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Discussion Starter · #1 ·
Reading around it seems that of you're running stock injectors you need a 12:! ratio to safely run a few PSI of boost. My question for you guys is what are your rail pressures at boost? I really have no way to measure mine (as the guage is on the rail). I hooked up a steady 4-5psi air tank on the FMU when it was idling and the rail went to 60PSI. My baseline FPR sets it to 19 PSI under 0 boost. To me this means I must have about a 10:1 ratio on my FMU.

I'm running rich as I threw a CEL with RICH as the trouble code. This only triggered with I was boosting up a hill constanly for a minute or so. It seems that it only throws the CEL when it's rich for an extended time.

I an going to lower my ratio but what would you guys reccomend, my wastegate is set at about 6.5psi, so I'm not running a bunch of boost.

A side question. Could slight pinging at 6psi be due to running too rich?

Again I'm running 24# mustang injectors.

Thanks,
Tim
 

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12:1 ratio is a bit much for larger than stock injectors. 60psi @ 6 psi sounds about right.
Being a little rich is OK, you might loose a bit of power, but it's safe. You should'nt be pining at 6psi, as long as your intercooled, runing 92-93 octane gas, colder spark plugs, and have the IAT sensor reading the air after the turbo some where.

What does your A/F gauGe read at WOT? (or what is the O2 voltage at WOT?)
 

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pinging is caused by a few things... lean condition, too advanced spark, or intake temp to hot from not having an intercooler. you said your at 19 psi of fuel at 0''hg?what is it at idle?also if your going from 19psi at 0'' to 60psi at 12''hg(6psi)that seams a bit rich for me.

normaly the fuel pressure is around 31 at idle and 42 at 0''hg that means that for 22''hg you use 11psi of fuel.... so that means for every inch of hg you use .5psi of fuel. and since 1psi equals 2''hg, you should be using 1psi(2 to be safe) fuel for every psi of boost. but this is all just theoretical......
 

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Discussion Starter · #4 ·
Jeff, 12:1 is too rich for 24# injectors, I think I'm about 10:1 now and at WOT the guage is pegged. These A/F LED guages don't seem too consistant. I think there are 10 LEDs and it seems to bounce back and forth between the 2nd one (lean) and the 9th one (rich) when I'm running around not letting the boost kick up. When it is boosting though the meter pegs.

If the FMU was set ideally would the meter bounce like it does unloaded or would it sit right dead smack in the middle?

I've never adjusted the FMU does turning the knob counter clockwise decrease the ratio and clockwise to increase it?

The pegged meter :lol: is a pretty good indication that it's running very rich at 10:1, again the car was idling and I used an air tank pressurizing the FMU to about 4 or 5psi, the rail went to 60psi but this is static under no load from the engine. My ratio of 10:1 is determined from 19psi idlling regularly and 60psi rail at 4psi (static from a air tank) 60-19 = 41psi additional rail pressure, this =10:1. Running 60psi @ 6psi boost would be 41/6 which is more like 7:1.

I really need to get a guage inside the car so I can see what the rail is doing realtime pressure values. Someone once mentioned running a fuel line into the car with a guage. I think this would be very unsafe and I'm not willing to do that. Are there guages that send a electronic signal from the rail? I'll do a little searching.
 

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tokstad said:
Jeff, 12:1 is too rich for 24# injectors, I think I'm about 10:1 now and at WOT the guage is pegged. These A/F LED guages don't seem too consistant. I think there are 10 LEDs and it seems to bounce back and forth between the 2nd one (lean) and the 9th one (rich) when I'm running around not letting the boost kick up. When it is boosting though the meter pegs.

If the FMU was set ideally would the meter bounce like it does unloaded or would it sit right dead smack in the middle?

I've never adjusted the FMU does turning the knob counter clockwise decrease the ratio and clockwise to increase it?

The pegged meter :lol: is a pretty good indication that it's running very rich at 10:1, again the car was idling and I used an air tank pressurizing the FMU to about 4 or 5psi, the rail went to 60psi but this is static under no load from the engine. My ratio of 10:1 is determined from 19psi idlling regularly and 60psi rail at 4psi (static from a air tank) 60-19 = 41psi additional rail pressure, this =10:1. Running 60psi @ 6psi boost would be 41/6 which is more like 7:1.

I really need to get a guage inside the car so I can see what the rail is doing realtime pressure values. Someone once mentioned running a fuel line into the car with a guage. I think this would be very unsafe and I'm not willing to do that. Are there guages that send a electronic signal from the rail? I'll do a little searching.
The A/F bounce is normal when just crusing. Thats a good sign, it's just closed loop searching and staying in the 14:1 A/F ratio for good gasmilage and power.
Keep in mind, narrow band O2 sensor's do a good job at the 14:1 A/F range, but cant read the 12-10:1 much at all. It would be like trying to measure a 12 volt battery with a volt meter that only goes to 11 volts. That is why the car's ECU completely ignors the O2 sensor when your at WOT. 14:1 is safe when just cruzing around, but dangerious at WOT or with any boost. So you want the A/F gauge pegged (or very close to pegged) at WOT.
Yes they make electronic senders for fuel presssure with a gauge, but they cost more then what most of us have spent on our turbo's.
 

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the narrow band does a good job ov giving an mean temp reading. the o2 location can have an affect on the reading also. mines located about 2 or 3 inches into the down pipe, and it keeps a relativly tight ratio pattern.
 

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Discussion Starter · #7 ·
Keep in mind, narrow band O2 sensor's do a good job at the 14:1 A/F range, but cant read the 12-10:1 much at all. It would be like trying to measure a 12 volt battery with a volt meter that only goes to 11 volts. That is why the car's ECU completely ignors the O2 sensor when your at WOT. 14:1 is safe when just cruzing around, but dangerious at WOT or with any boost.
So under boost we want to lower the A/F ratio so there is more fuel (running rich). I had assumed that all the additional air being compressed into the intake was mixing with all the additional fuel from the FMU to generate about a stoic condition of 14.7:1 ratio and that you always wanted to stay in that range for optimum performace.

You are saying that while boosted a stoic 14.7/1 ratio is too lean and that we actually want a 12:1 ratio. With that as the case then I will likely never get it to run boosted without throwing a CEL unless I'm currently running something like 8:1 under boost and the ECU can tolerate a range that would be acceptable for boost say 10:1 -> 15:1

(Does anyone know the specific condition that the ECU flags a "rich" trouble code to the CEL). Is is possible to stay out of the rich condition to the ECU and have the right fuel ratios across boosted and non boosted conditions?

As I understand the A/F LED meter measures a swing voltage from 0-1V. So what does this voltage represent, I can't see how oxygen content is translated into voltage?

Anyone know of any good online reading about A/F ratios and tuning?
 

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Jeff didn't you run a hose out the back of your hood and tape a gauge to the windsheild while you were tuning your car? I remember someone doing that. It's ghetto but it works for tuning and after you get it set up the gauge isn't absolutely necessary anymore.
 

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Discussion Starter · #9 ·
Reading a little from a dsm site http://www.dsm.org/archives/1999/01/19990104.txt/18.html

It seems that O2 sensors are in essence a switch. That is set for around 14.7/1. I think that the ECU under conditions other than WOT checks to see if the voltage is switching between 1V (Rich) and 0v Lean. Normaly if the car is running right it will switch between these values.

I'm assuming since I wasn't WOT and I was boosting for a minute or so up a hill the voltage was staying at a consistant high voltage. The ECU sensed this and tripped the CEL.

Since everywhere I've read it says that you shouldn't run more than 12:1 when boosted I'm assuming that even lowering my FMU ratio to 8:1 from 10:1 won't help the CEL issue as it's likely still going to be in the range of being rich.... Any thoughts?

It is not clear from anything I read that all O2 sensors
have the exact same characteristics (other than switching at 14.7:1)
or even that non-switching characteristics are controlled between
batch runs at the factory.

First, it must be stated: The O2 sensor on almost every production
vehicle today would never be any engineer's first choice to measure
A/F ratio. Or second choice. Or third choice. It would probably
fall behind the fourth choice - the engineer's nose.

The first choice would be the O2 sensor known to every dyno tuner as
the Horiba (they make the most popular one). It is a true instrument
that is calibrated and repeatable.

The O2 sensor included with almost every car is meant to act like a
switch. What this means is that the sensor is practically either
at one voltage or another (say, 0.2V and 0.8V). There is an extremely
steep-sloped area in the middle where a large change in voltage means
a tiny change in A/F ratio.

Now certainly, this wasn't the car manufacturers' first choice. Had
they had it their way, I'm certain that we'd all be running around on
nice wide-band sensors, and no one would ever know the name Horiba.
But like most things, the best way was also the most expensive - very
expensive. Like 10-20 times more expensive than cheap switch method.

It is very important to remember that this sensor is really only intended
to give an on-off, rich-lean indication. It was never made for exact
A/F measurements. In fact, Mitsubishi at no time relies on the O2
sensor for anything other than a rich-lean indication. The ECU is not
looking at the O2 sensor under WOT, making sure that mixture stays
rich enough, which would make good sense if it were accurate.

Let's list the various problems that get in the way of accurate A/F
measurements with O2 sensors.

1) Hysteresis. You may have seen graphs of A/F ratio vs O2 sensor
voltage. You basically see a nice S-type curve with a steep
sloped sensor. Well, that center is really just an average.
You see, when the O2 sensor goes from lean to rich, it snaps
almost instantaneously from 0.2V to 0.8V right when the A/F
ratio crosses 14.7:1. Basically infinite slope. However, when
the O2 sensor goes from rich to lean, it does so more slowly, so
you can actually see the descent. That basically means you need
a separate conversion table or chart when reading off A/F from
the voltages whether the car was going to or coming from a rich or
lean condition.

2) Temperature. We all know that O2 sensors don't work at all until
they are warmed up by the exhaust to a nice toasty 300C. But,
did you know that the O2 sensor voltage output also drifts with
temperature, often significantly? The voltage for 14.0:1 (slightly
rich) can drift from nearly 1.0V at 350C to 0.8V at 800C. The
voltage for 15.5:1 (slightly lean) only drifts from 0.05V to 0.07V
in the same temperature range. This is probably the worst problem.

3) Frequency response. The speed at which the O2 sensor reacts also
changes with temperature. At 350C, it takes 0.2sec to switch from
full lean to full rich, and vice versa. At 800C, it takes 0.1sec
to switch.

4) Voltage offsets. The low output voltage of O2 sensors makes minor
voltage offsets from mismatched grounds into potentially big headaches.
 
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