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AFR & camshaft overlap

EFI Tuning Fundamentals

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Could you please help me to understand why we should use rich lamda for idle on big overlapped camshafts?

When we have overlap - some oxygen will go into exhaust. WBO sensor senses oxygen. So if we have oxygen - it will show lean mixture? (WBO thinks that it's unburnt oxygen, but it's oxygen from intake manifold).

But video "Choosing The Correct AFR" mentions that becuase of inaccurate WBO readings we should add some fuel (use rich mixture), Why not vice versa?

Forget about the o2 sensor for a minute. One of the big things is the combustion stability. Basically, when the mixture is richer it will tend to misfire less when you have higher internal EGR rates (more exhaust gases from overlap).

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