Rusty 13B

Hi all,

I am 80% through my rebuild and have a query re injectors.

I have done some research and I am planning to keep the stock 550cc in the stock location.

I also want to run 4 secondary injectors in my xcessive LIM. So my question is:

1. Can the PFC run 4 high impedance secondary injectors?

2. if not - could it run 4 low impedance secondary injectors?

3. If low impedance is ok - does there need to be any 'additions' (resistors?) to run these.

Thanks in advance for your help.

Rusty

BLACK MAMBA

iTrader: (58)

You will have to splice the secondary injector harness and add 1 more injector pigtail on each secondary, I would recommend running high impedance to avoid using resistors.

Rusty 13B

Thanks - my questions really centres around the 'load' (ohms) that the power FC can handle. Wiring changes are easy!

Example - Siemens inj

High imp - 12 ohms X 4 = 48 ohm 'load' on the ecu secondary circuit

Low imp - 2.3 ohms x 4 = 9.2 ohm 'load' on the ecu secondary circuit.

So - that my math at this point - but I am not sure if its right. Basically I don't want to 'fry' the power fc ecu!

Cheers - Jeremy

cewrx7r1

iTrader: (2)

Use the Holley (previously FJO) driver.
For your reading:

The PFC And Secondary Low Impedance Injectors


There have been discussions on the big forum 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 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 secondary 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 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.
who wants to experiment? Using the FJO driver for the secondary solves the PFC load problem and maximizes secondary injector fuel delivery.