a/f ratio

**mark21742** · 02-12-2009, 04:37 AM

right now I'm running between 14-15:1 a/f across the board with 12.8-13.3:1 wot a/f.
my question is what is a "safe" lean ratio to cruise at. in other words, can I safely lean it out alittle more, also if I did could I see any gains in fuel mileage.

**Firebirdjones** · 02-12-2009, 05:48 AM

It's my understanding that in closed loop the car will constantly search for 14.7 AFR. I've verified that with my wideband.

There are tricks you can do like lowering the closed loop switching points of the 02 sensors to trick them into leaning the car. I've tried that with no change in mileage at all. Yet people report picking up 3-4 mpg. I got nothing.

However with the car constantly searching for 14.7 in closed loop,,,,Frost changed my MAF tables around for me while I was tuning and I picked up 2 mpg with no change in the AFR in closed loop.

So somehow the MAF table he sent me picked up some mileage for me,,,Yet I don't see any change in the AFR. I don't quite understand that one. How you can change the airflow tables in the MAF and lean the car yet in closed loop the computer is always looking for 14.7 AFR anyway,,,,and still see a gas mileage increase.

Maybe Frost will chime in and explain that.

**Frost** · 02-12-2009, 06:04 AM

You get the gain because even though you mostly see 14.7ish:1 at cruise, that is a result of closed loop trimming. The closer you get your underlying fueling values (really airflow), the less correction has to work to get it back to stoich. If your underlying tables are 10% off then there are times such as transients and immeadiately following throttle changes where fueling will be off for an instant waiting for closed loop to fix it.

For the best gas mileage and driveability, tune the VE first, then MAF. Use one of those cumulative airflow cfgs that are floating around and get your base airflow sorted. Once you have this and the rest of the driveability stuff done, work on making DFCO functional again and make it as aggressive as possible without it affecting the way the car drives. Get the VE and MAf in line and dfco working to get the most out of your mileage.

Also as mentioned, you have an airflow mode table that goes back to an O2 mV lookup table for closed loop operation. These tables can be worked to lean out cruise areas. As noted, this can provide some gain but it's not huge.

**mark21742** · 02-12-2009, 06:08 AM

I got one of those damn SLP MAF's and nomatter how I adjust the tables for it it sends me up to 20+:1 a/f, so I've been running MAFless SD tune

**Firebirdjones** · 02-12-2009, 07:16 AM

Originally Posted by Frost

You get the gain because even though you mostly see 14.7ish:1 at cruise, that is a result of closed loop trimming. The closer you get your underlying fueling values (really airflow), the less correction has to work to get it back to stoich. If your underlying tables are 10% off then there are times such as transients and immeadiately following throttle changes where fueling will be off for an instant waiting for closed loop to fix it.

For the best gas mileage and driveability, tune the VE first, then MAF. Use one of those cumulative airflow cfgs that are floating around and get your base airflow sorted. Once you have this and the rest of the driveability stuff done, work on making DFCO functional again and make it as aggressive as possible without it affecting the way the car drives. Get the VE and MAf in line and dfco working to get the most out of your mileage.

Also as mentioned, you have an airflow mode table that goes back to an O2 mV lookup table for closed loop operation. These tables can be worked to lean out cruise areas. As noted, this can provide some gain but it's not huge.

Okay that makes perfect sense. In a nutshell closed loop isn't working as hard to keep that 14.7 AFR.

I have never played with DFCO and don't quite understand it other than knowing it's some type of lean cruise mode that kicks in at some point.
Don't mean to change the idea of this post but since we are on the subject of DFCO and mileage,,,,care to give a few tips or tricks on what someone can do in DFCO for a mileage gain??? My wife would think I'm a hero if I squeaked more out of her car

**Frost** · 02-12-2009, 03:29 PM

DFCO = decel fuel cutoff

From the factory, if you watch your wideband (it happens after commanded fueling which is why it has to be disabled to tune or it will add a ton of false AFR error) on decel, you will see 20 or lean or whatever it maxes out at on the lean side. The injectors are cutoff until a certain point. With a cam it may not work at all. Even from the factory, it can be more aggressive. You have entry points based on RPM and MAP. It must exit DFCO or the car will stall. Start to open up the window on both ends to let it kick in as often as possible at no-throttle conditions. Log; you will see injector time fall to nothing and the O2s will sit at the bottom of the scale fully lean.