John Micheal Henry

I'd Like some opinions on the correct air/fuel ratio after tune. I'm having ARH with cats and 1 7/8" primaries and aFe cai installed. The C7 is otherwise stock. My tuner said the air/fuel ratio will run @ 13.2:1, that DI engines like to be a little leaner. Thanks for your time and replies..

T/A KID

All depends on the stoich ratio
If using 14.68 as stoich then .90 lambda is 13.2

If using the factory stoich of 14.10 then 12.69 is more in line

I haven't seen true 14.68 in years. Around my parts 93 is usually 14.3 to 14.4 stoich and other places it's closer to 14.0

John Micheal Henry

T/A KID, I have no idea what you just told me. I'm ignorant about all of this technical jargon. Could you please explain what the word stoich means. Thanks

Higgs Boson

stoichiometry is the air fuel mixture where all the fuel and air is most completely burned. it is the AF ratio a vehicle generally runs at all times except for special circumstances like cold start...warming up to operating temp....full throttle.....you get the idea.

the air fuel ratio you see on a dyno graph is full throttle af ratio which is richer (higher proportion of fuel) for several reasons.

the issue that confuses people is AF ratio is not a real number, it is a calculation. the real number is actually something called Lambda. This is because it is a consistent number.

Stoich AF ratio for gasoline is 14.68 parts of air for 1 part of fuel, or 14.68:1 AF ratio. However, stoich for E85 is 9.85. Stoich for VP C12 is 15:1. etc..... point being, stoich AF ratio changes depending on the fuel that is being used / what is in the tank.

When you are using lambda to measure, stoich is always 1.0. Lambda 1.0 is stoich no matter what fuel you use. Therefore, if you want to add a percentage of fuel for full throttle (just as one example), you would add, say, 12% and run Lambda .88. If you are using gasoline, this is 12.91 AFR.....BUT if you are using E10, which is in most pumps, this is 12.40 AFR (because stoich AFR is 14.1:1).

So really, you are running .88 either way, but the ending AFR's are different. Now, what lambda or AFR to run for full throttle depends on what mods you have, what fuel you are using, what activity you are participating in, etc.....

Different fuels like E0 pure gasoline vs race gas vs E85 will all call for different values.

Different modifications like superchargers, cam only, etc will call for different values.

Different uses like quick spurts like a dyno pull or drag race vs road racing vs towing vs daily driving will all call for different values.

Different areas in the RPM range will call for different values as well if you are doing it right. 3500-4500 is usually a higher combustion pressure zone and likes a richer mixture than 6000.

So asking what is the ideal AFR is a hard question to answer.....Personally, I do agree DI engines can run a bit leaner, but it also depends on a lot of other things, like how much spark timing is best for your combination, etc.....sometimes more spark lead and a richer mixture is better than less spark lead and a hotter, leaner mixture......

The best AFR is the one that works best for you and your car.

John Micheal Henry

Higgs Boson, Thank you sir for explaining all about the variables figured into tuning for the a/f ratio. There is so much to learn about tuning, that's why I'm having a well known tuner in my area take care of my car. Thank you sir for taking the time to explain.

tblu92

Stoich value is used when the car is running at Part throttle--
But when you floor the throttle the engine uses what they call the "power enrichment table" Your tuner is speaking about the AFR when the engine is in power enrichment.
On older C5 and C6's with port injection most tuners would shoot for a safe AFR of about 12.7-12.9
However with DI the AFR can be run somewhat leaner Just as your tuner did from 13.0 to 13.3- and still be safe---Yours is definitely in line---
ALSO stoich values change according to what % of ethanol your state is using for their pump gas-----In old days with 100% gas the stoich would be 14.68
Most states now use a 10% mix of ethanol even in their pump gas--This lowers the stoich value to 14.10--The more ethanol they use---the lower the stocih value--