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Originally Posted by steffen707
Try taking a stock d16z6 si engine and just lean the fuel injectors out to 20:1 air fuel ratio and you're going to destroy the engine. The engine has to be designed to go to lean burn in the first place.
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Fuel injectors don't work like that in modern EFI systems. The ECU will still follow its closed-loop programming until the peak capacity of the injector is reached, and then it will lean out, but in an uncontrolled manner.
While small bore engines do better at lean burn than larger bore engines (all other things being equal), and lean burn engines benefit from higher C/Rs, almost any engine can be converted to lean burn without inherent problems. (This is not to imply that it's
easy or emissions-legal to do, but it certainly can be done.)
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Sure leaning out the A/F ratio to like 14.7:1 from 13:1 will realize a gain, but from my limited tuning experienece, you don't want to mess with high AF ratios on a normal engine.
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The challenge with leaning an engine is to get leaner than 'peak egt' in a controlled manner. Aircraft engines have been doing this for decades by separating fuel metering from throttle position, but it can be done as well on EFI engines with a Megasquirt or similar controller.
Running an engine at peak egt (A/F ratio about 15:1...plus or minus) long term
might damage it, but I doubt it even then. Water cooled engines are pretty efficient at wicking away heat. Running lots of timing advance at peak egt can easily bring on preignition and detonation, which WILL damage and engine if permitted to persist, but modern ECUs sense this and pull advance out, returning the engine to the safe zone.
The lean condition itself is not inherently damaging. In fact, running the Insight's engine way lean of peak egt is one way that car achieves high MPGs. I don't know how lean they can go (anybody know?), but I wouldn't be surprised to learn that it's well over 16:1. As an engine goes leaner than peak, it runs cooler and cooler, and makes less and less power, until proper combustion can no longer be maintained and the engine roughens up and finally dies if you keep going leaner.
Reading between the lines on comments about the Insight engine, and how it won't stay in lean burn above about 75 mph, it's clear that it comes down to the 'power required' equation. When the car's total drag exceeds the max hp the engine can make in lean burn, whether during steady-state driving or while accelerating, the ECU converts to 'normal' mode to provide the power required and FE drops noticeably.