Air/Fuel Ratio Question

[clicklogical]

I know this is an old thread, but I have a question:

At idle and WOT my AFR hold around 13.9-14.3, all in range and normal.

Coasting maxes out at 20, also normal.

However, we I apply some gas coming off that coasting, the AFR will pitch down to 11.7 and bounce back and forth between that and 15.1 for a second or so before settling back in the normal range of 14 or so under load. I am not fluctuating the amount of fuel, just a set steady pressure on the accelerator.

Does this seem normal to you folks?
:shrug:

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[Jn2]

I know this is an old thread, but I have a question:

At idle and WOT my AFR hold around 13.9-14.3, all in range and normal.

Coasting maxes out at 20, also normal.

However, we I apply some gas coming off that coasting, the AFR will pitch down to 11.7 and bounce back and forth between that and 15.1 for a second or so before settling back in the normal range of 14 or so under load. I am not fluctuating the amount of fuel, just a set steady pressure on the accelerator.

Does this seem normal to you folks?
:shrug:

you'll be fine, the car is in DFCO while coasting so your AR will be 20, tapping the gas takes it out of DFCO, it starts spraying more fuel, depending on the accuracy of your MAF curve, it could spray a bit too much, then use STFT to correct itself and settle out...How ever what you are describing is a non-issue...

 

[clicklogical]

you'll be fine, the car is in DFCO while coasting so your AR will be 20, tapping the gas takes it out of DFCO, it starts spraying more fuel, depending on the accuracy of your MAF curve, it could spray a bit too much, then use STFT to correct itself and settle out...How ever what you are describing is a non-issue...

Thanks!!:cheers:

 

[Albertcado]

Old thread but does the AFR dip below 14 when accelerating or throttling during normal driving or from a stop (not WOT) ?

 

[Albertcado]

Here's a good basic explanation of AFR: http://www.turbobygarrett.com/turbobygarrett/airfuel_ratio_tuning_rich_vs_lean

The lower the number the richer (more fuel/less air) the mix is. Motors don't pop because they are too rich, they fail because timing is too far advanced or the AFR is too lean (higher number). Too rich of an AFR will cause issues over time though...fouled plugs, injector issues, etc. For the record, I don't think your current AFR is anywhere near too RICH to actually cause issues, but you aren't maximizing potential power.

It also depends when you are taking the reading. On a naturally aspirated car (no supercharger, turbos or nitrous) you should be fine with a lambda of .85 at WOT. If you're using actual stoich of 14.7:1 then a .85 lambda would be an AFR of ~12.5. Basically, on an NA motor you should shoot for a lambda of .85 under WOT...that stays constant. Then you convert it to AFR depending on your type of fuel is. Pure gas (no ethanol), pump gas with some ethanol, E85, race gas...they all have different stoich points.

Since gasoline has up to 10% ethanol these days that old standard of 14.7:1 for stoichiometric AFR is actually slightly high. I think you can use something around 14.3 for current gas blends with up to 10% ethanol (someone correct me if wrong here). If that is what you use and you go for a lambda of .85 then .85 x 14.3 = 12.15:1 AFR at WOT.

Long story short, seems like you are still pretty rich (safe) an probably have more room left for power if they lean out the AFR a little more....assuming you are seeing this reading under WOT, the timing is within check (you'd have to log, no factory gauge for that) and you're not getting constant or excessive KR (again, you'd have to log for that).

EDIT: just saw you said under full throttle....I'd say you have room left for power if they lean it out a little...talk to your tuner.

What should normal AFR be when simply accelerating in the city or from a complete stop at idle?
Does it stay at 14 or should it blip a bit rich momentarily when giving it gas?

 

[EFI]

Old thread but does the AFR dip below 14 when accelerating or throttling during normal driving or from a stop (not WOT) ?

Yes it will.

The target AFR is ~14.1 and it's never going to be perfect...so it will swing between the high 13s and mid 14s during light operation.

 

[Angrey]

Running rich at WOT isn't an issue, just a little more safety and a little less power/torque. Running rich everywhere else however isn't good. It's not as bad as lean, but still has negative effects.

First, it's hard on your catalytic converters. The alloy catalysts will burn hotter with any raw fuel or incomplete combustion that comes through. This eventually results in premature failure.

Secondly, enough extra fuel over time contaminates and "cuts" the motor oil. It thins it out. If you run the car hard enough (and sustained high temps) it'll help to evaporate off lower weight hydrocarbons, but still isn't great for the motor. It also can "wash down" the cylinder walls. This is where the extra fuel actually thins the oil/film that's protecting your cylinder walls as the combustion and wiper rings scrape against it. Unlike iron blocks, Aluminum blocks need to either be sleeved with a harder/stronger material inserted, or as Ford has done to save cost and weight, they use a newer technology whereby they take high temp "plasma" and coat the Aluminum cylinder bores with iron. This "coating" is extremely thin. Over time it can wear and even fail all the way through with any sort of ring deformation (like when the motor is cold and the pistons haven't warmed fully and expanded and are scraping around at less than perfect angles). Basically, washing down the cylinders can end up jacking up your plasma coating or accelerating it's wear. Will the motor fail? Not necessarily, but it will cause less perfect mating between the rings and the cylinder walls which results in less compression and more oil blow by.

Basically, it's okay to run rich for short periods, but running rich all the time or for long periods isn't good for the motor either. It's not as bad as running lean (and risking preignition or detonation) but the best practice is to have a tuner adjust it so it's on the rich side but close to proper stoichiometric ratios for the fuel you're running.

 

[EFI]

First, it's hard on your catalytic converters. The alloy catalysts will burn hotter with any raw fuel or incomplete combustion that comes through. This eventually results in premature failure.

If cats run hotter with raw fuel in them, then why does the PCM dump extra fuel as a means to cool them off when the cats get too hot aka cat overtemp protection?

 

[NightmareMoon]

stoichiometric is only the target when cruising and under light acceleration. When running hard, its going to be tuned to run rich.

 

[Angrey]

If cats run hotter with raw fuel in them, then why does the PCM dump extra fuel as a means to cool them off when the cats get too hot aka cat overtemp protection?

To cool the exhaust temp, but it's only temporary in effect. Unspent fuel will eventually melt cats.

 

[Albertcado]

Yes it will.

The target AFR is ~14.1 and it's never going to be perfect...so it will swing between the high 13s and mid 14s during light operation.

Could use some enlightenment here. Just drove in the city with lots of stop and go streets. Most of the time my AFR is staying in that ideal high 13s ~ mid 14 range, BUT I noticed that whenever I go into neutral (MT) after accelerating, AFR will immediately drop to around 12.3~12.5 for a few seconds before climbing to that steady 14/14.1 at idle. Happened every time when going into neutral from moderate acceleration.

Is this a normal characteristic for a manual transmission? Why would the gauge show it running rich for a couple seconds as soon as it went into neutral when there's no throttle being applied? Confused and hoping there's a logical reason and not an issue...

 

[EFI]

I noticed that whenever I go into neutral (MT) after accelerating, AFR will immediately drop to around 12.3~12.5 for a few seconds before climbing to that steady 14/14.1 at idle. Happened every time when going into neutral from moderate acceleration.

Is this a normal characteristic for a manual transmission? Why would the gauge show it running rich for a couple seconds as soon as it went into neutral when there's no throttle being applied? Confused and hoping there's a logical reason and not an issue

Anytime you abruptly change the throttle input and load on the engine, it will have a spike like that simply because it has not caught up to the new load factor yet. If you're accelerating for example, then you let off the gas and put the clutch in, that kills alot of the load and for a moment it will run rich then recover.

Don't over think this, it's all normal operation of an engine. If it stuck at 12.5 for more than a few seconds then you have an issue. If it recovers, it's fine.

 

[Albertcado]

Cat protection.

Can you elaborate? You're saying this is normal and a beneficial function? Does AFR dip like that briefly for all manual s550s?

 

[WildHorse]

Can you elaborate? You're saying this is normal and a beneficial function? Does AFR dip like that briefly for all manual s550s?

Watch your AFR when you upshift and see that 20:1 "POOF" mind blown...

As EFI said, your over thinking this.

 

[Albertcado]

Anytime you abruptly change the throttle input and load on the engine, it will have a spike like that simply because it has not caught up to the new load factor yet. If you're accelerating for example, then you let off the gas and put the clutch in, that kills alot of the load and for a moment it will run rich then recover.

Don't over think this, it's all normal operation of an engine. If it stuck at 12.5 for more than a few seconds then you have an issue. If it recovers, it's fine.

Thanks so much! Yes it recovers to 14 every time in 2-3 seconds after the drop in neutral.
I was thinking that when it goes into neutral, all the air is cut off at that moment from the closed throttle, so air needs to catch up to the fuel being used to idle, which would read it rich until it balanced out again

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