asianspeed

I'm looking to use an afc & either 390/370cc injectors to control my fuel, until I can save more money. I'm looking to get the 1st gen s-afc, which is analog and only controls +/- 30%. I've heard of people running with 450cc injectors and using the afc to take out -40% of fuel, leaving them with 270cc being injected. And then at a higher rpm, take out -35% of fuel, leaving them with 292.5cc's. This is all settings to boost about 10 psi.

Now this is my theory:
I am only going to be running about 6-7 psi, so I wouldn't need that much fuel for 10 psi. Hence the smaller injectors. Because I'm using smaller injectors, I wouldn't need to take out as much of a percentage. Infact, if I were to get 390cc to get around 270ccs flowing, I would need to take out 120cc's, which is 30%. Then at higher rpms, I would need to take out less, about 100cc's, which would be about 27%. Now these numbers are within the -30% range. I think this gives me 2 benefits: 1) Smaller injectors = easier to idle, 2) Cheaper!!

So that is my quest for temperary fuel management at a economical cost. Does this sound feasible?

filetofit

The -35% and -40% that you are referring to is not subtracted from the injector flow rate. What the AFC hack does is reduce the MAP sensor voltage that the ECU sees. The maximum voltage the MAP sensor can put out is 5 volts. Reducing that by 40% puts you at 3 volts, which is the upper limit of voltage that the ECU can read from the MAP sensor before it trips a CEL. When running the hack, at full boost, your ECU will be maxing out whatever injectors you have in there, it doesn't know that you have 240cc or 1000cc injectors in your fuel rail. At full boost with the hack the injectors will be running at whatever the max duty cycle is in the stock ECU, roughly 80% duty cycle.

Idleing problems with larger injectors can easily be solved by adding back more fuel at idle. This is because larger injectors take longer to open so for short duration fuel pulses they flow less fuel than what you would expect based on injector size alone.

If you don't want to buy an AFC, I designed a custom circuitboard to control 450cc injectors with boost for $20. IM me if interested. It works quite well, I run 9 psi of boost and use 450cc injectors, it can be tuned for whatever size injectors and or boost you want to run. See my website for the circuit schematic:

http://www.angelfire.com/ab7/diymaplimiter

liam821

Your correct for the most part. The smaller injectors you wont have to give such a large negative valve. Althought the part about the smaller injectors are cheaper im not so sure. I bought my 450CC for $50 off a 99 eclipse GSX...it doesnt get much cheaper then that. Personally i would get the larger injectors and still boost only 6-7.

liam

liam821

Im curious to know what your little circuit board does. Does it just lower the map voltage, ie like a afc set at -40% ? Or does it just limit your map sensor from reading past x volt?

liam

filetofit

I can't IM you, your IM is disabled. What's your e-mail?

fastludeh22

iTrader: (1)

i think we all would likwe to know about this lottle board. why dont you post what it does in here?

asianspeed

The part that I meant cheaper is the cost for the afc. I just saw the 1gen afc go for $102.50 on ebay.

1.8T_EG

... i agree with 93ludevtec... please share... i think others (myself included) would be interested in knowing how this circuit board works too... thanks...

filetofit

The circuit board does the same thing as the VAFC hack. It reduces the MAP sensor voltage that the ECU "sees" so that you can run boost with 450cc injectors.

More technically speaking it applies a linear correction to the MAP sensor output of the form

f(x) = mx+b

where x is the unaltered MAP sensor voltage, f(x) is the altered MAP sensor voltage and m and b are constants that are determined by the amplification characteristics of the op amps and potentiometers that make up the circuit.

I run it with 450cc injectors and 9 psi boost on a turbo LS/Vtec. Works well, totally adjustable to run whatever MAP correction you want. Exact same principle as running the VAFC hack.

1.8T_EG

... you said you are selling these things for $20 shipped?... can they be made to custom specs?... for example 1-4k= -40%, 4.5k= -38%, 5k= -36%, 5.5-7k= -35%... lemme know... i might be interested...