Anyone running any different types of resistors for after-market injectors? I have one of the rx7store Ultimate system and would like smaller resistors that i could put some shrink tube over. if not no big deal.

 

people run certain 1680's without resistors at all actually. what is the resistance across the terminals of your injectors? other option is to use an OEM resistor pack, like the 86-87.5 FC3S resistor pack.

 

You can get them at Radio Shack

 

The one's Ive found at radio shack are ceramic and the wire can very easily break off. I use mil spec resistors from mouser electronics. Just get larger heat shrink.

 

I'm actually in the process of doing my fuel system this weekend and am going with the rx7store.net gold anodized resistors myself. My plan is to put them on the ecu side of the harness and put the wire through the hole and solder it then slide the heat shrink as close as I can.

 

ECU side is fine as well. Doesn't really matter.

 

I am curious who and why you would run a saturated 1680 injector, for that matter I am curious as to who even makes a saturated 1680.

 

it is not "as low" of an ohm as a standard peak and hold injector, thus not having a large enough amp draw on driver to burn them out... its been done that way for years, not uncommon

 

Some of the 1680's actually read more than the usual 2-3 ohms across the terminals, although they are not in the 12-15 ohm range of the stockers. I've seen varying numbers. But there are a couple variations/manufacturers of the 1680s and you would have to check resistance across your 1680s first then calculate how much wattage you would be up to. By these types of calculations, I would say you would want at least 6 or 7 ohm 1680's to run no resistors. I am unsure which particular 1680s have the higher resistance values.

I'll explain and go through a calculation of injector wattage with the stock injectors, and then stock primary's and 1680 secondarys rated at 7 ohms resistance. The original source of the information for these calculations is from page 80 of the Datalogit FC-Edit manual.



The max wattage allowed is 3 watts. First let's go through the easier calculation using the stock injectors first, which we are going to assume all have the same resistance. We have two formulas:

V=I*R (Ohm's law. in this case voltage equals the injector's current multiplied by resistance of the injector itself)
P=I^2*R (Joule's law. In this case, the injector's power/wattage equals the injector's current multiplied by itself, then multiplied by the resistance of the injector driver transistor)


V=I*R .

let's set V to 14V , probably the max you'd get to your PFC on crusty 15 year old wires.

Let's say that all the stock injectors would have 12 ohms of resistance each, which is a safer assumption than the 15 ohms used in that document ^ .

V=I*R ---> 14=I*12 . I=1.17A. so each stock injector needs about 1.17 amps.

Now we take the 1.17 amps and plug it into our power formula.

At 100% injector duty, the wattage is P=I^2*R . We plug in the 1.17amps we just calculated as being needed for each injector. Then for the resistance, we use .25 ohms across the transistor. That .25 ohms is a given value that comes from that document ^ where it talks about "FETs [a type of transistor] rated at .25 ohms"

so P=I^2*R ---> P=(1.17)^2*.25 = .34 watts per injector.

So each of the stock injectors is going to use .34 watts . We multiply by 4 injectors, and we get:

.34 watts * 4 = 1.36 watts @ 100% duty cycle . Most people run up to about 85% duty cycle.

So 1.36 @ 100% duty cycle, multiplied by .85 , gets us the 85% duty cycle wattage of 1.156 watts. The max allowable is 3 watts according to the Datalogit document ^ .


Ok now let's do a calculation at 100% duty cycle for stock primary's and 1680 secondary's with 7 ohms of resistance. We just calculated that each stock injector is .34 watts . So multiply by 2 and we have .68 watts on the primary's.

Now we re-do the calculations for the secondary injectors:

V=I*R ---> 14=I*7 , I=2 . we are using 14 battery volts and 7 ohms across the 1680s . With a little algebra that gets us to 2 amps of current for each 1680.

P=I^2*R ---> P=(2)^2*.25 = 1 watt per 1680 injector @ 100% duty. We plugged in the 2 amps from the previous calculation, then multiplied by the .25 ohms that was given to us from the Datalogit document.

So stock primary's and 1680 secondarys (measuring 7 ohms resistance) at 100% duty gives us:

Front primary: .34 watts
Rear primary: .34 watts
Front secondary: 1 watt
Rear secondary: 1 watt

Total wattage at 100% duty: 2.68 watts . Total wattage at 85% duty: 2.68 * .85 = 2.278 watts . Max allowable is 3 watts.

So running a 550/1680 setup (secondary's measuring 7 ohms each) @ 85% duty would still put you significantly below the max power output of the injector driver circuit. Given the mathematical relationships though as resistance drops across those secondarys the current will shoot up quickly, so I wouldn't run less than 6 ohms. Someone else can post up more details about the different brands of 1680's and what resistance they measure.

 

Or get an FJO injector driver and forget the math, resistors or anything else for that matter other than some wiring.

As I and several others will tell you, it's the proper way to toss in some aftermarket (read: low impedance) injectors.

 

Agreed, thats what I did. Any other way you're just not using the injectors properly.

 

if you've got money to burn and you're willing to add another box to the car than yeah, go for it. but Mazda uses 6 ohm resistors on their factory low impedence EFI rotary cars (84-87 1/2). So resistors can't be THAT bad, and I've never had any hesitation or staging problems using an OEM resistor pack. But I am not a fan of the 10 watt 10 ohm radioshack ones.