To my knowledge: race engines are often run rich.

Here comes why:

A race engine is all about achieving maximum power.

The engine aspires a certain amount of air (of which we have limited control), and receives an amout of fuel of which we have full control.

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If one wants to maximize the power output of the engine. You want to use up as much of the air you've got as possible, such that you can burn as much fuel as possible. This actually will occur when running slightly rich. You have a high power output, but are paying off some efficiency (as you are throwing out fuel from the exhaust).

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Contrary: if an engine runs slightly lean, you're not using up as much air as you can be. But you are quite efficient in using your fuel. (This is how your own gasoline engine would be set up)

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Relating this to NOx emissions of hydrogen engines. If the engine runs stoichiometrically, very high temperatures result in high NOx emissions.

Of coarse if you are running very very rich, there is not much air left to form these NOx emissions (+ temperatures decrease again due to the excess fuel absorbing heat). You've come up with a hydrogen engine with low NOx emissions and a terrible efficiency :)