Sharado

S4 NA 13B, with a custom turbo running low boost
engine is striped bare, running the minimum
it's a 88 so the ecu runs on high imp injectors
when i had the stock injectors,everything worked like a dream

due to a break-in, rush to make it to a meet, and my planned mods.. etc
i hooked up 10w10ohm resistor on the power wire of each injector
and now running low imp 550 primary and 720 secondary
i was able to start the car once via turning the key
but that never happened again

now, the car runs just fine after you push start it
a tad rich, but that can be tunned out using the safc
however, the car can no longer be started via turning the key

anyone has an idea how to solve this?
a car that can only be push started is no good...

Go48

iTrader: (7)

Tell us a little more. What happens when you turn the key to start the car? Does the starter crank the engine or not? If the starter doesn't even turn the engine over, fully charge the battery, check all the ground points and try starting the car.

RETed

Are you still running the stock non-turbo ECU?
Put the stock 460's back into the primaries.


-Ted

Sharado

-starter does crank
-there is spark when cranking
-i am still on the stock na ecu
-putting the stock primaries back on would be last resort
as that takes alot of work to remove manifold and stuffs
and the 550 injectors was suppose to prepare to run more boost
also i would have to worry about the resistors, but i do have low imp 420's sitting around

RETed

Your stock ECU cannot compensate for the larger 550's ON CRANKING.
It's flooding the engine.
An S-AFC or any other fuel computer does not adjust fuel on cranking.
You need a stand-alone EMS to do that properly.
In fact, I'd recommend getting one.


-Ted

Sharado

oh man, that's gay
but thanks for identifing the problem so precisly
i will use big primaries when i get a standalone then

Sharado

Ok, I thought of an alternative solution to this startup issue
please bare with me and correct my calculations if i am wrong
(it's been over 6 years since i last used ohm's law)

first of all, my injector size has been increased by 31% = (550-420)/420
then we calculate the old current using V=iR
(i use just a V and not 12 because the injectors' V might be less than 12)
V = i * 13Ohms --> i = V/13
to decrease the current by 31% we go
i = V/13 x 0.69
armed with this new current, we find the new resistance required using R=V/i
R = V / (V/13 x 0.69) = 18.85Ohms
If the above calculation is correct, that means I can reduce the injector size from the ECU's POV by increasing the total resistance to 18.85 Ohms

Now, how to make it work:
I considered having a switch and parallel resistors
but that would not work for the combined resistance would be too low when the switch is on

on the other hand, resistors in series with a switched bypass should work
with the 10 Ohm that I have now, add in series a 5 Ohm
and then add a switched wire to bypass the 5 Ohm
result: (including the 3 Ohm on the injectors)
18 Ohm when switch is off
13 Ohm when switch is on

I need confirmation that my calculation methods and math operations are in fact correct before I go ahead and put in the circuitary
so, blast away!

powrdby13B

The current shouldn't be changing proportionately to the size of the injector upgrade I don't think. I'm not up on how injectors work for sure, but I would imagine that they use voltage to control how far they open, and the same amount of voltage is completely open no matter what the size of the injector is. If it's not apparent, I'm talking out of my *** but I think it's something worth looking in to.

arghx

iTrader: (3)

uhh why not a turbo II ECU?? you can get them used for $50 or less...

Sharado

one more question
does the variable resistor do anything during cranking?
if so, i can probably adjust that and get the car to start

RETed

No, variable resistor only adjust fuel delivery on idle.
This means it is only in effect when the RPM's are over 250RPM and under 1200RPM.
Since cranking is typically under 250RPM, the variable resistor is ignored.



-Ted

NZConvertible

There are errors in your math, but that doesn't matter because current through the injector is not what determines how much fuel is injected. That's done by varying how long the injector is open for each cycle. Changing that for the cranking maps would require a programmable ECU. Reducing injector current would only slow how quickly the injector opens, which will reduce flow a bit but certainly not by the amount you need.

Sharado

forget it, way too much headack
i reinstalled stock sized primaries
car started on first rotation