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Why Lean Engines Do NOT Run Hot – Myth Busted

If you’ve ever tuned a seriously modified performance car,
Or even just fiddled with a carburetor, you’ve probably heard an old bit of advice:
Running lean will make your engine overheat. Its good guidance to follow, but it’s a little over-simplified. A very lean air-fuel mix will make your engine run cooler What gives?

Jason Fenske is here to demystify all the details in his latest Engineering Explained video. As you probably know, the ideal air-fuel ratio is 14.7 parts air to one part fuel.
But that’s in an ideal world,
One where every molecule of fuel and oxygen is completely consumed
During every single combustion event.

Also. read:

In a real engine, a 14.7:1 air-fuel ratio is a little too lean.
To compensate for incomplete combustion, and to reduce NOx emissions,
Modern automobiles are tuned to run richer,
Sometimes dipping as low as 12:1 or even richer during high-load situations.
Adding in extra fuel prevents detonation and just generally makes the engine happier.

So, yes, if you lean out your engine, going from 12:1 to around 14:1, your engine will run a bit hotter. But 14:1 is still “rich” by stoichiometric standards. If you lean out past 14.7:1, all the way to something like 17:1, your engine will run cooler again. It just won’t run smoothly.

For a more detailed explanation with his trademark,
White-board charts and illustrations, check out Fenske’s video below.

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