Ignition Timing and AFR
 

I've seen debate on a couple forums about whether changing ignition timing affects A/F Ratio.

I've also seen mentioned that although pulling/adding timing may change the 02 Sensor readings, it doesn't necessarily mean there is an overly rich or lean mixture happening in the cylinder.

My reason for looking into this is that in my latest datalog at WOT, my AFR reading is slightly richer than the commanded AFR reading. Also, where the Primary Open-Loop Fueling table in RomRaider shows I should be at 11.76, my AFR reading is 11.25..slightly rich.

I'm running the OFT stage 2 with a drop in filter and OFH, with .35-1 degree of timing pulled at engine loads .8-1.4 depending where I saw slight FLKC. Is the pulled timing causing a slightly rich AFR reading?

I know slightly rich is better than lean but I'd like to figure this out.

No, it will be because of MAF scale, LTFT and primary OL fueling additive more than timing.

I guess it'd make sense to include my datalog here. I do see that my LTFT during WOT is +2.34% when the datalog is showing AFR 11.25 and Commanded AFR around 12. Should I be concerned or just live with it?

http://datazap.me/u/joshbustos86/206...4-7-9-12-13-14

1) Do you know that you O2 sensor scale is accurate, so can you trust the reading?
2) 11.25 is the lowest value on your O2 scale, it may be reading richer than that.
3) Mild changes to timing wont have a significant impact on AFR.
4) Do you know that the OL Fuel table is actually the target AFR?

The question should be is whether running "overly rich" is costing you power, or whether the timing and AFR combination is ideal.

What @Wayno and @Kodename47 say above is correct.

Have noticed the OFT guys appear to deliberately increase the maf flow numbers in the OL section of the scale. making it run richer than target on many cars.

I think they do this for a couple of reasons
1. rich is safer than lean
2. MAF sensors vary a bit mine and @Wayno s mafs are about 8% different so maf scaling for my car on wayno maks his car run lean. My car always ran rich like yours on oft tunes.
OFT guys need to account for maf sensor variation the maf scaling is adjusted as a compromise for variations in MAf sensors
3. Rich mix generally buys you a bit of knock resistance at upper rpm.

If you do scale you maf and correct the OL maf scaling you will then be closer to target or commanded AFR. However on 91 octane your likely going to run into knock issues and will probably have to pull a degree or two of timing over 5000 rpm.

Or is your O2 sensor reading 8% different?? I'll post up something later about my recent findings, but your MAF scaling is only as good as your Front O2 one.

True could be 02 sensor differences or differences in readings as we have different header.

Firstly, lots of good info already posted here, no intention to contradict any of it. Second, if you have ever logged a stock car you will see that the commanded AFR is NOT in correlation with the actual afr, especially when in open loop. You can get close with careful maf scaling but you will rairly ever be dead in when in open loop. The values in the open loop fueling table are just amounts that translate to pulse width once you are in open loop. The number in the cell should not be thought of a the target afr once open loop, but rather a raw value that has been scaled to resemble an AFR value.

If all of your calibrations are correct the commanded AFR should be the actual AFR. If it is not then there are still some calibrations to be corrected. I'm not sure if we have access to all if the necessary calibrations, and it's obvious that the commanded AFR does not need to be dead nuts accurate, but it would be incorrect to say that it's just s random scalar. If you're seeing a difference, then something is off. That doesn't mean you need to be chasing it, but it's something to be aware of.

But it IS just a scalar or sorts, the ECU doesn't look at the table and try to hit the AFR in the table. You could say that the table is merely a fuel adding algorithm on anything richer than 14.7, but mentally it's easier to view it in lambda or AFR ratios. There is nothing that states you have to hit the AFR in the table, so long as the AFR in the cylinders/exhaust are the desired effect then why does it matter? It's only figures in a log that won't match. Don't forget we are talking open loop fueling here and not closed loop.

I'm sure the open loop fueling additive table is what's actually needed to correct the fueling to the stock MAF scale, rather than moving the MAF to correct the fueling. However to get that correct then you really need a steady state dyno or lots and lots of data collection in open loop running.

It IS trying to hit that AFR. It has a known mass air flow and known RPM, so it knows what mass of fuel it needs to deliver every rotation to achieve the AFR the table is asking for. There are other factors to account for and it becomes much more difficult in transient conditions, but in rough terms, yes; That's exactly what it's trying to do.

The big thing is that it's not perfect, but there are things which can be done to help it, such as correcting the MAF scaling based on logs. The factory tuning is based on nominal values for sensors, whereas we have the capability to tune for the actual sensors in our individual cars.

In my limited knowldge (learning how to tune my car, so I've been reading all of mad_sb's posts, as well as the stickies) I noticed that with the OFT tune my car was hitting -17% LTFT's which is insane, I decided against using the OFT tune and took my time to scale my maf using yikes and getting a general idea of how to scale my maf with james vid.

After I got my LTFT's less than 5% on the stock tune, I basically imported the timing map, fuelling table, cam timings, and O2 sensor scaling from OFT to my scaled tune.

With the OFT map as it sat my O2 would go as low as 11.25, which is the O2 sensors limit, even though the lowest requested AFR was 12, now with my rescaled maf my O2 reports AFR readings almost bang on compared to the fuel map, with a .2 variance at some points.

nice work

if you use the VGI utility it heaps easier and quicker than yikes spreadsheet :)

I used it at first, but it would always create a funky looking maf curve. ie. a bump on the curve around the 2.5-3v area

Decided to use yikes as it would show me where the fueling errors where located so I adjusted the maf scale based on a general area in order to keep the MAF scale as perfectly logarithmic as I could.