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Working with a BMW 335i and on a jb4 off the shelf map the car is hitting mid 13 afr at redline. It doesn't even drop below 14 till about 4 grand well into boost which is 15 psi. I would think it should be a lot lower at 11 something but many people in the 335 tuning community claim direct injection can run lean mixtures without causing detonation or potential damage to the engine. Anyone have experience with direct injection or 335s specifically who could confirm whether this is true?
I haven't had any experience with the 335 unfortunately so I can't say too much about the AFR your'e seeing. DI is quite different in a number of important ways compared to port injection and it's always advisable to test and see what sort of AFR the factory mapping is using. This isn't a definitive guide but it's always interesting to know what the factory consider to be safe.
What you are seeing could be explained if the 335 uses a stratified charge injection strategy as in this instance the mixture in the cylinder is not homogenous and there will be a rich kernel of fuel and air located around the spark plug while the overall mixture will still show lean. This is a situation that we don't come across with PI engines as the mixture for the most part is homogenous.
