How lean is OK?

So I gather a good afr to run is around 14/15/16:1, and you want to be running on the lean side.

If you have no way of calculating AFR, could you just tune to add roughly more air than that it needs?

If diesels like to run lean anyway, is there no problem running 1.6 bar of boost when the fuel is only calling for 1.2 bar for example?

I have a big turbo installed that can take 2.5 relative boost, so as long as it's not more than the turbo can take, theoretically it should be OK?

Depends...

I'm not sure how well you understand how diesels, AKA compression ignition, engines work?

While you're checking that out, I would point out that engines don't make 'power" from the fuel, they make power from the burning fuel increasing the temperature, and hence pressure, of the gases in the combustion chamber. In practice, this means that even with the same amount of diesel fuel added, you should gain some power because there in a greater mass with the additional boost, to be expanded - but that is rather situation dependent, as the variables swing it one way or the other.

This means that, as some friends have demonstrated, simply adding a turbo' with no other tuning, etc, changes, can make a marked improvement - in principle, adding a larger turbo', assuming it's characteristcs are suitable, should have the sme affect.

I would strongly suggest you check out Mr Banks' videos on his testing and development of diesel engines - https://www.youtube.com/@bankspower

I'd be interested to read what the gurus have to say, including any corrections where I may be wrong.

My $.02,

In situations where a diesel vehicle has fuel rate as the limiting variable on horsepower as a consequence of injector size, HPFP capacity, etc. Then adding more airflow to lean out the mixture at WOT can be an effective way to gain HP. For instance, if the factory fuel system and stock turbo result in lambda 1.05 @WOT and power output of 500rwhp then adding a turbo capable of moving enough air to get combustion closer to 1.2 lambda will likely pick up 50-60HP or more. You'll reach a point of diminishing returns as you go leaner than 1.25 lambda.

Hope this helps,

Nick