I am going to simulate an engine intake manifold by star ccm+ .I have msss flow at inlet and pressure at outlet. Please answer to these questions:

1- These analysis should be compressible or incompressible?

I ran incompressible analysis , and as expected I found little pressure change. But I suspect that, this may be compressible analysis. Kindly guid me for this.

You REALLY need to do a compressible analysis!
In an intake system it's very important the dynamics of the fluid and the pressure-wave propagation phenomena, if you're interested in a transient analysis of the intake phase of the engine's cycle.
If instead you want to know the efflux coeff. of the intake duct, or something like that for example, also in this case the compressible fluid is very important: as you know the fluid will have a different behaviour inside the intake duct when that fluid is compressible, and you'll get also a different behaviour inside the boundary layer.
I suggest you do do a sim with compressible flow, try to use the solution of incompressible flow as initial condition for the new sim (so you lower the problem in the start up phase of the sim)
Regards,
Andrea

Hi Andrea, Thanks a lot. I will run with comp flow and will update you if required.

hello sir, i am mtech student undrgoing industrial traning & would like to do project on hydraulic manifold analysis. i have got the idea of design of hydraulic ckt,as well as manifold.please guide me