A/F ratio on DS, is it accurate and how is it measured?

[2QUIK6]

Is the Air/Fuel Ration on Direct Scan accurrate and how is it computed/measured? solely off of the O2 sensor I guess, but the reason I ask is I've noticed my recent runs, the O2s are in the 780-790 range on my GN, but the A/F on DS is really low, like 8.0-9.0 indicating a really fat mixture

Yet, on my TTA, the O2s are in the 800-810mv, and the A/F is around 11.0-11.7 which seems about right....so how could lower O2s readings give a much fatter A/F on the GN??

 

[TurboDave]

Originally posted by 2QUIK6
Is the Air/Fuel Ration on Direct Scan accurrate and how is it computed/measured? solely off of the O2 sensor I guess, but the reason I ask is I've noticed my recent runs, the O2s are in the 780-790 range on my GN, but the A/F on DS is really low, like 8.0-9.0 indicating a really fat mixture

Yet, on my TTA, the O2s are in the 800-810mv, and the A/F is around 11.0-11.7 which seems about right....so how could lower O2s readings give a much fatter A/F on the GN??

Obviously it's not accurate since there's no wideband O2 sensor involved. It's what is commanded by the ECM, based on certain inputs. What's commanded and what's absolute can, at times, be very different!

 

[bruce]

Originally posted by 2QUIK6
like 8.0-9.0 indicating a really fat mixture

It's not INDICATING ANYTHING! !!

That's just what the commanded AFR is.

The ecm in open loop, *thinks*, and tries to maintain 14.7:1. Then when in Power Enrichment, there is a correction made based on the assumption that close loop is 14.7:1. If you say ADD 10% more fuel during PE, the ecm is going to move the AFR by 10%, 10% of 14.7 is 1.47, so the new PE AFR is 13.23, so when in PE the commanded AFR to a 10% enrichment would be 13.2, and that's what DS would show.
*It has little to do with actual AFR, in most cases*.

With all your smaller injectors they get run static. So after a point commanded AFR, is totally meaningless.

What can be done is just hammering the PE enrichment really hard so that the injector doesn't spend much time, operating in an erratic condition.

 

[ijames]

The main reason the commanded AFR looks wacky is that the maf reading is pegged at 255 gm/sec when in reality the ecm has no idea of the true airflow (extenders excepted), so to get a true afr of say 12:1 when the true airflow is 512 gm/sec the ecm has to command an afr of 6:1 when it thinks the airflow is 255 gm/sec. And in reality, of course, the ecm will command the fuel and the user has to set the boost to give that airflow to get that afr. So in ds don't worry about the reported afr. Concentrate on the knock retard, esc counts, O2 volts (as a rough guide of afr and reproducibility), and the injector duty cycle (primarily just after a chip change).

 

[bruce]

Originally posted by ijames
The main reason the commanded AFR looks wacky is that the maf reading is pegged at 255 gm/sec when in reality the ecm has no idea of the true airflow (extenders excepted), so to get a true afr of say 12:1 when the true airflow is 512 gm/sec the ecm has to command an afr of 6:1 when it thinks the airflow is 255 gm/sec. And in reality, of course, the ecm will command the fuel and the user has to set the boost to give that airflow to get that afr. So in ds don't worry about the reported afr. Concentrate on the knock retard, esc counts, O2 volts (as a rough guide of afr and reproducibility), and the injector duty cycle (primarily just after a chip change).

Well for the guys that get that get to the level of really flowing alot of air (512, that is).

BTW, has anyone reading this, seen a 512 gm/sec.?.

 

[TurboDave]

Originally posted by bruce
Well for the guys that get that get to the level of really flowing alot of air (512, that is).

BTW, has anyone reading this, seen a 512 gm/sec.?.

I'm sure there are those that have, and a lot more! That's the reason the Extender Extreme was born. I'm not sure how much Neal has seen from his Stage motor, but I'd bet it's significantly more than 512.
Heck, even my little weeny motor can move well over 320!

 

[2QUIK6]

Thanks for the explainations. I've never really paid much attention to it in the past, I just happened to notice it was really low compared to what it used to be, and everything makes sense now because I jacked the PE fueling up alot with the bigger turbo's I've been experimenting with, and yes the MAF is maxxed on the GN and it is not on the TTA, hence the difference is AF commanded readings between them now.

 

[bruce]

Originally posted by TurboDave
I'm sure there are those that have, and a lot more! That's the reason the Extender Extreme was born. I'm not sure how much Neal has seen from his Stage motor, but I'd bet it's significantly more than 512.
Heck, even my little weeny motor can move well over 320!

Most guys had guite using the MAF based systems before getting to the 512 mark, was my point, since this thread did originate with trying to understand DS.

While 320 is nothing to make light off, it's still aways from 512.

 

[TurboDave]

Originally posted by bruce
Most guys had guite using the MAF based systems before getting to the 512 mark, was my point, since this thread did originate with trying to understand DS.

While 320 is nothing to make light off, it's still aways from 512.

Understand

My only point was that 512 isn't really that hard to hit assuming one is making upwards of mid 500's in the horsepower arena.