I was talking to my buddy and he said that a digital air/fuel meter(gauge) would not work on a non-computer car. He said that the minute you stepped on the gas it would surge to rich etc.. it does make sense ..oh by the way i'm talking about any of in car gauges found at summit or jegs...is this true, anyone have experience..

thanx rob

 

Most A/F guages can only accurately tell you whether the current A/F Ratio is richer than or leaner than 14.7:1. They don't have enough
resolution. The different leds or lights are not calibrated to a specific A/F ratio. Try reading the output of the O2
sensor with a digital volt meter. Voltage
will raise as it gets richer. Apparantly
.8 to .9 v is a good reading to tune to
on a naturely aspirated car at WOT.
Even with it's limitations, you'll find an
A/F ratio gauge a very usefull tool.
http://photos.yahoo.com/russ_q

 

thanx

 

Like F-BIRD said, the AFR gauge will not be a very useful tuning tool aside from sustained WOT tuning. The problem is that the aftermarket AFR gauges use the factory O2 sensor for it's mixture information. The O2 sensor that is on our cars was never designed to constantly read the air fuel ratio and give an accurate output (that is what a Wide Band O2 sensor does)... It was designed to act like an umpire and tell the engine "rich" or "lean" so that the engine can adjust the fuel trim accordingly. The idea is that the O2 sensor will oscillate back and forth from rich to lean and log what are called 'cross counts' which indicate that the air fuel ratio has gone from rich to lean or vice versa (crossed ~.450 mv). The ECM needs to see a decent amount of cross counts to be happy. If the O2 sensor stays rich or lean too much (ie not enough cross counts), the ECM will flag a DTC for the rich or lean condition that exists (thus the common O2 sensor rich or lean codes when it is actually some other componant that has failed, causing the improper mixture).

So by understanding what the job of the factory O2 sensor is, you can see why the AFR gauges that rely on it for input are not very useful tuning tools at anything but WOT. At WOT the mixture will stay a steady state of rich (if everything is working properly) and you can tune how rich the engine runs at WOT with Fuel pressure regulator settings and EPROM tuning. The idea (way to make the best power) is to run it as lean as posssible without detonating or pre-igniting the mixture....

Hope that all helps a bit
Good Luck...

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1987 GTA L98 MD8
355, TFS Heads, LT4 Hot Cam
My GTA

The Minnesota F-body Club

 

well let me ask you this: would an AFM work on a computer controlled carbed car? i think mine's comp controlled, even though i havent seen the comp. i'd like to get the guage and learn a little about carbs so i could tune it for fuel economy and change it when i want for performance. hook it up to the O2 sensor? any thoughts?

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1984 Z-28, 305HO 4bbl, t-tops, K&N, Flowmaster 80 Series with 3" tips. Planning a massive modfest. It's gonna be one gangsta of a street demon.

 

Matt87GTA ,jeez i think you've got this subject DOWN......yah i cant do any prom tuning seeing i dont have one all i have is a 305 planted in the car no wires nothing its pretty plain...i've got a 78 305 ...but since i'm not buying the guage i can spend that money on better toys....


Rob

 

Yes, if the car is a computer controlled carb engine it will have an O2 sensor and will allow you to run an AFR gauge just like an EFI engine... It will also have the same drawbacks of not being all that accurate since the mixture control solenoid is constantly changing the mixture (to some extent anyways, from what I know about them...). The other unfortunate thing about the carbed system is the extreme rich mixture you get when the throttle is depressed a decent amount and the accelerator pump is actuated..... talk about flushing the toilet with fuel!!!!! The gauge will still help you understand what is going on with the engine much better and will offer some tuning abilities at sustained WOT.... The install of the gauge is the same as a carbed engine - yes, the signal wire will need to be tapped into the O2 sensor wire.

I guess you can say that I have it down... I did go to school for two years to learn about GM-specific cars.... I graduated from a GM sponsered technician program called ASEP.... A very good program if anyone is interested in being a service tech at a dealer...... I liked wrenchin for a while, but it just isn't the right career for me... need more money to mod the GTA

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1987 GTA L98 MD8
355, TFS Heads, LT4 Hot Cam
My GTA

The Minnesota F-body Club

 

umkay, i dont think im understanding this "accurate reading at sustained WOT" deal. would the guage be accurate at idle? if i was racing (knowing only to engage if i've got a good mixture), when would I know if i'm getting a bad mix at WOT? almost immedietly after i hit the throttle? i've read at a site where this guy had twin-turbos on his mustange and he had a simple little box w/ LEDs on it to read the mixture which only cost him $20 or so by using some chip he got somewhere (which i could locate) to read the voltage. seems like a pretty cheap way to get a reading. at least, cheaper than buying a guage designed to read AF readings. looked kinda gangsta. i'm taking an auto-tech class in high school, so i probly could get some help from the teacher on installing it to the O2 sensor. think i should invest in a fuel-pressure guage?

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1984 Z-28, 305HO 4bbl, t-tops, K&N, Flowmaster 80 Series with 3" tips. Planning a massive modfest. It's gonna be one gangsta of a street demon.

 

That's on thing this gauge will do.
It will show if you have moved away from
rich and crossed over to lean. Lean at full throttle needs to be avoided, always.
The gauge should stay rich (richer than 14.7)
the whole time you at at WOT.

 

The stock O2 sensor is a switching design like I was stating earlier. Thus, it will be going from rich to lean and back again very quickly as the computer 'fine tunes' the air fuel ratio. The overall 'mixture' of the engine will be where it needs to be (close to 14.7:1) if everything is running right but the O2 sensor will oscillate back and forth - lean to rich and vice versa - since the ECM will be making those fine adjustments to get it as close to the ideal air fuel ratio (14.7:1) as it can. The sensor will behave like this at part throttle as well (atleast it should but I haven't actually watched one on a computer controlled carbed engine).

But when you go to WOT, the accelerator pump will be dumping huge amounts of fuel into the engine so it responds to all of the added air that is coming into the engine through the opened throttle plates... This sudden rush of fuel will make the air fuel ratio read extremely rich for the first few seconds after hitting WOT. Now if/when you stay at WOT for a decent amount of time, the mixture should level out at a sustained air fuel ratio that is more of a true indication of how rich or lean the engine is running.

That is the point that you can actually get a little bit of useful info from an air fuel ratio gauge. The mixture will not be oscillating back and forth at WOT. It will sit at one ratio (this is all assuming the engine is in good working order) and you want that ratio in the rich area a decent amount (~.850 mv to be safe. You can go leaner, but it can be dangerous....) to avoid having any detonation which can burn and/or destroy pistons due to the extremely high heat of a lean burning cumbustion process.

The reason that the engine doesn't need to run on the rich side when it isn't at WOT has to do with spark timing and engine load and how they increase the stress on the internal engine componants.... ie. when the load on the engine is at it's highest, it is the most sensitive to the air fuel ratio....

Well, I am no trained instructor and it is kinda hard to explain in just words, but I hope that helps a bit....

Good Luck
Matt

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1987 GTA L98 MD8
355, TFS Heads, LT4 Hot Cam
My GTA

The Minnesota F-body Club