3 Ohms Resistors Sold @ Rx7store.net
 

Are the resistors sold on Rx7store.net too low of a resistance for the PFC injector drivers? It is my understanding that if the drivers see more than 2 amps, it will fry the circuits?


I have been having issues with ignition break up @ 3000 rpms as well as my wide band showing that the air fuel ratio is off the charts lean 19+ when crusing at any speed above 2000 rpms

This is what is stamped onto the resistors: HEI TMC - 25 3 Ohms 1%

Link is as follows:
http://www.rx7store.net/product_p/in...20resistor.htm

So my question is... could running these resistors with the Bosch 1680 injectors have fried my secondary injector drivers?

measure the resistance of your injectors. i think my bosch injectors were 5.1 ohms and i'm running 4 ohm resistors cause thats what i had, but 3 ohm should be fine if your injectors measure out the same as mine. the way to do the math is:

battery voltage / inj ohm + resistor ohm = AMPS
SO
12-14 volts / 5.1 +3ohms = 1.48 - 1.73

So even at peak battery voltage the 3ohm injectors are plenty safe with 5.1 ohm injectors.

Some brand injectors are around 2 ohm though which would put it at 2.8 amps with 3ohm resistors.

Unlikely. i bet if you datalogged injector duty cycle whenever the problem occured you will find that the secondary's aren't even on trying to come online. It just sounds like a bad tune or some other unrelated mechanical/electrical problem.

There have been discussions on how much power the PFC injector drivers can safely handle. Some say the low impedance Bosch injectors don't need resistors. Some say they do; 3 or 5 or 10 ohms. I need to go back and reread it.

I was reading pages 80 and 81 of the Datalogit manual. It states that the PFC injector driver's IC contains 4 FETS (field-affect transistors). Each has an impedance of .25 ohms, and that the IC has a total power dissipation safe level of 3 watts. If power is not accumulative, then it will be .75 watts per driver/FET.

Let's do some calculations. I will use a voltage of 14.3 VDC as that is the highest I have seen on any FD. The stock injectors are listed as 13.8 ohms. My Bosch 1600/1680 with the light gray top are 5.1 ohms.

Stock injectors first:
(14.3 / 13.8) * 4 * .25 = 1.04 watts. 1.04 / 4 = .26 The total wattage of 1.04 is way less than 3 watts, and the individual wattage of .26 is way less than .75.

Stock leading and Bosch 1600 secondaries with 3 ohm resistors:
(14.3 / 13.8) * 2 * .25 = .52 watts
(14.3 / 8.1) * 2 * .25 = .88 watts
Total wattage = 1.4 watts. Less than 1/2 of the 3 watt maximum.
Secondary individual wattage = .44 is less than the individual max of .75 watts.
We are safe by both total and individual wattage.

If we do not use the resistors:
(14.3 / 13.8) * 2 * .25 = .52 watts
(14.3 / 5.1) * 2 * .25 = 1.40 watts
The 1.92 watts total is safely less than 3 watts.
The .7 watts per secondary is close to the max of .75.
I would not use this combination as the individual wattage is too close unless it can be verified that total wattage is the main factor.

BUT; if we use 1 ohm resistors we have 1.69 total or .59 per 1600 individual.
1.69 total is less than 3 and .59 is less than .75.

2 or 1 ohms should be safe and give better response than 3 ohms.
Or just get the FJO driver and have safety and more flow.

If the injectors react sooner, what benefit is there? Does it just allow the fuel maps to be smoother especially in transition? does it help with smoother cruise or lower duty cycles? I'm just curious if the benefits are functional or just cosmetic.

It reduces duty cycle. In reality, before they were running as if they were smaller.

I understand. Because they open sooner they will flow more fuel during a given cycle which lowers the overall duty cycle. So there isn't a huge benefit to lower resistance unless your running out of fuel due to high duty cycles. I suppose the fuel maps will have less of a hump during transition as well.

Since we can tune the transition there is no issue with transition regardless of which resistor or injector driver we use on the bosch injectors.

i'm just saying visually on the map with slower reacting secondaries you would have to add more fuel right at transition and your map will have a large hill. Its no big deal, but it bothers some people.

The only transistion problem is caused by the PFC's very bad stock transistion values for % DC and initial ms. I have posted about this crap before when I first logged it and showed how to overcome it.

My tuning notes package shows how to best correct it.

Chuck as usual is spot on to what I was talking about ... Chuck, the man, the myth, the lecher