is measured as a DC value. Since the injector presents an inductive load, it draws quite a bit more current than Ohms Law (at DC) would suggest, similar to starting an electric motor (for example a fan).

Adding ballast resistors to low impedance injectors will protect the driving circuit, but also provide less current to the injectors, than they are designed to operate under.

Sorry to “muddy up the waters” with facts, but transient responses of inductive loads require a little more complex analysis, than I have immediate access to.

Hosrom is right, Tony G. did drop in his 72 lb Siemens injectors and since he had no problems on reliability (or idle) I, for simplicity reasons, followed suit and am happy to report no problems (yet).

Laust

 

Me 2! Running 72lbs Siemens without resistors. No problem yet! Time will show if we get any problem I quess Is there any way we could check if its to hot in there?

 

Thanks for the input
Had a feeling that impedance was a bit more complicated, my 50ohm RF cables show 0ohms when you measure them with a multimeter.

Could it be the standard injectors require more energy to fire than the lower impedence ones so they don't mind losing a bit to the resistors?

This does answer another question I had forming - why not stick a 3ohm resistor and halve the current again? - cos the injectors wont get enough current to drive them.

Tony