Is the Mixture broken?

[Viper2000]

Actually it's more complicated than that.

Auto leaning does work, but not on all aeroplanes.

If you take the Tiger Moth for a flight, and have plenty of time on your hands, you can climb it above the altitudes at which the fighters start to suffer backfires and suffer no ill effects. If you fly in no cockpit mode then you can look at the exhaust flames and see that they stay blue.

Also, I tried moving the mixture control in the Tiger Moth and it appeared to be locked in the rich position.

Make of it what you will, but I think that either there has been a mix-up, such that the Tiger Moth has ended up with an automatic mixture control that a fighter should have, and the fighters have ended up with something weird, or else the actual culprit is the supercharger.

Obviously I don't know how the model is coded. However, if they were trying for really high fidelity then a real SU carburettor has two needles, one controlled by ambient pressure and the other controlled by supercharger boost pressure. The second needle therefore enriches the mixture to protect against detonation at high boost pressures, whilst the first needle provides the basic fuel demands of the engine and also provides altitude compensation.

It seems to me that what's happening is that the model is enriching the mixture as a function of the ratio*:

P[intakemanifold]/P[ambient]

This means that we're getting boost compensation in the correct sense, but no altitude compensation. What should actually happen is that we should have

Needle 1
Spring acting to close needle, vs atmospheric pressure acting to open it

Needle 2
Spring acting to close needle, vs boost pressure acting to open it

*NB - this ratio is not the supercharger pressure ratio. To a first order, the supercharger pressure ratio is a simple function of rpm because the throttle is upstream of the supercharger. The second order correction is that its running line is also going to be affected by exhaust back pressure. For this reason I'm not entirely convinced by the boost increase with rpm reduction that we see, because supercharger pressure ratio varies as rpm^2, whilst engine air consumption varies roughly as rpm^1. But that's another debate.