Hello, 

On FSX, using the default baron 58, I barely can get over 12000 feet at full power and a V/S of 100 feet yet baron's altitude ceiling is 20668!  Doew anyone know why?

Are you adjusting the Mixture levers as you go up in altitude?  The higher you climb, the leaner the mixture needs to be.  Make sure your Pitot Tube heat is on.  Although it's not really critical on the default piston props, it's a good idea, shortly after takeoff and wheels up, to adjust the prop levers to lower the prop RPM down into the green zone.

No I wasn't!  I just got to playing with it right now, it has improved.  After researching a little into it, what I've got so far is I need to keep my rpm around 2300 by adjusting the prop lever and to save fuel I need to adjust the mixture lever, what I do I bring it from full right rich to a point where I see it effecting my RPM and then go back untill my rpm stays the way it was.  I don't know if that's the proper way of doing it but it improved quite a bit.  The da42 doesn't have these levers though, does this mean that the airplane does it automatically?

Don't know anything about the DA-42.  You might check the aircraft.cfg under [engine] to see if the AirFuel Mixture=0 or 1.  If 1, then the mixture is adjusted automatically.

I'm never sure if an aircraft with a fuel-injected engine needs to have mixture levers or to manually adjust the mixture.

The Diamond features an EECU Single Lever Control System. It is much different.  Is it not also a diesel engine aircraft?

The early ones might of been but not the latest one!  I believe they changed the engines a couple of times already.

http://www.diamond-air.at/content/diamond-air/downloads/DA42-VI/DA42-VI_Factsheet.pdf

Your PDF file describes the diesel engine mentioned by fppilot. Is yours one of the "early ones" or is yours different?  List the [engine] section from the aircraft.cfg for further clarification.  This way we can see why you don't have to adjust the mixture for the DA-42.

[GeneralEngineData]
//0=Piston, 1=Jet, 2=None, 3=Helo-Turbine, 4=Rocket, 5=Turboprop
engine_type = 0
fuel_flow_scalar =      0.6
engine.0 = 4, -4, 0,
engine.1 = 4,  4, 0,
min_throttle_limit=0                        

[piston_engine]
power_scalar=           0.74
cylinder_displacement= 90.17 
compression_ratio=       8.5 
number_of_cylinders=       4 
max_rated_rpm=          2300   
max_rated_hp=            135    
fuel_metering_type=        0  
cooling_type=              0 
normalized_starter_torque= 30 
turbocharged=              1 
max_design_mp=            29 
min_design_mp=             5 
critical_altitude=     13000 
emergency_boost_type=      0 
emergency_boost_mp_offset= 0
emergency_boost_gain_offset= 0
fuel_air_auto_mixture=     1 
auto_ignition=             0 
max_rpm_mechanical_efficiency_scalar=  1 
idle_rpm_mechanical_efficiency_scalar= 1 
max_rpm_friction_scalar=   1
idle_rpm_friction_scalar=0.7 
 

Feathering and air mixture is automatic on this airplane.

 

And here's why......fuel_air_auto_mixture=     1

Just thinking that you can make the same change in the Baron's aircraft.cfg and never have to worry about adjusting mixture.

The reason I asked is that there are no levers for either one in the cockpit!  Now I understand as to why they claim their plane is easier to fly!

In the Baron, you will need to lean the mixture as you increase your altitude, otherwise, the mixture becomes too rich for the lower oxygen content of higher altitudes.  The result is poor power and fouling.