I have heard a lot of people say that you run lean when using an FCD on a TII, but isn't the AFM what determines how much fuel the engine gets? If boost is over the cut-off, the engine should still be seeing the proper amount of fuel to mix with the air that the AFM is reporting.

Can someone tell me for sure if an FCD leans out a car or not? It doesn't matter to me, because I don't need one, but I still want to know.

 

No, an FCD doesn't cause the car to run lean, contrary to popular belief. It does, however, prevent the engine from retarding ignition to the degree (no pun intended) necessary for safety at these higher boost levels.

The main thing that causes you to run lean is the maxing out of the stock injectors and the overworking of the stock fuel pump.

Brandon

 

This can and will still cause you to blow your motor however. Ignition is just as if not more important than fuel when it comes to keeping the engine together.

 

I'm glad I took my FCD off, then. I don't need any more reasons my engine could die.

 

I think the confusion about FCD’s comes from that fact the everyone knows leans mixtures cause detonation and hence engine failure, but less people realise that over-advanced ignition also causes detonation. So engine failures caused by detonation on FCD-equipped engines were incorrectly blamed on mixtures assumed to be lean.
Either way, you just need to be aware that while an FCD allows you to exceed 8.6psi, you shouldn’t go much past that.