FMU is fuel management unit, it's rising rate fuel pressure regulator which raise the fuel pressure relative to boost pressure, a 10:1 fmu raise the fuel 10 psi for every 1 psi boost, for example if your car has 40psi fuel pressure, when running 5psi boost a 10:1 fmu it will raise the fuel pressure to 40+5x10=90psi, in simple terms increasing the fuel pressure make the injectors acts as bigger injectors (increase flow). as i mentioned before this it's not ideal but for low boost (like 0.5bar of boost) it's better than nothing.

this book, will be a good start for turbo system
Maximum Boost: Designing, Testing, and Installing Turbocharger Systems by Corky Bell

Maximum Boost: Designing, Testing, and Installing Turbocharger Systems (Engineering and Performance): Corky Bell: 9780837601601: Amazon.com: Books

and Performance Fuel Injection Systems by Matt Cramer
which is great book that i highly recommend it for better understanding of EFI tuning with simple explanation

Performance Fuel Injection Systems HP1557: How to Design, Build, Modify, and Tune EFI and ECU Systems.Covers Components, Sensors, Fuel and Ignition ... Tips, Aftermarket ECUs, and EFI Convers: Matt Cramer, Jerry Hoffmann: 9781557885579: Amazon.com: B

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W123 - M111+Turbo