Mazda Rotary Club

Hi,

Does anybody know the impedance on the primary and secundary injectors on the 3rd gen, its a JDM R1

Regards,

Rogier

Rogier, they are high impedance injectors.

Dan

Thanks

so when fitting low resistance injectors , say 1600cc you have to put resistors on the + wires to the injector

You got it. ;)

Dan

can you fit the standard 850 secundaries as primaries

I have 1600cc secondaries (bought from rx7store.net) and was not told any info about the impedance (even tho\' I did ask them!).  I measured them to be 4.8ohms, and the std 850cc ones are 13.7ohms. I\'ve had a few problems getting the 1600\'s to stage correctly (I\'m running a Microtech LT8).
Would I be damaging the LT8 or injectors by running them without a resistor?

Adrian

I found Rx7 difficult to deal with to be honest.

I dont buy from them anymore.

I have just bought  1600cc injectors that are low resistance  4.6 Ohms.  If I fit a 10 Ohm resistor in line too bring the resistance up too the standard ones there will be a problem.

As the resistor is two thirds bigger than the injector  it will take two thirds off the voltage  say  8 Volts  leaving only 4 volts too switch the injector on !!.

Many people have fitted these injectors before  can you tell me what size resistor was fitted and any issues.

Thanks  crispyj.

Ps  I am running a Apexi PFC

Quote from: crispyjAs the resistor is two thirds bigger than the injector  it will take two thirds off the voltage  say  8 Volts  leaving only 4 volts too switch the injector on !!.

Many people have fitted these injectors before  can you tell me what size resistor was fitted and any issues.

It\'s not the voltage you need to worry about - just the current.  The current passing through the coil inthe injector will need about 1 Amp to fire it.  Since the low impedance ones would run several Amps without the resisitors in series, you risk burning out the injector coils or frying the power transistors in the ECU or piggy back.

You are correct about the resistor taking 2/3 of the voltage, but since you only need to worry about the current, the lower voltage available to the injector is not really an issue.  One thing to keep in mind is the heat generated by the resistor.  If you assume 1 Amp flowing in 10 Ohms - this gives a power dissipation of 10W  (P = I x I x R).  The resistor needs to be big enough to handle the current, and needs some airflow around it to get rid of the heat.  The old FC resistors were about 7-8 Ohms.  RS do some nice Power resistors with heatsink bodies, and these have mounting lugs as well.

Hope this helps...

Thanks for the response.

I have ordered some 10 Ohm  15 W  wirewound resistors.  I am hoping that due too the fact the secondaries only operate at higher revs that the resistors wont get too hot.

15 Watt will be ok

Besides they will only be getting hot when the duty cycle is right up  - so unless you drive everywhere on WOT, they should be plenty cool  ;)

Just a thought, will wirewound resistors cause problems due to their larger \'inductance\' (eg. 5mH) than just normal ceramic resistors (ie. low inductance)?

Could each injector coil see less current (or even suffer from EM interference) due to current being induced in the wirewound resistor (if this significant)?

Also, do you really need that much power handling because the injectors are only open part (not all) of the time. Even at injector duty cycle of 70% say, would the heat dissipated be equivalent to 70% of the max current (eg. 1.1Amps)? So if W=I2 x R , then if R=14ohms and I=1.1amps, W=1.1 x 1.1 x 14 = 16.9watts, then 70% of 16.9watts = 11.9watts ...

With that low a resistance, the inductance is going to be negligable - probably in the uH range.  But it\'s a good point.  You\'ll probably find the coil in the injector and the wiring loom offer more of an inductive load !

It\'s always a good idea to over rate the power resistors in any case, as they will maintain their resistance and stability over their lifetime much better.

The \'Ceramic\' resistors are normally either wirewound resistors or metal film resistors in a ceramic mount.  The ones to avoid are Carbon - as these will not like the constant temperature variation and their resistance varies more over the operating temperature range.

By far the best are the ceramic sealed wirewound resistors in the heatsink packages - they are very reliable in harsh envrronments.

As to the interference, it\'s more likely that the injector circuits will radiate out into the ICE in the car, rather than be affected by EM radiation, as the circuit impedances are so low.  - Just keep the resistors away from any unsheilded signal wiring.

Cheers,

Prof  :cheers

can someone explain what the advantages/disadvantages to in the different impedance injectors? it\'s a recent curiosity of mine. i figured higher impedance was better because Mazda changed from low to high - mid 1987 (i think). however, my simple thinking can\'t see the advantage if they are prone to getting hotter.

can someone clear it up for me, please?

I used these (http://tinyurl.com/zke7r) when I did my 1600cc upgrade.

You can get bigger non-metal cased, but I thought they looked better.

Fish

Diabolical1-

It\'s a while since I studied this, but from memory:

High impedance injectors (around 16 ohms) are easy to drive, since even fed directly across the battery voltage, the current is not too high. The disadvantage is that the low current causes extra delay in opening the injector (lag) especially at low battery voltages.

Low impedance injectors (around 4 ohms) would draw excessive current if fed directly from the battery voltage. If used with an ECU having appropriate peak and hold injector drivers, the ECU can allow the injector to draw a high initial current, thus opening quickly, and then regulate the current to a sustainable hold value for the rest of the pulse. When done correctly, this maximises the open period for a given pulse width, and reduces sensitivity to battery voltage.

Using low impedance injectors with resistors of course does not give the fast opening times that might otherwise be available. However, some manufacturers have used this method in the past.

There were at some time injector driver boxes that you could drive with your ECU to overcome this. I haven\'t looked around for a few years, but they may still be available if you are really interested. I think that I would try the resistors first.

Keith

thanks, kbareham! :) things make more sense to me now.

I have spoken too Re-Worx about the 1600cc injectors and he has told me that they have fitted lots off these without resistors and they have not had any issues with the PFC.

So i dont know if to fit the 10 Ohm resistors or not.  If I dont fit them and it melts the ecu will the car run lean and melt a tip or  if I do fit them will it make setup for the guys at Re-worx more difficult as a resistor may slow the injector opening times.   :rolleyes:

I would find out what the power FC can tolerate current wise.  You might find you can get away without resistors, because for the most part the duty cycle on the secondaries is going to be low.

For example, if you had 4 ohm injectors, then across 12 volts they would draw 3 Amps.  If the duty cycle is only 10%, then the \'effective\' current drawn (for the purpose of working out heat dissipation and so on) is only 0.3 Amps.

The main things to watch for are the maximum allowable current through the injector, and the maximum current that the ecu can sink (injector current is drawn to ground through the transistors in the ECU).  Provided you don\'t go outside these limits, you should be ok.  :)

I\'m guessing that you wont be sitting at WOT for long extended periods  :Nono!

Best off asking Re-Worx what they think.

Some of the threads on the RX7Club forums, people have had issues with the PFC and not running resistors on the 1600\'s.

Fish

We have fitted quite a few of these kits with never a problem as well, but can only comment on carts with a PFC.

Hope this helps

We have fitted quite a few of these kits without resistors with never a problem as well, but can only comment on cars with a PFC.

Hope this helps

Re-worx said it would be OK without resistors.  

Fish just for reference can you tell me the size of the resistors you have fitted on your 1600 injectors and who did the tune/setup.

Thanks crispyj

I think they are the 10W 10 Ohm type. I got the details from a thread on the RX7Club forums.

Clicky to the KGParts fitting site (http://www.3rdgenrx7.com/keithshowto.htm)

Fish