Any easy trick to measuring A/F ratio?

Like most I've finished a stroker build on a NON-HO 4.0L. I have accell 24 lb injectors and an adjustable MAP. I want to be completly sure that my engine isn't running lean at any times as I don't want to cook my nice new motor

Anyway, is there any cheap way of measuring A/F ratios or is it a special machine?

Thanks

You should be able to splice the sensing wire from this gauge right into your O2 sensor wire:
Air/Fuel Ratio Gauge

If you don't want to splice into your O2 wire, you'll need this then:
Air/Fuel ratio kit w/ O2 sensor

You can't splice into your existing o2 because the ECU is using the information to reach stoichiometric. If you read the a/f ratio in parallel, it will read good. You have to unhook the o2 so the engine is running open loop to see what the engine is really doing.

The cheapest and easiest would be use a volt meter and run the engine fully warmed up without the o2 hooked up (electrically) and test the voltage coming from the o2 to determine if you are rich or lean.

If this sounds over your head, the second option would be to get an a/f ratio gauge like Greg said (consists of an o2 sensor and a box that reads what the a/f ratio is). You'd remove your o2 sensor from the vehicle and run the engine (fully warm) with this other o2 hooked up (nothing hooked to the ECU). This will keep the engine in open loop like it does the first few minutes of warm up each time you start the engine. With this gauge, you can see on the readout what a/f ratio you are running. If it's close to stoichiometric (14.7:1), then leave it. The o2 can make up for quite a bit, but if it's way off, you can get bigger or smaller fuel injectors, and/or adjust your map to get it good.

Steve

I do all my own wiring, repair, engine building etc so splicing a few wires is nothing.

What is the voltage output of the 02 sensor at a A/F ratio of 14.7:1?

Bender said:

I do all my own wiring, repair, engine building etc so splicing a few wires is nothing.

What is the voltage output of the 02 sensor at a A/F ratio of 14.7:1?

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.5 volts, but it won't stay there. It should sweep from .2-.8 volts. If I remember correctly, if it stays closer to .2, it's rich, if it stays closer to .8, it's lean. Don't quote me on that, I might be backwards. But, if it seems to sweep properly and evenly, you should be ok. If it hangs close to .2 or .8, there is a problem. Be advised though, that cheap meters do not read in real time. There is a delay. If you have use of an oscilloscope, that's the best, and most accurate way to view what the o2 is doing.

Maybe I'm missing something here, but wouldn't the best way to see what your fuel ratio is under various driving conditions be checking your fuel trims? 02 voltage swings around so much its hard to see what is really going on. But looking at your short and long term fuel trims will tell you what kind of compensations the computer is giving (rich or lean).

what are "fuel trims"?

The main area you want to keep an eye on is under full throttle. It should be around the .8 v reading then during other driving it is going to change. you can tap dircetly of your 02 sensor to a guage or meter with out hurting anything. Unfortunatly I haven't found a resaonable scanner to read the renix system like i can on a obd system witch my scanner will give me direct readings. Th renix system is a very adaptive system unlike the HO so its not much of an issue unless you are realy running to rich and you would know.

Hijack!

I've been wanting to do something similar, but I'd prefer a direct-reading numeric display rather than the dot/bar graph displays that are so common. I've seen a couple "wideband" lambda sensor kits out there, have any of our racing gurus worked with anything like that? Supposedly, they WL sensors are more accurate around stoich and can give a more direct reading...

Regarding computer scanners & Renix - there are no codes, and the Renix setup is pre-OBD. There are two scanners I know of that will work - the Snap-On MT2500 and one by Mac (I don't remember the model number.) New, they go for about $1400 and $2500 respectively. Ouch.

5-90

OK, I guess I come from the old school. I just do a spark plug reading and check the tail pipe color. My Renix 4.7L stroker is running just fine with no adjustments with the Ford 24lb injectors.

Old Man,
That is exactly what I wanted to hear. Do you have an adjustable MAP sensor? I kept my displacement down to 4.6L with a compression in the 9.5 range. Most I know have HO strokers and I wanted to be sure the 24# injectors would be sufficient at WOT on a NON-HO. I'm going to check the O2 reading at WOT anyway for interests sake.

89xj said:

what are "fuel trims"?

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As the computer looks at o2 voltge and notices a tendancy toward being lean or rich it will make changes to the fuel maps to get the mixture closer to stoich. These are the trims. Short term trims are constantly changing while long term trims take a few seconds to a few minutes under certain conditions to be changed. So say if you had unmetered air entering the engine somehow, when the computer looks at the o2 voltage, it will notice that it is unexpectedly lean. In its own mind (of sorts) it will say, "hmm... from now on I need to add a little more fuel at this rpm and load to get us closer to stoich." And it will remember that change as long as the battery isn't disconnected or the computer notices that the change is accurate. I'm not great at explaining things so let me know if that was just rambling.

Israel said:

As the computer looks at o2 voltge and notices a tendancy toward being lean or rich it will make changes to the fuel maps to get the mixture closer to stoich. These are the trims. Short term trims are constantly changing while long term trims take a few seconds to a few minutes under certain conditions to be changed. So say if you had unmetered air entering the engine somehow, when the computer looks at the o2 voltage, it will notice that it is unexpectedly lean. In its own mind (of sorts) it will say, "hmm... from now on I need to add a little more fuel at this rpm and load to get us closer to stoich." And it will remember that change as long as the battery isn't disconnected or the computer notices that the change is accurate. I'm not great at explaining things so let me know if that was just rambling.

Click to expand...

thanks, good info.

The fuel trim basically tells you in which direction the PCM is adjusting the A/F mixture to bring it back to stoichiometric. If the engine has a tendency to run lean, the PCM will add more fuel (positive fuel trim). If it has a tendency to run rich, the PCM will remove fuel (negative fuel trim).
An O2 sensor reading of 0.2v or less indicates a lean A/F ratio, while 0.8v and above indicates full rich. Stoichiometric is 0.4-0.6v. When the PCM is in closed loop, the O2 sensor readings will fluctuate constantly between 0.2-0.8v so using the O2 sensor voltage to determine the A/F ratio for tuning purposes is almost useless. The reason is that it takes only a small deviation from a 14.7:1 A/F ratio to produce a large swing in O2 sensor voltage in either direction. The only way around that problem is to use a wideband O2 sensor and they're not cheap.
The A/F ratio gauges that are commonly sold on the market such as Autometer's produce a nice light show but little else. When the PCM is in closed loop operation, the gauge will swing wildly from lean to rich and back all the time so it won't tell you anything useful other than the fact that the O2 sensor is working properly. Their only real use is to let you know that your engine should be running rich under WOT (critical information for turbo/supercharged applications).

Re: Any easy trick to measuring A/F ratio? NO!

You can get an air fuel guage and tap onto the O2 sensor and the guage will operate. However they don't give exact readings (just red yellow and green bands) and do need a few seconds of throttle to stabilize (going uphill works best - otherwise you run out of road!!!).
If you have a stock renix head (Not ported or big valves) #24 lbs should be about right. I have a ported head with Mopar Performance valves that flows considerably more than stock and am using 26.5 lbs injectors and have an AF ratio of 13.7 on the chassis dyno. 12000 miles 15 MPG around town 19 - 20 on highway trip.