what size resistor for 1600cc & PFC?
 

i posted this in the pfc forum also, i jsut need a quick answer..
i got a set of the bosh injectors and wanted to know what size resistor to use with the PFC?
and also do i put one in each wire to the injectors?
thanks.so, i jsut need a quick answer..

Get 2 10 ohm, 20 watt resistors. You can just put one on each positive lead.

Another option would be to find a resistor pack from an 86- early 87 2nd gen RX-7.

G-dam. 20 Watt would be huge...

I'm running 9 Ohm .5 watt resistors with no problems. put them in the power wires (the thick wire I think it's black with a white or yellow strip if I remember right) You will also need injector connectors for the 1600cc injectors.. I bought a HKS AIC 2 injector harness for $20 and cut the connectors off it...

The reason I went with the 9 ohm is because I took the stock injector Ohm reading - the 1600 cc injector Ohm reading and it came out to 9.1 Ohm's

ok, you need 10 ohm - 10 watt resistors ... do the math.

14V system and old system used 15 ohm injectors, so approximately 1 amp of current 14V/15ohm. New system used 5 ohm injectors and you use 10 ohm resistors to simulate the same load, so 5 ohm + 10 ohm = 15 ohm. Now you have approximately the same 1 amp running through the injector and the resistor, so (1 amp)^2 * 10 ohm is 10 watts. I'm sure Dragon's setup is fine with 9 watt resistor because we approximated the current, but going with an extra watt is extra protection ... plus 10 watt is probably easier to find!

dragon is using 9 ohm- .5 watt
i have 10 ohm .5 watt, i can't find 10 ohm-10 watt resistors anywhere(radio shack only go's up to .5 watt)

Hmm hold up a sec...

I just got a set of 1600 cc bosch injectors and am planning on installing them soon (powerfc in the car already). I didn't know that the resistance was different!

What happens if you don't install the resistors? Will the injectors wear out or something? Too much juice?

Brian

You definitely need the resistors. One time I gave my friend some 720's to use as secondaries, and I SPECIFICALLY told him to use resistors. A shop told him he didn't need them, and so he didn't use them. The car ran like total crap. after he got the resistors it was ok.

I dont know if this has been brought up already, but I just wanted to make sure I got this straight:

1) Does The Apexi Power FC only work with low impedance injectors or does it work with only high impedance injectors? or does it work with both?

2) what impedance are the stock injectors?

I keep getting them mixed up but let me see if I got this right.

The FD has a "saturated" or high impedance injector driver...so the stock injectors are also high impedance. IF you use "peak-and-hold" or low impedance injectors, you will need to wire a resistor box or resistors between the low impedance injector and the high impedance injector driver (ECU).

was that how it goes? or am I mixing up the injector types? also is this the "problem" people encounter with the PFC, the fact that its wont handle the type injectors UNLESS you wire in some resistors?

sorry for all the questions :o:

You're correct. The stock injectors are high impedance injectors.

If you run low impedance injectors in a high impedance system such as the FD and FD PFC you will burn out the injector driver in the computer. That ='s BAD. If you were running High impedance injectors in a low impedance system you'd be fine...

My stock injectors measured around 14.3 and 14.5 Ohm's

Also I made a mistake in my above post:
Me, IGY and FDOkinawa are all running the 9 Ohm 1 watt resistors and have had no problems at all with them in the last 10 month's so I would assume that there will never be a problem running them...

this might sound like a stupid question, but how much "current" is supposed to be flowing between the injectors and the drivers?

VOLT = force
AMP = current
OHM = resistance

so

VOLT - OHM = AMP

I just recently learned a little about the whole Ohms law, but what I cant figure out is, what's the VOLTAGE for the PFC system?

so the reason the high impedance driver ECU burns out is because there is too much CURRENT going to it right? that CURRENT is either resisted by (1)resistor (2)resistor box (3)high resistance (hi-OHM) injectors.

I know this stuff might seem pretty basic to you guys but to me I am just getting the hang of it. :)

Ludeowner,

I think you got that wrong or was just a mistype
V=I*R

so V/R=I

Just to clear things up

I don't know how it is on an FD, but I had to do that on my TII. No matter what wattage resistors I used, it ran like shit.

FWIW the Mazda resistors are only like 7 ohms.

I would have figured they were 10-12 ohms. I'll check my friend's when I see his car.

...and soon to be me too Chris...:p:

opps! thanks for clearing that up, I double checked and you were right.

here is another "silly" question, can you request a specific resistance injector? not high or low, but rather a high impedance 14 OHM injector or a high impedance 16 OHM injector? I dont know if it's necessary but I just thought I'd ask.

EDIT: the reason I asked, is cause I want to know what resistance (in OHM's) injectors to get IF i get 1600 cc's. I'm still unsure if I will need 1600cc or bore out the stock secondaries to 1300cc (400 WHP goal).
I'd rather stick with bored out secondaries if at all possible.

- Apexi PFC
- HKS twin power ignition
- stock 550 primaries and ??? secondaries

Nope



The 1600's only come in low resistance so you will have to use resistors to use them...



You should be ok running 4 850cc injectors up to 400 hp, that would be the cheapest way to do it..

hmm...that's a new suggestion.

I've never heard of anyone suggesting changing ALL the injectors to 850cc (only 2 injectors per rotor chamber right?). It seems like a lot of people stick with the stock 550 primary and just upgrade the secondaries to either 1300 or 1600.

BTW will this allow me to keep the stock fuel rail? I'm guessing the answer is no...:(

TIA

Actually yes you can use the stock rail... pull out the 550 primarys and then gind off about 1/16th of and inch of metal off the top part of the fuel rail (the part where the hold down caps mount), you need to keep it as even and smooth as possible. This will allow the bottom O-ring of the shorter 850cc injectors to seal. Then stick in a set of stock 850's and put the hold down caps on.. Myself and 4 other people I know have all done this mod and it works great and weve had no problems with fuel leaking or anything else for that matter... I'm running 2 x 850cc and 2 x 1600's myself...

neat idea! one thing though, I'm guessing that after I modify the fuel rail by grinding of that 1/16 in, then I wont be able to back to using the stock primary 550's? (in case I ever have to convert to EVERYTHING to stock)

TIA

I never tried, but you should be able to use a small washer as a spacer between the fuel rail and the injector hold down caps if you need to put the 550's back in with no problem...

Chris

OK...

I'm looking at some Bosch 1600s I got from k2rd. They have a brown top and they are about 2.5 ohm.

So, I wire in some 10 ohm, 10 watt resistors and I'm fine right? Oh wait... that will lower the voltage across the pins of the injector. Does this matter? How much voltage do they need?

Anyone know where to find Bosch's actual tech specs?

Thanks,
Brian

Decided on this...
 

Well here's what I'm doin...

I figure the stock setup draws approx 1 Amp of current because:

I = V/R

so

I = 14/14 (approx values of 14 V for car power, and 14 Ohm injector resistance)
I = 1 Amp

If I now put in a 10 Ohm resistor with the new Bosch 1600s, we can get about the same overall resistance as the stock setup... so the current for the circuit should still be about 1 amp.

Now the power levels are something to consider here...

P = IV

P = (1)(14) (again, using our approx values)
P = 14 Watts

By my calculations, to be SAFE, you had better be able to dissipate about 14 Watts of power. Probably since the injectors aren't on 100% of the time, you can get away with a lower wattage resistor... this is likely why people here are using lower values and seem OK... but why risk it?

So what I did is bought four 10 Ohm, 10 Watt resistors at Radio Shack PER INJECTOR. This is a good resistor to pick because it's dirt common. I will wire them up as follows:

For each injector ..

connect two of them in series and the other two also in series.. then wire the two new bits together in parallel...

(not sure if the picture below will come out OK)

__////___////___
------< >-------
\_////___////___/

This should give us the overall effect of a single 10 Ohm, 20 Watt resistor.

I will bundle the groups of 4 for each injector together in a block similar to the resistor packs on an early 87 Turbo II car. Then I will make a bracket to wrap around them and bolt down to some metal on the car like a heat sink.

Anyhow, that's what I'm going with... if it's overkill, so be it. Only cost me 7 dollars in parts from the store, so that's cheap insurance.

Brian

if you have datalogit to use with your PFC, you dont have to use resistors. you guys can call or email KDR for details. my car is running 1600cc secondaries without resistors on the PFC. its been running this way for about 1500 miles now.