Adjusting Stoich on tune

[Fastmax32168]

I originally tuned my car in at 14.7 A/F ratio. I want to adjust that to 14.4 to compensate for the 10% ethanol in my fuel now. If I change the Stoich to 14.4, can I simply adjust the entire VE and MAF tables by the equivalant 2% change to stay in the correct ballpark and then fine tune from there? I have read some confusing posts, some that say that adjusting these tables isnt even necessary, but it seems to me it is. Thanks

[steveo]

in a lot of ways you're just changing 4+5 to 6+3. i'd leave it alone. but yes if you change the afr values for stoich in this case you would alter your ve table as well

[Fastmax32168]

I guess this is the part I dont understand. If I adjust the Stoich to 14.4 and then I adjust the tables around that figure, does the motor not now run at 14.4 instead of 14.7 roughly 2% richer everywhere? Or do the narrowbands not allow this to happen?

[dfarr67]

I have adjusted the stoich to 14.5 then I have a open loop AFR table I adjusted accross the board- currently learning in VE.

[steveo]

I guess this is the part I dont understand. If I adjust the Stoich to 14.4 and then I adjust the tables around that figure, does the motor not now run at 14.4 instead of 14.7 roughly 2% richer everywhere? Or do the narrowbands not allow this to happen?

if you tuned your VE tables with 10% ethanol and stoich for 10% ethanol is 14.4:1 your car is now running at 14.4:1

it doesn't matter if your 'stoich' value in the ecm was 14.4:1 or 14.7:1 or 9999999:1 if you set your VE table based on E10 gas with narrowbands then you tuned to actual stoich of your fuel, whatever that was.

stoich and the current ve table cell are just two values (there are more, of course, such as the injector size constant) that are used together to arrive at an amount of fuel that is injected.

narrowbands only 'allow' a car to run at stoich or damn close to it

your 'trims' are a representation of how far it had to adjust fuel to achieve that value

so when you 'tune' a car to have little to no trimming, you are tuning it to stoich

[Fastmax32168]

Got it. Best explanation I have seen. Thanks

[Turnshredder]

14.7 is only for closed loop. I have tried to adjust mine to say 15.5 or 15 to 1, for better fuel economy, and it didn't seem to do anything. I did this under scalars/open loop afr parameters/stoichiometric afr. ??

[steveo]

most fuels do not burn at 14.7:1 anymore. no matter what you do to 'change the stoich ratio' in your ecm, when in closed loop your narrowband o2 sensors will try to get to half a volt or whatever is configured, which is stoich of whatever fuel you're burning. o2 sensors can't target a ratio, only stoich.

the only reason you would change the 'stoich' parameter in your ECM is so it doesn't have to trim as much to get there, the ecm does not have the ability to maintaining that ratio in closed loop.

if you want to try to burn a bit leaner than stoich, you can try to tune the o2 voltage that closed loop tries to get to. you might be able to tune it to 420-450mv without losing stability which ends up being very slightly leaner.

lets say you want to run 15:1 on gasoline that burns at 14.7:1 stoich so you adjust your o2 voltage

15:1 with regular gasoline usually ends up being like 200mv on the o2 sensor

that's fine but on a lot of o2 sensors, 17:1 is like 150mv and 20:1 is around 100mv

so what ends up happening is it just flutters back and forth from lean to barely running at all and runs like crap

the only real way to run leaner than stoich is open loop or get some kind of closed loop with a wideband rig of some kind