What is the point of tuning fuel?

[Ajax]

I have a theoretical question here, didn't know where to post it.

Why do we bother tuning fuel maps when we could just tune the AFR on the O2 sensor?

[2xLS1]

Not sure exactly what you are asking but.... Which O2 sensor? Wide band or narrow band? Narrow bands just display richer/leaner than stoich. They are designed to be highly accurate at stoich A/F. Wide bands display the actual A/F ratio. And it is the fuel map, which is actually the air flow model that determines what a wide band would display in open loop. While in closed loop they should display close to stoich with the help of fuel trimming. I really don't even like the term map and rarely use it. I prefer tables.

[kingtal0n]

oxygen sensor reads the past and gives you a number

then you decide how to fuel the engine based on that number + your other observations such as plug reading, engine roughness, fuel economy, torque/power curves, and compare to dyno wideband.

An oxygen sensor is merely a tool and cannot be used by itself for anything than interpreting the past. Different sensor designs look at the past differently as do their counterpart microprocessor controllers.

[Ajax]

Thanks for the reply guys.

What I meant is, the O2 sensor's input controls the STFT, which is essentially the car tuning itself to adjust for more/less air. So, for example, if I put a turbo on a NA car, when there is more air in the cylinders, the O2 sensor will read a leaner mix and inject more fuel by increasing the STFT.

So, what's the point of tuning if the O2 sensor is essentially doing it for you?

Edit: I meant, what is the point of tuning fuel if the O2 is doing it for you?
I know there are many more variables to tune.

[5FDP]

But you have to wait for the o2 sensor to see that it's lean and by that time it can be too late.

Having the airflow model correct means that you are injecting the correct amount of fuel before the o2 sensor or wideband sensor even reads that information.

[gtstorey]

You really aren’t tuning fuel, you are tuning air.

[2xLS1]

Thanks for the reply guys.

What I meant is, the O2 sensor's input controls the STFT, which is essentially the car tuning itself to adjust for more/less air. So, for example, if I put a turbo on a NA car, when there is more air in the cylinders, the O2 sensor will read a leaner mix and inject more fuel by increasing the STFT.

So, what's the point of tuning if the O2 sensor is essentially doing it for you?

Edit: I meant, what is the point of tuning fuel if the O2 is doing it for you?
I know there are many more variables to tune.

The goal is to keep the fuel trims close to zero so less correction. But yes that is what the fuel trims do in closed loop.

[Ajax]

But you have to wait for the o2 sensor to see that it's lean and by that time it can be too late.

Having the airflow model correct means that you are injecting the correct amount of fuel before the o2 sensor or wideband sensor even reads that information.

Sure but the effect is the same no? The only difference is a few miliseconds between the O2 sees it and it actually changes the STFT?

So, essentially all you are doing is avoiding that delay. Not only avoiding that potentially harmful delay, but also maybe getting more performance out of the engine since the correct fuel is being injected without the "test cycles" that precede it.


I guess that makes sense.

Still, in non-critical situations it shouldn't really make that much of a difference.

For example, how many times have I heard "it is pointless to get a high-flow air filter without a tune"? Why? The O2 sensor will automatically adjust your AFR, and since it has no way to limit air intake, it should simply balance it out by injecting more fuel and thus making more power.

Sure, it is not optimal but it should still be a noticeable improvement (assuming the filter or whatever mod you install actually allows for more air and that your injectors have the capability of injecting more fuel).

Am I wrong?

[2xLS1]

It is also less critical if using a MAF as it will sense any additional airflow and adjust fueling accordingly. Where it is more important when in speed density mode where the airflow model is all based on a table.

[GHuggins]

The goal is to keep the fuel trims close to zero so less correction. But yes that is what the fuel trims do in closed loop.

What he said. AND to emphasize on it more the CLOSED LOOP part of this statement. O2's at least on the majority of the controllers will not do anything to wot fueling. LTFT's are the only ones that play into that unless you have that one oddball controller that uses stft's too


To go into this even further. Everything is decided off of air calculations from fueling, idle controls, cyl airmass calcs to torque and so on, so it needs to be tuned in.

[kingtal0n]

Thanks for the reply guys.

the O2 sensor's input controls the STFT,

Nah the Oxygen sensor does what YOU tell it to do. You can disable STFT LTFT Closed loop if you want. You can also install different oxygen sensors with their own gauges that the computer doesn't know about, just to watch.
You could cut the wires or unplug it. It just does whatever you tell it to do.

which is essentially the car tuning itself to adjust for more/less air.

At idle and cruise a computer can take an error signal from the past and use it to correct for the error to reach some goal which is really a state space for vectors to operate within on a Root locus.
https://lpsa.swarthmore.edu/Root_Locus/RLDraw.html

On OEM computers this automatic adjusting only works for idle/cruise. Since there is no wideband there is no way to tune for wide open throttle which is where you need to target specific air/fuel ratios. You use the oxygen sensor to tell you the air fuel ratio and adjust as needed. If the engine is a stock engine you can make good guesses without a wideband for wide open throttle. But if you have any kind of performance engine, like an actually modified setup to make more than 20+hp than factory, you will absolutely need the wideband and cannot just guess the wide open throttle ratios. The more power:displacement ratio the more important air fuel ratio and spark timing and injector firing time will matter and make or break the engine. Thus the useful of a wideband scales with the performance aspect of a vehicle. Its obviously the first thing I always buy when building a car, and it sits in it's little box next to the turbo until the car is ready running and driving nice and clean already because you don't want dirty time on that precious wideband device unnecessarily. The better you treat the wideband the longer it will last. They can last over 20 years I've never had one from AEM fail since I started using them around 2005.

So, for example, if I put a turbo on a NA car, when there is more air in the cylinders, the O2 sensor will read a leaner mix and inject more fuel by increasing the STFT.

Adding a turbo will add exhaust gas pressure which will make the engine breath harder and get worse fuel economy thus increasing the fuel requirements a few percent which would lead to leaner air fuel ratios based on old information from befoer the turbo obstruction and yes that is correct the oxygen sensors will notice and correct the base fuel map for idle/cruise situations where the turbo is making the engine less efficient by adding a tiny bit of extra fuel for the cruise and idle. Very good if you meant that way. Make sure you understand we are only talking around 1.5% or 3% change in efficiency for typical performance-size turbochargers when added to the engine, brake specific will get worse only a couple or few percent, its not a huge drain. Turbochargers do not improve fuel economy of that engine but at idle and cruise they don't take much either. It could also allow you to use a smaller engine to make more power. Turbocharging is the most reliable engine you can operate. Exhaust gas pressure on exhaust stroke protects the rod caps making the engine the most reliable performance configuration because you can rev out a stock rod cap to way higher rpm than would normally be possible without touching the rod bolts. Intercooling has come a long way making it extremely safe if you know what you are doing and use the right engine.

So, what's the point of tuning if the O2 sensor is essentially doing it for you?

Edit: I meant, what is the point of tuning fuel if the O2 is doing it for you?
I know there are many more variables to tune.

There is a base map with values that the computer uses. STFT is only a adjustment of the base values. The adjustment only has to many percentage it can move, it can only make like a 10 or 20% difference to the base values, so if they are way off it can never correct all that And the engine wont run smooth if all the cells are lopsided and jagged because of all the crazy corrections. You dont really want corrections in the map it makes a jagged appearance. What you want is a smooth map and to prevent too much if any corrections, I turn off closed loop, because Idon't want jagged edges on my fuel map. For example some people put 440ccinjectors into a nissan sr20det 2.0L with 370cc injectors then the extra fuel is noticed by the oxygen sensors and you get a correction that is within the boundary. So now you get the same fuel for idle and cruise as it should like factory but the wide open throttle has more fuel than before and doesn't auto- correct. There is no auto correct for wide open throttle. Not for these OEM Ecu, not yet anyways. All aftermarket stand-alones can do wide open throttle correcting though. Its just the stock ecu doesn't do it so you need to really get the base map within a range and fine tune the wide open throttle is easier when you can see all the other cells already filled in, like i said before you can tune without a wideband if its a mostly stock engine by making a good guess and reading plugs and by looking at the torque curve, there is a lot more of information in a dynojet curve than people realize. If the engine is really going to be a performance engine though it should always have a wideband no questions it is the most important piece of equipment in a performance oriented vehicle because it directly protects the engine from disaster the more performancy it is.

[saucemolepl]

Tuning fuel maps and adjusting the air-fuel ratio (AFR) through the O2 sensor are two aspects of optimizing engine performance, but they serve different purposes and are often used together for comprehensive engine tuning. Let's break down the key differences:

Fuel Maps:

Overview: Fuel maps represent the relationship between engine parameters (such as RPM and throttle position) and the amount of fuel injected into the engine.
Customization: Tuning fuel maps involves adjusting the injection duration to optimize the fuel delivery at different operating conditions. This allows for fine-tuning the fuel curve for specific performance goals.
Application: Fuel maps are particularly important in systems that use port fuel injection or direct fuel injection, where the injectors are precisely timed to deliver fuel into the combustion chamber.
O2 Sensor and AFR:

Overview: The O2 sensor measures the oxygen content in the exhaust gases, providing feedback to the engine control unit (ECU) about the combustion efficiency.
AFR Adjustment: The ECU uses the O2 sensor data to make real-time adjustments to the air-fuel ratio. AFR adjustments aim to maintain the optimal ratio for combustion (typically around 14.7:1 for stoichiometric combustion, but this can vary).
Closed-Loop and Open-Loop: The engine operates in closed-loop mode when the ECU uses O2 sensor feedback to adjust AFR. In open-loop mode, the ECU relies on pre-programmed fuel maps without real-time O2 sensor input.
Why Both?

Comprehensive Tuning: Tuning fuel maps and adjusting AFR through the O2 sensor are complementary processes. Fuel maps provide a baseline for the engine's fuel delivery strategy, while O2 sensor feedback allows real-time adjustments to optimize AFR for varying conditions.
Precision: Tuning fuel maps allows for precise control of the injected fuel quantity, taking into account factors such as injector characteristics, fuel pressure, and engine efficiency.
Adaptability: O2 sensor feedback ensures adaptability to changing conditions, compensating for factors like altitude, temperature, and fuel quality.
In summary, both tuning fuel maps and adjusting AFR through the O2 sensor are essential for comprehensive engine tuning. Fuel maps provide a baseline for fuel delivery, and the O2 sensor ensures real-time adjustments to maintain optimal combustion efficiency under dynamic operating conditions.

[AMCJavelin74]

Nah the Oxygen sensor does what YOU tell it to do. You can disable STFT LTFT Closed loop if you want. You can also install different oxygen sensors with their own gauges that the computer doesn't know about, just to watch.
You could cut the wires or unplug it. It just does whatever you tell it to do.

[removed text to save space]

All aftermarket stand-alones can do wide open throttle correcting though. Its just the stock ecu doesn't do it so you need to really get the base map within a range and fine tune the wide open throttle is easier when you can see all the other cells already filled in, like i said before you can tune without a wideband if its a mostly stock engine by making a good guess and reading plugs and by looking at the torque curve, there is a lot more of information in a dynojet curve than people realize. If the engine is really going to be a performance engine though it should always have a wideband no questions it is the most important piece of equipment in a performance oriented vehicle because it directly protects the engine from disaster the more performancy it is.

Tuning fuel maps and adjusting the air-fuel ratio (AFR) through the O2 sensor are two aspects of optimizing engine performance, but they serve different purposes and are often used together for comprehensive engine tuning. Let's break down the key differences:

[REMOVED TEXT TO SAVE SPACE]

Fuel maps provide a baseline for fuel delivery, and the O2 sensor ensures real-time adjustments to maintain optimal combustion efficiency under dynamic operating conditions.

These two guys are on point.

[Ajax]

Ok guys, thanks for all the replies. It's been very informative.

I have one last, kind of unrelated, question, maybe you can answer this: How often does the ECU read the O2 value? Does it read it on every cycle? Or does it have a certain delay to it?

[kingtal0n]

There is a delay called transient delay. With enough experience some people can decipher transient delay based on user inputs and (oxygen) sensor response character. Most ECU you can change the delay vs RPM because as RPM increases the delay becomes less and less. The delay is based on sensor distance from the engine and exhaust gas velocity mostly.

Any computer sampling of sensors could be 'fast' or 'slow'. This can also be set by the end-user in some ECU. I would guess the factory ECU will sample somewhere between 500uS to 1ms minimum with as high as 5ms to 100ms , I doubt its any slower than 400ms for example it wouldn't be fast enough. 1uS is 1 microscond or 1/1000 of a millisecond for reference and 1ms is 1 millisecond or 1/1000 of a second.