I'm kinda new, so help me if you can. What does an air/fuel ratio guage measure? how do you read it? any more info would be grand.. thanks

 

It measures the amount of air and fuel being mixed in your engine. There are three areas on an average guage: Lean, Stoich and Rich.

Lean = more air than fuel
Rich = more fuel than air
Stoich = still have no idea, but I guess since it's in the middle it probably means the ratio of air to fuel is 1:1 or in that general area.

 

so im guessin you wanna set your car up to where you run stoich? how do you get there.. on my car i have an intake and exhaust, but no fuel system upgrades, my a/f guage scans from stoich to lean when im running at a constant steady speed.. it just goes back and forth.. what should i do? thanks

 

I know very little about cars and I dont pretend too...but I do know you DO NOT want a 1:1 ratio, you will flood the engine. I think its something like 9:1 or 11:1 is the proper ratios but hopefully a mod or senior member will answer your question better then I can.

 

A/F gauges that hook up to your stock O2 sensor are narrow band and will not give you a constant reading, unless at WOT, so they're basically useless for tuning part-throttle. They're basically just good as an indicator of an overly lean or overly rich condition during WOT acceleration. GReddy makes an A/F ratio meter with a semi-wideband O2 that will work better thn your typical Autometer unit. If you have more money, you can dish out for a wideband O2 sensor and display unit, but those are pretty expensive, and dyno shops usually have wideband O2's that you can rent to tune with.

 

If i remember correctly:

12.6 or 12.7 : 1 makes the most power
15 : 1 is the best for fuel economy. anymore more then that will over heat the engine
anything below 12.6 or 12.7 : 1 will kill both fuel economy and power ^_^ hope thats help heehee:thumbup:

 

As far as using something that taps into the stock oxygen sensor as a part-throttle tuning tool, inspyral is right. The ECU can run in either "closed-loop" or "open-loop" mode. In closed-loop the ECU picks how much fuel to use pased on RPM and MAP sensor voltage, that's it. At WOT it switches over to open-loop and uses O2 sensor data to compensate for any difference from the intended ratio. Since the ECU is only in open-loop mode at WOT, the sensor is only really designed to give an accurate reading at WOT. A "wide band" sensor simply means that it gives an accurate reading at more than just WOT.

Stoichiometric ratio is when you have exactly enough air to combust all of the fuel. For gasoline this is 14.6:1 (air:fuel). Rich means you have more fuel than that, lean means you have less fuel. Running lean (higher than 14.6:1) has the advantage of more fuel economy and at the same time produces more energy in the form of heat. More energy = more horsepower. If you run rich (lower than 14.6:1) not all of the fuel is combusted. If you stand behind an old carbeurated car and can smell gasoline in the exhaust that's because it's running rich. You don't really gain any power from running rich because no additional fuel is being combusted. The extra fuel helps to lower cylinder temps which allows more aggressive tuning of things like compression ratio and ignition timing.

 

If it's a street car, especially if it's running high compression or FI, you want to tune the A/F to somewhere around 12.5:1. If you tune it to be stoich, there is little room for error and more of a risk of running lean. Running lean with FI = detonation. Detonation under boost is not a good thing.

 

wow, thanks guys. you really cleared it all up for me. so would a device like the Apex-i Vtec controller help. doesnt it have a setting where you can change your air-fuel and some other settings? if not, you said the greddy one is gonna be better? sorry for all the questions.. im learning

justin

 

Yes, an AFC will allow you to adjust your fuel curve based on throttle position and RPM.

Yes, the GReddy A/F ratio meter is better than an Autometer unit because it comes with its own semi-wideband O2 sensor and displays A/F ratio directly instead of having bouncing LEDs like the Autometer.