Discussion and questions related to the course Understand AFR
Just trying to understand. Andre said that for turbo applications 0.80(11.76 AFR) lambda is enough. But on factory ECU of Toyota 3S-GTE gen3 (ST205 Celica GT-Four) AFR reachs 10.0 under high load (0.9bar and 5000+ RPM).
Why they did so? It's safety margin for improved exhaust and etc?
Mixtures richer than 0.8 lambda are typically used to control exhaust temperatures, in respect to exhaust valves, turbochargers and catalytic converters. On the dyno a manufacturer would typically tune to MBT and then add fuel until the temperatures are within spec. Over the last 20 years or so this has become harder and harder as OEMs choose higher compression ratios, smaller turbines, etc so exhaust temperatures are higher. You'll find quite a few manufacturers now have special high-temperature turbochargers (e.g. the KKK used on the Volvo 5 cylinders since 2006).
The Toyota Celica is running an ECU from the early '90s, most if not all cars of this time ran hugely rich on full load and high rev's. This is most likely due to the car being shipped worldwide and there is a huge variance in quality of fuels, so the extra fuel could have been used to bring down charge temps also to prevent the onset of knock.
When you stick in a standalone to a 3S motor and tune it you can see some good gains by bringing the mixture into a more sensible range at the same boost levels.
Thanks for such a interesting answers.
Chris250, interesting idea about huge variance in quality of fuels worldwide, but we can see such AFR on JDM ECU(european version of ST205 Celica has it's own ECU). Also 3S-GTE tend to brake ring lands. How do you think, is it safe to try to move from 10.0 to lean with standalone?
I can not see a single reason not to have a leaner lambda (say 0.76-0.8) that will only benefit you with better fuel consumption and spool (if that's a turbo engine).
Probably you would never see a professional tuner run richer than 0.73-0.75 lambda unless it is necessary for a specific race discipline to and the engine can't hold the EGT. Otherwise i can't see any other benefit.