Thread: What cold start AFR to aim for?

Lynch's thread got me wondering how to improve MPG's, and I think I'm running too rich during cold startup

Current tune:
40*F(5*C) = AFR @ 10.5 initial startup, creeps to 14.7 after a few mins.

What AFR do you guys aim for at cold start? Some say 10, some say 11, some say 13?

I don't know if cold start up is where I would be putting the effort. The sr tends to warm up pretty quick. But in any case an engine will always run rich when cold to protect the motor. Also, take into consideration the oil viscosity for the temperature range. Too thick an oil will cause an increase in parasitic loss. If I were to pursue this though, I would get an egt and start to mess with the cruising afr's. Also reducing rotating mass will help.

I run 14.0 to 14.7 while warming up, from what I can tell is a cold engine requires more fueling to achieve the same afr a warm one would have.. atleast for idle.. correct me if im wrong here but the car starts up fine and is immediately drivable tho I usually give it a min or two.

The engine requires more fuel during warmup because some of the fuel condenses on the cold manifold walls and therefore doesn't make it into the combustion chamber. In other words, it takes a longer IPW to maintain 14.7 AFR when cold. As it warms up, it takes less and less additional IPW. This is also why ECT is a factor in all tunable parameters that affect IPW.

So technically, even though he is aiming for 14.7 afr when cold he is still dumping more fuel when cold?

Yes, an engine runs richer while warming up just to maintain a normal burn because a lot of the fuel stays liquid. The AFR readings should be as close to 14.7 as you can get while keeping the engine running properly.

Thanks for enlightening me! Superb info OnTheChip!

What about wideband readyness? Can you trust a wideband a few seconds after startup? The MTX-L takes about 15-20s to fully warmup, but should you still be cautious when interpreting early values?

If you have a good AFR gauge, shoot for 12:1 range right after engine start. I played around with the after engine start enrichment setting and 12 afr was stable. Any leaner and it was rough. The factory setting is really rich from 10-11 like you have seen.

This setting is coolant temperature dependent, so the colder the temperature the richer the engine wants to be stable.
My suggestion is based on an average 60*F cold engine. Over 100*F you can run 14 AFR.